sperm start the journey in the coiled ducts of what

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Sperm release pathway

Sperm are produced and released by the male reproductive organs.

The testes are where sperm are produced. The testes are linked to the rest of the male reproductive organs by the vas deferens, which extends over the base of the pelvic bone or ilium, and wraps around to the ampulla, seminal vesicle, and prostate. The urethra then runs from the bladder through the penis.

Sperm production in the testes takes place in coiled structures called seminiferous tubules.

Along the top of each testicle is the epididymis. This is a cordlike structure where the sperm mature and are stored.

The release process starts when the penis fills with blood and becomes erect. Continuing to stimulate the penis will cause an ejaculation.

Mature sperm begin their journey by travelling from the epididymis to the vas deferens, which propels sperm forward with smooth muscle contractions.

The sperm arrive first at the ampulla just above the prostate gland. Here, secretions from the seminal vesicle located next to the ampulla are added.

Next, the seminal fluid is propelled forward through the ejaculatory ducts toward the urethra. As it passes the prostate gland, a milky fluid is added to make semen.

Finally, the semen is ejaculated from the penis through the urethra.

Review Date 10/15/2023

Updated by: Kelly L. Stratton, MD, FACS, Associate Professor, Department of Urology, University of Oklahoma Health Sciences Center, Oklahoma City, OK. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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14.3: Structures of the Male Reproductive System

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• Suzanne Wakim & Mandeep Grewal
• Butte College

Rocky Mountain Oysters

First, they are peeled and pounded flat. Then, they are coated in flour, seasoned with salt and pepper, and deep-fried. What are they? They are often called Rocky Mountain oysters, but they don’t come from the sea. They may also be known as Montana tendergroin, cowboy caviar, or swinging beef — all names that hint at their origins. Here’s another hint: they are harvested only from male animals, such as bulls or sheep. What are they? In a word: testes.

Testes and Scrotum

The two testes (singular, testis) are sperm- and testosterone-producing gonads in male mammals, including male humans. These and other organs of the human male reproductive system are shown in Figure \(\PageIndex{2}\). The testes are contained within the scrotum, a pouch made of skin and smooth muscle that hangs down behind the penis.

Testes Structure

The testes are filled with hundreds of tiny tubes, called seminiferous tubules , which are the functional units of the testes. The seminiferous tubules are coiled and tightly packed within divisions of the testis called lobules. Lobules are separated from one another by internal walls (or septa).

One or more seminiferous tubules are tightly coiled within each of the hundreds of lobules in the testis. A single testis normally contains a total of about 30 m (90 ft) of these tightly packed tubules! As shown in the cross-sectional drawing of a seminiferous tubule in Figure \(\PageIndex{2}\), the tubule contains sperm in several different stages of development

Other Scrotal Structures

Besides the two testes, the scrotum also contains a pair of organs called epididymes (singular, epididymis) and part of each of the paired vas deferens (or ducti deferens). Both structures play important functions in the production or transport of sperm.

The seminiferous tubules within each testis join together to form ducts (called efferent ducts) that transport immature sperm to the epididymis associated with that testis. Each epididymis (plural, epididymes) consists of a tightly coiled tubule with a total length of about 6 m (20 ft). As shown in Figure \(\PageIndex{2}\) the epididymis is generally divided into three parts: the head (which rests on top of the testis), the body (which drapes down the side of the testis), and the tail (which joins with the vas deferens near the bottom of the testis). The functions of the two epididymes are to mature sperm, and then to store that mature sperm until they leave the body during an ejaculation when they pass the sperm on to the vas deferens.

Vas Deferens

The vas deferens, also known as sperm ducts, are a pair of thin tubes, each about 30 cm (1 ft) long, which begin at the epididymis in the scrotum and continue up into the pelvic cavity. They are composed of ciliated epithelium and smooth muscle. These structures help the vas deferens fulfill their function of transporting sperm from the epididymes to the ejaculatory ducts, which are accessory structures of the male reproductive system.

Accessory Structures

In addition to the structures within the scrotum, the male reproductive system includes several internal accessory structures. They include the ejaculatory ducts, seminal vesicles, and the prostate and bulbourethral (Cowper’s) glands. See Figure \(\PageIndex{3}\). The major reproductive structures represented in this figure are explained below.

Seminal Vesicles

The seminal vesicles are a pair of glands that each consist of a single tube, which is folded and coiled upon itself. Each vesicle is about 5 cm (2 in.) long and has an excretory duct that merges with the vas deferens to form one of the two ejaculatory ducts. Fluid secreted by the seminal vesicles into the ducts makes up about 70 percent of the total volume of semen, which is the sperm-containing fluid that leaves the penis during an ejaculation. The fluid from the seminal vesicles is alkaline, so it gives semen a basic pH that helps prolong the lifespan of sperm after it enters the acidic secretions inside the female vagina. Fluid from the seminal vesicles also contains proteins, fructose (a simple sugar), and other substances that help nourish sperm.

Ejaculatory Ducts

The ejaculatory ducts form where the vas deferens join with the ducts of the seminal vesicles in the prostate gland. They connect the vas deferens with the urethra. The ejaculatory ducts carry sperm from the vas deferens, as well as secretions from the seminal vesicles and the prostate gland that together form semen. The substances secreted into semen by the glands as it passes through the ejaculatory ducts control its pH and provide nutrients to sperm, among other functions. The fluid itself provides sperm with a medium in which to “swim.”

Prostate Gland

The prostate gland is located just below the seminal vesicles. It is a walnut-sized organ that surrounds the urethra and its junction with the two ejaculatory ducts. The function of the prostate gland is to secrete a slightly alkaline fluid that constitutes close to 30 percent of the total volume of semen. The prostate fluid contains small quantities of proteins, such as enzymes. In addition, it has a very high concentration of zinc, which is an important nutrient for maintaining sperm quality and motility.

Bulbourethral Glands

Also called Cowper’s glands, the two bulbourethral glands are each about the size of a pea and located just below the prostate gland. The bulbourethral glands secrete a clear, alkaline fluid that is rich in proteins. Each of the glands has a short duct that carries the secretions into the urethra, where they make up a tiny percentage of the total volume of semen. The function of the bulbourethral secretions is to help lubricate the urethra and neutralize any urine (which is acidic) that may remain in the urethra.

The penis is the external male organ that has the reproductive function of delivering sperm to the female reproductive tract. This function is called intromission. The penis also serves as the organ that excretes urine.

Structure of the Penis

The structure of the penis and its location relative to other reproductive organs are shown in Figure \(\PageIndex{4}\). The part of the penis that is located inside the body and out of sight is called the root of the penis. The shaft of the penis is the part of the penis that is outside the body. The enlarged, bulbous end of the shaft is called the glans penis.

The urethra passes through the penis to carry urine from the bladder — or semen from the ejaculatory ducts — through the penis and out of the body. After leaving the urinary bladder, the urethra passes through the prostate gland, where the urethra is joined by the ejaculatory ducts. From there, the urethra passes through the penis to its external opening at the tip of the glans penis. Called the external urethral orifice, this opening provides a way for urine or semen to leave the body.

Tissues of the Penis

The penis is covered with skin (epithelium) that is unattached and free to move over the body of the penis. In an uncircumcised male, the glans penis is also mainly covered by epithelium, which (in this location) is called the foreskin, and below which is a layer of the mucous membrane. The foreskin is attached to the penis at an area on the underside of the penis called the frenulum.

As shown in Figure \(\PageIndex{5}\), the interior of the penis consists of three columns of spongy tissue that can fill with blood and swell in size, allowing the penis to become erect. This spongy tissue is called corpus cavernosum (plural, corpora cavernosa). Two columns of this tissue run side by side along the top of the shaft, and one column runs along the bottom of the shaft. The urethra runs through this bottom column of spongy tissue, which is sometimes called corpus spongiosum. The glans penis also consists mostly of spongy erectile tissue. Veins and arteries run along the top of the penis, allowing blood circulation through the spongy tissues.

Feature: Human Biology in the News

Lung, heart, kidney, and other organ transplants have become relatively commonplace, so when they occur, they are unlikely to make the news. However, when the nation’s first penis transplant took place, it was considered very newsworthy.

In 2016, Massachusetts General Hospital in Boston announced that a team of its surgeons had performed the first penis transplant in the United States. The patient who received the donated penis was a 64-year-old cancer patient. During the 15-hour procedure, the intricate network of nerves and blood vessels of the donor penis were connected with those of the penis recipient. The surgery went well, but doctors reported it would be a few weeks until they would know if normal urination would be possible, and even longer before they would know if sexual functioning would be possible. At the time that news of the surgery was reported in the media, the patient had not shown any signs of rejecting the donated organ. The surgeons also reported they were hopeful that such transplants would become relatively common, and that patient populations would expand to include wounded warriors and transgender males seeking to transition.

The 2016 Massachusetts operation was not the first penis transplant ever undertaken. The world’s first successful penis transplant was actually performed in 2014 in Cape Town, South Africa. A young man who had lost his penis from complications of a botched circumcision at age 18 was given a donor penis three years later. That surgery lasted nine hours and was highly successful. The young man made a full recovery and regained both urinary and sexual functions in the transplanted organ.

In 2005, a man in China also received a donated penis in a technically successful operation. However, the patient asked doctors to reverse the procedure just two weeks later, because of psychological problems associated with the transplanted organ for both himself and his wife.

• What are the testes? Where are they located?
• Describe the structure of a testis.
• Identify the epididymis and its functions.
• What are the vas deferens? What do they do?
• Where are the seminal vesicles located? What is their reproductive role?
• Which parts of the male reproductive system are connected by the ejaculatory ducts? What fluids enter and leave the ejaculatory ducts?
• Identify the location of the prostate gland relative to other male reproductive organs. What is the prostate’s function?
• Where are the bulbourethral glands? What is their function?
• Relate the structure of the penis to its two basic functions.
• Sperm are produced here.
• Sperm mature here.
• Sperm are transported through the penis in this structure.
• This is a gland that produces fluid that is a major component of semen.
• A vasectomy is a form of birth control for men that is performed by surgically cutting or blocking the vas deferens so that sperm cannot be ejaculated out of the body. Do you think men who have a vasectomy emit semen when they ejaculate? Why or why not?
• seminal vesicles
• glans penis

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Morning erections are part of the normal sleep cycle in men. Learn more here:

Attributions

• Lamb Fries by Paul Lowry, CC BY 2.0 via Wikimedia Commons
• Testicle by (public domain; National Cancer Institute via Wikimedia.org )
• Male Reproductive Anatomy by OpenStax College licensed CC BY 3.0
• Anatomical Illustration by Grant, John Charles Boileau licensed public domain, via Wikimedia Commons
• Cross-section of the penis , by Gray's Anatomy, licensed public domain, via Wikimedia Commons
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

Duct System

Sperm cells pass through a series of ducts to reach the outside of the body. After they leave the testes, the sperm passes through the epididymis , ductus deferens , ejaculatory duct , and urethra .

Sperm leave the testes through a series of efferent ducts that enter the epididymis. Each epididymis is a long (about 6 meters) tube that is tightly coiled to form a comma-shaped organ located along the superior and posterior margins of the testes. When the sperm leave the testes, they are immature and incapable of fertilizing ova . They complete their maturation process and become fertile as they move through the epididymis. Mature sperm are stored in the lower portion, or tail, of the epididymis.

Ductus Deferens

The ductus deferens, also called vas deferens , is a fibromuscular tube that is continuous ( or contiguous ) with the epididymis. It begins at the bottom (tail) of the epididymis then turns sharply upward along the posterior margin of the testes. The ductus deferens enters the abdominopelvic cavity through the inguinal canal and passes along the lateral pelvic wall . It crosses over the ureter and posterior portion of the urinary bladder , and then descends along the posterior wall of the bladder toward the prostate gland . Just before it reaches the prostate gland, each ductus deferens enlarges to form an ampulla. Sperm are stored in the proximal portion of the ductus deferens, near the epididymis, and peristaltic movements propel the sperm through the tube.

The proximal portion of the ductus deferens is a component of the spermatic cord , which contains vascular and neural structures that supply the testes. The spermatic cord contains the ductus deferens, testicular artery and veins , lymph vessels, testicular nerve , cremaster muscle that elevates the testes for warmth and at times of sexual stimulation, and a connective tissue covering.

Ejaculatory Duct

Each ductus deferens, at the ampulla , joins the duct from the adjacent seminal vesicle (one of the accessory glands) to form a short ejaculatory duct. Each ejaculatory duct passes through the prostate gland and empties into the urethra.

The urethra extends from the urinary bladder to the external urethral orifice at the tip of the penis . It is a passageway for sperm and fluids from the reproductive system and urine from the urinary system . While reproductive fluids are passing through the urethra, sphincters contract tightly to keep urine from entering the urethra.

The male urethra is divided into three regions. The prostatic urethra is the proximal portion that passes through the prostate gland. It receives the ejaculatory duct, which contains sperm and secretions from the seminal vesicles , and numerous ducts from the prostate glands. The next portion, the membranous urethra , is a short region that passes through the pelvic floor . The longest portion is the penile urethra (also called spongy urethra or cavernous urethra), which extends the length of the penis and opens to the outside at the external urethral orifice. The ducts from the bulbourethral glands open into the penile urethra.

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Producing and Delivering Sperm: The Male Reproductive System

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The testes are the primary reproductive organs and generate sperm cells through a process called spermatogenesis . The glands of the male reproductive system produce sperm and seminal fluid. The prostate gland, the seminal vesicles, and the bulbourethral glands contribute seminal fluid to semen, which carries and protects the sperm. During sexual intercourse, semen moves through a series of ducts to deliver the semen directly into the female reproductive system.

1. The Testes Produce Millions and Millions of Sperm Each Day

The testes (or testicles) are the male gonads and sit below the penis within a sac called the scrotum. They are 4-5 cm long, 2.5 cm in diameter, and covered with two membranous layers, the tunica albuginea and the tunica vaginalis. The testes generate sperm, the male sex cells, as well as testosterone and other sex hormones. The production of sperm is constant and occurs within numerous lobules in each testis. First, structures called seminiferous tubules generate stem cells. These cells, the spermatogonia, divide into spermatocytes, and then divide further to become spermatids. The process is called spermatogenesis . Spermatids move from the testis to the epididymis and mature into sperm.

2. Reproductive Ducts Push Sperm and Semen Through the Internal Genitalia

The epididymis, vas deferens, ejaculatory ducts, and urethra form a four-part transportation system. The epididymis sits directly on top of each testis. Sperm from the testis mature as they move through the coiled duct of the epididymis. During sexual intercourse and ejaculation, they are expelled into the vas deferens. The vas deferens pushes the sperm up over the bladder and down toward the prostate gland. There, the vas deferens joins the ends of the seminal vesicles (accessory reproductive glands) to form the ejaculatory ducts. The ejaculatory ducts receive seminal fluid from the vesicles, pass through the prostate, and move semen into the urethra. The male urethra extends from the bladder, through the prostate, to the external orifice at the end of the penis. It receives additional seminal fluids from the prostate before it expels semen out of the body.

3. Accessory Reproductive Glands Add Seminal Fluid to Semen

Sperm cells depend on seminal fluid to keep them moving and alive. This fluid is produced during ejaculation by accessory glands: the seminal vesicles, the prostate, and the bulbourethral glands. The seminal vesicles, two saclike structures, sit close behind the bladder and extend toward the bladder. There they each join one of the vas deferens to form the ejaculatory ducts. The vesicles secrete a whitish-brown fluid containing sugars, prostaglandins, and other substances that makes up two-thirds of the semen volume. The prostate, located under the bladder and above the start of the penis, contains the ejaculatory ducts and the prostatic urethra. As semen enters the urethra, the prostate secretes enzymes that help activate the sperm. The bulbourethral glands (or Cowper’s glands) are pea-sized, with single ducts that connect to the urethra where it emerges from the prostate. These glands add mucus that helps with sperm motility.

4. Spongy Internal Tissue Is Key to the Reproductive Function of the Penis

The penis is part of the male external genitalia, suspended from the body at the front and sides of the pubic arch. Internally, the penis consists of three connected columns of tissue. The paired corpora cavernosa extend together from the root of the penis through the body. The corpus spongiosum runs along the underside of the cavernosa. It contains the spongy urethra and expands past the body of the penis to form the glans penis (the tip). The tissue of the corpora cavernosa and the corpus spongiosum is spongelike in structure. During arousal and intercourse, the spaces in the tissue fill with blood. The penis stiffens. Now it can penetrate the vagina to deliver semen directly into the female reproductive system.

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External Sources

Information on prostate cancer screening from the CDC.

Male reproductive system terms from the Des Moines University Online Medical Terminology Course.

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Pathway of sperm

The testes are where sperm is manufactured. The epididymis is a long coiled structure topping the testis, and it receives immature sperm from the testis and stores them as they mature. When ejaculation occurs, sperm are forcefully expelled from the tail of the epididymis into the vas deferens. Sperm then travel through the vas deferens through up the spermatic cord into the pelvic cavity, over the ureter to the prostate behind the bladder. Here, the vas deferens joins with the seminal vesicle to form the ejaculatory duct, which passes through the prostate and empties into the urethra. When ejaculation occurs, rhythmic muscle movements propel the sperm forward.

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Review Date: 1/25/2013  

Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.

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23.2: Anatomy and Physiology of the Male Reproductive System

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Learning Objectives

• Describe the structure and function of the organs of the male reproductive system
• Describe the structure and function of the sperm cell
• Explain the events during spermatogenesis that produce haploid sperm from diploid cells
• Identify the importance of testosterone in male reproductive function

Unique for its role in human reproduction, a gamete is a specialized sex cell carrying 23 chromosomes—one half the number in body cells. At fertilization, the chromosomes in one male gamete, called a sperm (or spermatozoon), combine with the chromosomes in one female gamete, called an oocyte. The function of the male reproductive system (Figure \(\PageIndex{1}\)) is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and androgens, the hormones that support male reproductive physiology. In humans, the most important male androgen is testosterone. Several accessory organs and ducts aid the process of sperm maturation and transport the sperm and other seminal components to the penis, which delivers sperm to the female reproductive tract. In this section, we examine each of these different structures, and discuss the process of sperm production and transport.

The testes are located in a skin-covered, highly pigmented, muscular sack called the scrotum that extends from the body behind the penis (see Figure \(\PageIndex{1}\)). This location is important in sperm production, which occurs within the testes, and proceeds more efficiently when the testes are kept 2 to 4°C below core body temperature.

The dartos muscle makes up the subcutaneous muscle layer of the scrotum (Figure \(\PageIndex{2}\)). It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each housing one testis. Descending from the internal oblique muscle of the abdominal wall are the two cremaster muscles, which cover each testis like a muscular net. By contracting simultaneously, the dartos and cremaster muscles can elevate the testes in cold weather (or water), moving the testes closer to the body and decreasing the surface area of the scrotum to retain heat. Alternatively, as the environmental temperature increases, the scrotum relaxes, moving the testes farther from the body core and increasing scrotal surface area, which promotes heat loss. Externally, the scrotum has a raised medial thickening on the surface called the raphae.

The testes (singular = testis) are the male gonads —that is, the male reproductive organs. They produce both sperm and androgens, such as testosterone, and are active throughout the reproductive lifespan of the male.

Paired ovals, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum (see Figure \(\PageIndex{2}\)). They are surrounded by two distinct layers of protective connective tissue (Figure \(\PageIndex{3}\)). The outer tunica vaginalis is a serous membrane that has both a parietal and a thin visceral layer. Beneath the tunica vaginalis is the tunica albuginea, a tough, white, dense connective tissue layer covering the testis itself. Not only does the tunica albuginea cover the outside of the testis, it also invaginates to form septa that divide the testis into 300 to 400 structures called lobules. Within the lobules, sperm develop in structures called seminiferous tubules. During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. This is called the “descent of the testis.” Cryptorchidism is the clinical term used when one or both of the testes fail to descend into the scrotum prior to birth.

The tightly coiled seminiferous tubules form the bulk of each testis. They are composed of developing sperm cells surrounding a lumen, the hollow center of the tubule, where formed sperm are released into the duct system of the testis. Specifically, from the lumens of the seminiferous tubules, sperm move into the straight tubules (or tubuli recti), and from there into a fine meshwork of tubules called the rete testes. Sperm leave the rete testes, and the testis itself, through the 15 to 20 efferent ductules that cross the tunica albuginea.

Inside the seminiferous tubules are six different cell types. These include supporting cells called sustentacular cells, as well as five types of developing sperm cells called germ cells. Germ cell development progresses from the basement membrane—at the perimeter of the tubule—toward the lumen. Let’s look more closely at these cell types.

Sertoli Cells

Surrounding all stages of the developing sperm cells are elongate, branching Sertoli cells . Sertoli cells are a type of supporting cell called a sustentacular cell, or sustenocyte, that are typically found in epithelial tissue. Sertoli cells secrete signaling molecules that promote sperm production and can control whether germ cells live or die. They extend physically around the germ cells from the peripheral basement membrane of the seminiferous tubules to the lumen. Tight junctions between these sustentacular cells create the blood–testis barrier , which keeps bloodborne substances from reaching the germ cells and, at the same time, keeps surface antigens on developing germ cells from escaping into the bloodstream and prompting an autoimmune response.

The least mature cells, the spermatogonia (singular = spermatogonium), line the basement membrane inside the tubule. Spermatogonia are the stem cells of the testis, which means that they are still able to differentiate into a variety of different cell types throughout adulthood. Spermatogonia divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. The process that begins with spermatogonia and concludes with the production of sperm is called spermatogenesis .

Spermatogenesis

As just noted, spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis (see Figure \(\PageIndex{3}\)). The process begins at puberty, after which time sperm are produced constantly throughout a man’s life. One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules. Sperm counts—the total number of sperm a man produces—slowly decline after age 35, and some studies suggest that smoking can lower sperm counts irrespective of age.

The process of spermatogenesis begins with mitosis of the diploid spermatogonia (Figure \(\PageIndex{4}\)). Because these cells are diploid (2 n ), they each have a complete copy of the father’s genetic material, or 46 chromosomes. However, mature gametes are haploid (1 n ), containing 23 chromosomes—meaning that daughter cells of spermatogonia must undergo a second cellular division through the process of meiosis.

Two identical diploid cells result from spermatogonia mitosis. One of these cells remains a spermatogonium, and the other becomes a primary spermatocyte , the next stage in the process of spermatogenesis. As in mitosis, DNA is replicated in a primary spermatocyte, and the cell undergoes cell division to produce two cells with identical chromosomes. Each of these is a secondary spermatocyte. Now a second round of cell division occurs in both of the secondary spermatocytes, separating the chromosome pairs. This second meiotic division results in a total of four cells with only half of the number of chromosomes. Each of these new cells is a spermatid . Although haploid, early spermatids look very similar to cells in the earlier stages of spermatogenesis, with a round shape, central nucleus, and large amount of cytoplasm. A process called spermiogenesis transforms these early spermatids, reducing the cytoplasm, and beginning the formation of the parts of a true sperm. The fifth stage of germ cell formation—spermatozoa, or formed sperm—is the end result of this process, which occurs in the portion of the tubule nearest the lumen. Eventually, the sperm are released into the lumen and are moved along a series of ducts in the testis toward a structure called the epididymis for the next step of sperm maturation.

Structure of Formed Sperm

Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas women typically ovulate only one oocyte per month as is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, mid-piece, and tail region (Figure \(\PageIndex{5}\)). The head of the sperm contains the extremely compact haploid nucleus with very little cytoplasm. These qualities contribute to the overall small size of the sperm (the head is only 5 μ m long). A structure called the acrosome covers most of the head of the sperm cell as a “cap” that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. Tightly packed mitochondria fill the mid-piece of the sperm. ATP produced by these mitochondria will power the flagellum, which extends from the neck and the mid-piece through the tail of the sperm, enabling it to move the entire sperm cell. The central strand of the flagellum, the axial filament, is formed from one centriole inside the maturing sperm cell during the final stages of spermatogenesis.

Sperm Transport

To fertilize an egg, sperm must be moved from the seminiferous tubules in the testes, through the epididymis, and—later during ejaculation—along the length of the penis and out into the female reproductive tract.

Role of the Epididymis

From the lumen of the seminiferous tubules, the immotile sperm are surrounded by testicular fluid and moved to the epididymis (plural = epididymides), a coiled tube attached to the testis where newly formed sperm continue to mature (see Figure \(\PageIndex{3}\)). Though the epididymis does not take up much room in its tightly coiled state, it would be approximately 6 m (20 feet) long if straightened. It takes an average of 12 days for sperm to move through the coils of the epididymis, with the shortest recorded transit time in humans being one day. Sperm enter the head of the epididymis and are moved along predominantly by the contraction of smooth muscles lining the epididymal tubes. As they are moved along the length of the epididymis, the sperm further mature and acquire the ability to move under their own power. Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg. The more mature sperm are then stored in the tail of the epididymis (the final section) until ejaculation occurs.

Duct System

During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the ductus deferens (also called the vas deferens). The ductus deferens is a thick, muscular tube that is bundled together inside the scrotum with connective tissue, blood vessels, and nerves into a structure called the spermatic cord (see Figure \(\PageIndex{1}\) and Figure \(\PageIndex{2}\)). Because the ductus deferens is physically accessible within the scrotum, surgical sterilization to interrupt sperm delivery can be performed by cutting and sealing a small section of the ductus (vas) deferens. This procedure is called a vasectomy, and it is an effective form of male birth control. Although it may be possible to reverse a vasectomy, clinicians consider the procedure permanent, and advise men to undergo it only if they are certain they no longer wish to father children.

Watch this video to learn about a vasectomy. As described in this video, a vasectomy is a procedure in which a small section of the ductus (vas) deferens is removed from the scrotum. This interrupts the path taken by sperm through the ductus deferens. If sperm do not exit through the vas, either because the man has had a vasectomy or has not ejaculated, in what region of the testis do they remain?

From each epididymis, each ductus deferens extends superiorly into the abdominal cavity through the inguinal canal in the abdominal wall. From here, the ductus deferens continues posteriorly to the pelvic cavity, ending posterior to the bladder where it dilates in a region called the ampulla (meaning “flask”).

Sperm make up only 5 percent of the final volume of semen , the thick, milky fluid that the male ejaculates. The bulk of semen is produced by three critical accessory glands of the male reproductive system: the seminal vesicles, the prostate, and the bulbourethral glands.

Seminal Vesicles

As sperm pass through the ampulla of the ductus deferens at ejaculation, they mix with fluid from the associated seminal vesicle (see Figure \(\PageIndex{1}\)). The paired seminal vesicles are glands that contribute approximately 60 percent of the semen volume. Seminal vesicle fluid contains large amounts of fructose, which is used by the sperm mitochondria to generate ATP to allow movement through the female reproductive tract.

The fluid, now containing both sperm and seminal vesicle secretions, next moves into the associated ejaculatory duct , a short structure formed from the ampulla of the ductus deferens and the duct of the seminal vesicle. The paired ejaculatory ducts transport the seminal fluid into the next structure, the prostate gland.

Prostate Gland

As shown in Figure \(\PageIndex{1}\), the centrally located prostate gland sits anterior to the rectum at the base of the bladder surrounding the prostatic urethra (the portion of the urethra that runs within the prostate). About the size of a walnut, the prostate is formed of both muscular and glandular tissues. It excretes an alkaline, milky fluid to the passing seminal fluid—now called semen—that is critical to first coagulate and then decoagulate the semen following ejaculation. The temporary thickening of semen helps retain it within the female reproductive tract, providing time for sperm to utilize the fructose provided by seminal vesicle secretions. When the semen regains its fluid state, sperm can then pass farther into the female reproductive tract.

The prostate normally doubles in size during puberty. At approximately age 25, it gradually begins to enlarge again. This enlargement does not usually cause problems; however, abnormal growth of the prostate, or benign prostatic hyperplasia (BPH), can cause constriction of the urethra as it passes through the middle of the prostate gland, leading to a number of lower urinary tract symptoms, such as a frequent and intense urge to urinate, a weak stream, and a sensation that the bladder has not emptied completely. By age 60, approximately 40 percent of men have some degree of BPH. By age 80, the number of affected individuals has jumped to as many as 80 percent. Treatments for BPH attempt to relieve the pressure on the urethra so that urine can flow more normally. Mild to moderate symptoms are treated with medication, whereas severe enlargement of the prostate is treated by surgery in which a portion of the prostate tissue is removed.

Another common disorder involving the prostate is prostate cancer. According to the Centers for Disease Control and Prevention (CDC), prostate cancer is the second most common cancer in men. However, some forms of prostate cancer grow very slowly and thus may not ever require treatment. Aggressive forms of prostate cancer, in contrast, involve metastasis to vulnerable organs like the lungs and brain. There is no link between BPH and prostate cancer, but the symptoms are similar. Prostate cancer is detected by a medical history, a blood test, and a rectal exam that allows physicians to palpate the prostate and check for unusual masses. If a mass is detected, the cancer diagnosis is confirmed by biopsy of the cells.

Bulbourethral Glands

The final addition to semen is made by two bulbourethral glands (or Cowper’s glands) that release a thick, salty fluid that lubricates the end of the urethra and the vagina, and helps to clean urine residues from the penile urethra. The fluid from these accessory glands is released after the male becomes sexually aroused, and shortly before the release of the semen. It is therefore sometimes called pre-ejaculate. It is important to note that, in addition to the lubricating proteins, it is possible for bulbourethral fluid to pick up sperm already present in the urethra, and therefore it may be able to cause pregnancy.

Watch this video to explore the structures of the male reproductive system and the path of sperm, which starts in the testes and ends as the sperm leave the penis through the urethra. Where are sperm deposited after they leave the ejaculatory duct,

The penis is the male organ of copulation (sexual intercourse). It is flaccid for non-sexual actions, such as urination, and turgid and rod-like with sexual arousal. When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract.

The shaft of the penis surrounds the urethra (Figure \(\PageIndex{6}\)). The shaft is composed of three column-like chambers of erectile tissue that span the length of the shaft. Each of the two larger lateral chambers is called a corpus cavernosum (plural = corpora cavernosa). Together, these make up the bulk of the penis. The corpus spongiosum , which can be felt as a raised ridge on the erect penis, is a smaller chamber that surrounds the spongy, or penile, urethra. The end of the penis, called the glans penis , has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation (see Figure \(\PageIndex{1}\)). The skin from the shaft extends down over the glans and forms a collar called the prepuce (or foreskin). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. A surgical procedure called circumcision, often performed for religious or social reasons, removes the prepuce, typically within days of birth.

Both sexual arousal and REM sleep (during which dreaming occurs) can induce an erection. Penile erections are the result of vasocongestion, or engorgement of the tissues because of more arterial blood flowing into the penis than is leaving in the veins. During sexual arousal, nitric oxide (NO) is released from nerve endings near blood vessels within the corpora cavernosa and spongiosum. Release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis and induces the endothelial cells in the penile arterial walls to also secrete NO and perpetuate the vasodilation. The rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the penis. The result of this increased blood flow to the penis and reduced blood return from the penis is erection. Depending on the flaccid dimensions of a penis, it can increase in size slightly or greatly during erection, with the average length of an erect penis measuring approximately 15 cm.

DISORDERS OF THE Male Reproductive System

Erectile dysfunction (ED) is a condition in which a man has difficulty either initiating or maintaining an erection. The combined prevalence of minimal, moderate, and complete ED is approximately 40 percent in men at age 40, and reaches nearly 70 percent by 70 years of age. In addition to aging, ED is associated with diabetes, vascular disease, psychiatric disorders, prostate disorders, the use of some drugs such as certain antidepressants, and problems with the testes resulting in low testosterone concentrations. These physical and emotional conditions can lead to interruptions in the vasodilation pathway and result in an inability to achieve an erection.

Recall that the release of NO induces relaxation of the smooth muscles that surround the penile arteries, leading to the vasodilation necessary to achieve an erection. To reverse the process of vasodilation, an enzyme called phosphodiesterase (PDE) degrades a key component of the NO signaling pathway called cGMP. There are several different forms of this enzyme, and PDE type 5 is the type of PDE found in the tissues of the penis. Scientists discovered that inhibiting PDE5 increases blood flow, and allows vasodilation of the penis to occur.

PDEs and the vasodilation signaling pathway are found in the vasculature in other parts of the body. In the 1990s, clinical trials of a PDE5 inhibitor called sildenafil were initiated to treat hypertension and angina pectoris (chest pain caused by poor blood flow through the heart). The trial showed that the drug was not effective at treating heart conditions, but many men experienced erection and priapism (erection lasting longer than 4 hours). Because of this, a clinical trial was started to investigate the ability of sildenafil to promote erections in men suffering from ED. In 1998, the FDA approved the drug, marketed as Viagra®. Since approval of the drug, sildenafil and similar PDE inhibitors now generate over a billion dollars a year in sales, and are reported to be effective in treating approximately 70 to 85 percent of cases of ED. Importantly, men with health problems—especially those with cardiac disease taking nitrates—should avoid Viagra or talk to their physician to find out if they are a candidate for the use of this drug, as deaths have been reported for at-risk users.

Testosterone

Testosterone, an androgen, is a steroid hormone produced by Leydig cells . The alternate term for Leydig cells, interstitial cells, reflects their location between the seminiferous tubules in the testes. In male embryos, testosterone is secreted by Leydig cells by the seventh week of development, with peak concentrations reached in the second trimester. This early release of testosterone results in the anatomical differentiation of the male sexual organs. In childhood, testosterone concentrations are low. They increase during puberty, activating characteristic physical changes and initiating spermatogenesis.

Functions of Testosterone

The continued presence of testosterone is necessary to keep the male reproductive system working properly, and Leydig cells produce approximately 6 to 7 mg of testosterone per day. Testicular steroidogenesis (the manufacture of androgens, including testosterone) results in testosterone concentrations that are 100 times higher in the testes than in the circulation. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility. In addition to intratesticular secretion, testosterone is also released into the systemic circulation and plays an important role in muscle development, bone growth, the development of secondary sex characteristics, and maintaining libido (sex drive) in both males and females. In females, the ovaries secrete small amounts of testosterone, although most is converted to estradiol. A small amount of testosterone is also secreted by the adrenal glands in both sexes.

AGING AND THE Male Reproductive System

Declines in Leydig cell activity can occur in men beginning at 40 to 50 years of age. The resulting reduction in circulating testosterone concentrations can lead to symptoms of andropause, also known as male menopause. While the reduction in sex steroids in men is akin to female menopause, there is no clear sign—such as a lack of a menstrual period—to denote the initiation of andropause. Instead, men report feelings of fatigue, reduced muscle mass, depression, anxiety, irritability, loss of libido, and insomnia. A reduction in spermatogenesis resulting in lowered fertility is also reported, and sexual dysfunction can also be associated with andropausal symptoms.

Whereas some researchers believe that certain aspects of andropause are difficult to tease apart from aging in general, testosterone replacement is sometimes prescribed to alleviate some symptoms. Recent studies have shown a benefit from androgen replacement therapy on the new onset of depression in elderly men; however, other studies caution against testosterone replacement for long-term treatment of andropause symptoms, showing that high doses can sharply increase the risk of both heart disease and prostate cancer.

Chapter Review

Gametes are the reproductive cells that combine to form offspring. Organs called gonads produce the gametes, along with the hormones that regulate human reproduction. The male gametes are called sperm. Spermatogenesis, the production of sperm, occurs within the seminiferous tubules that make up most of the testis. The scrotum is the muscular sac that holds the testes outside of the body cavity.

Spermatogenesis begins with mitotic division of spermatogonia (stem cells) to produce primary spermatocytes that undergo the two divisions of meiosis to become secondary spermatocytes, then the haploid spermatids. During spermiogenesis, spermatids are transformed into spermatozoa (formed sperm). Upon release from the seminiferous tubules, sperm are moved to the epididymis where they continue to mature. During ejaculation, sperm exit the epididymis through the ductus deferens, a duct in the spermatic cord that leaves the scrotum. The ampulla of the ductus deferens meets the seminal vesicle, a gland that contributes fructose and proteins, at the ejaculatory duct. The fluid continues through the prostatic urethra, where secretions from the prostate are added to form semen. These secretions help the sperm to travel through the urethra and into the female reproductive tract. Secretions from the bulbourethral glands protect sperm and cleanse and lubricate the penile (spongy) urethra.

The penis is the male organ of copulation. Columns of erectile tissue called the corpora cavernosa and corpus spongiosum fill with blood when sexual arousal activates vasodilatation in the blood vessels of the penis. Testosterone regulates and maintains the sex organs and sex drive, and induces the physical changes of puberty. Interplay between the testes and the endocrine system precisely control the production of testosterone with a negative feedback loop.

Review Questions

Q. What are male gametes called?

D. testosterone

Q. Leydig cells ________.

A. secrete testosterone

B. activate the sperm flagellum

C. support spermatogenesis

D. secrete seminal fluid

Q. Which hypothalamic hormone contributes to the regulation of the male reproductive system?

A. luteinizing hormone

B. gonadotropin-releasing hormone

C. follicle-stimulating hormone

D. androgens

Q. What is the function of the epididymis?

A. sperm maturation and storage

B. produces the bulk of seminal fluid

C. provides nitric oxide needed for erections

D. spermatogenesis

Q. Spermatogenesis takes place in the ________.

A. prostate gland

B. glans penis

C. seminiferous tubules

D. ejaculatory duct

Critical Thinking Questions

Q. Briefly explain why mature gametes carry only one set of chromosomes.

A. A single gamete must combine with a gamete from an individual of the opposite sex to produce a fertilized egg, which has a complete set of chromosomes and is the first cell of a new individual.

Q. What special features are evident in sperm cells but not in somatic cells, and how do these specializations function?

A. Unlike somatic cells, sperm are haploid. They also have very little cytoplasm. They have a head with a compact nucleus covered by an acrosome filled with enzymes, and a mid-piece filled with mitochondria that power their movement. They are motile because of their tail, a structure containing a flagellum, which is specialized for movement.

Q. What do each of the three male accessory glands contribute to the semen?

A. The three accessory glands make the following contributions to semen: the seminal vesicle contributes about 60 percent of the semen volume, with fluid that contains large amounts of fructose to power the movement of sperm; the prostate gland contributes substances critical to sperm maturation; and the bulbourethral glands contribute a thick fluid that lubricates the ends of the urethra and the vagina and helps to clean urine residues from the urethra.

Q. Describe how penile erection occurs.

A. During sexual arousal, nitric oxide (NO) is released from nerve endings near blood vessels within the corpora cavernosa and corpus spongiosum. The release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis, and induces the endothelial cells in the penile arterial walls to secrete NO, perpetuating the vasodilation. The rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the penis. An erection is the result of this increased blood flow to the penis and reduced blood return from the penis.

Q. While anabolic steroids (synthetic testosterone) bulk up muscles, they can also affect testosterone production in the testis. Using what you know about negative feedback, describe what would happen to testosterone production in the testis if a male takes large amounts of synthetic testosterone.

A. Testosterone production by the body would be reduced if a male were taking anabolic steroids. This is because the hypothalamus responds to rising testosterone levels by reducing its secretion of GnRH, which would in turn reduce the anterior pituitary’s release of LH, finally reducing the manufacture of testosterone in the testes.

Contributors and Attributions

OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at  https://openstax.org/books/anatomy-and-physiology

All about sperm

Pathway of Sperm from Testes to Urethra: Understanding the Male Reproductive System

Short answer pathway of sperm from testes to urethra: Sperm travel through the epididymis and vas deferens to reach the ejaculatory ducts, where they mix with seminal fluid. The mixture then passes through the prostate gland and into the urethra before being expelled during ejaculation.

Understanding the Pathway of Sperm from Testes to Urethra: An Overview

The step-by-step guide to the pathway of sperm from testes to urethra, common faqs about the pathway of sperm from testes to urethra answered, troubleshooting problems that can impact the pathway of sperm from testes to urethra, examining the role of ejaculation in facilitating the movement of sperm along the pathway.

Table of Contents

As we all learned in our high school biology class, the process of procreation involves the fusion of a male sperm and female egg to create a new life. But have you ever wondered about the journey that these tiny little swimmers undertake before reaching their final destination? It may seem like a simple concept, but understanding the pathway of sperm from testes to urethra is a crucial aspect of human anatomy and reproductive health.

So, let’s get down to business. The journey of a million sperms begins in the testes- two egg-shaped glands located within the scrotum. These organs are responsible for producing both sperm and testosterone hormone required by males.

From there, matured Sperm cells move on to Epididymis where they undergo maturation .The epididymis is a tightly coiled tube attached to each testicle, and it is here that sperm enters after its production in the testes. The epididymis acts as an incubator, nourishing and bathing them with fluids that help them mature over several weeks until they are ready for ejaculation out into semen.

Next stop – Vas Deferens!. Once fully developed inside epididymis sperm travel through vas deferens tubes.It’s not just one & like 20 muscular watery tubes connect to another duct called Ejaculatory Duct forming thick tube travelling towards ejaculatory process.

Just before entering into prostate gland ejaculatory ducts fuse together creating space for Seminal Vesicles which secrete primarily carbs on which sperms feed.In accordance with this,the next pitstop would be Prostate Gland, secreting watery substance making up almost 30% of semen volume along with seminal vesicle fluid providing additional nutrient-rich material serving as fuel source during their voyage towards goal

Last but not least-Urethra: The moment we’ve all been waiting for – when it finally enters into Urethra.The last leg of their journey is less than an inch long covered by Glans Penis aka the Tip!. And voila! The last push of semen from the urethra results in ejaculation, which marks the end of a sperm’s incredible voyage.

In conclusion, understanding the pathway of sperm from testes to urethra may seem like a trivial matter – but when it comes to reproductive health and fertility, knowledge is power. Knowing how different parts work together can help both men and women protect themselves from sexually transmitted diseases (STDs), infertility issues and other problems related to reproduction. So next time you are on your biology lesson or having a conversation about fertility – remember to appreciate just how amazing this intricate process truly is!

The journey of sperm from the testes to the urethra is an incredibly complex and fascinating process. In this step-by-step guide, we’ll take a closer look at what happens along the pathway and how everything works together to enable reproduction.

Step 1: Production in the Testes

Sperm production starts in the testes – two small, oval-shaped glands located within the scrotum. Testicles are packed with tiny coiled tubes called seminiferous tubules that are responsible for sperm production.

Here, immature sperm cells start to develop into fully matured, fertilization-ready gametes. The process of spermatogenesis takes approximately three months, and newly produced sperm is continuously released into the male reproductive system.

Step 2: Maturation in Epididymis

After leaving the seminiferous tubules, immature sperm cells travel through a network of ducts and then enter a storage area known as the Epididymis; a coiled tube around 20 feet in length that lies on top of each testicle.

In this organ, they undergo maturation and become mobile as they grow tails or flagella to swim towards the female egg. It is also where they develop their capacity for fertilization via interaction with prostatic secretions later during ejaculation

Step 3: Transport Via Vas Deferens

Once maturation is complete, long thin tubes known as Vas deferens begin transporting these matured sperms from both testicles toward prostate gland which lies just beneath urinary bladder

The vas deferens can be described as muscular ducts capable of propelling waves of peristalsis or contraction down its walls pushing sperms forward by rhythmic muscle contractions.

Through an ejaculatory reflex action coming from spinal cord when an orgasm arrives during intercourse or masturbation activity both vas deference will push semen containing products from prostate past vasectomy site out through urethra surrounded by penis shaft

Step 4: Mixing with Seminal Fluids from Prostate and Seminal Vesicles

As sperms travel through vas deferens towards the urethra, they mix with seminal fluids produced by prostate gland and seminal vesicles. These fluids provide a nutrient-rich environment for the sperm cells, providing them with energy to propel themselves forward during ejaculation.

The prostatic secretions contain buffers that neutralize vaginal acidity which would otherwise denature sperm in an acidic environment while also acting as a lubricant to ease transport/movement of semen. The various enzymes within it help keep the tubes free from clots or jams allowing fluid movement required leading up to fertilization process.

In contrast, seminal vesicle contains fructose sugar rich in ATP that helps increase mitochondrial activity while at the same time reserves glycogen stores in order to provide long-term survival energy for sperms

Step 5: Ejaculation Via Urethra

Finally, when all these individual pieces come together, ejaculation occurs – forcing the semen out of penis shaft and through the urethra opening located

The journey of sperm from testes to urethra is quite a fascinating one for people who are interested in the anatomy of the human body. However, even though it is such a crucial process, many people have questions that often remain unanswered. In this blog post, we will address some common FAQs about the pathway of sperm from testes to urethra and provide detailed explanations.

1. What is the pathway followed by sperm as it travels from the testes to the urethra?

Sperm’s journey begins in the seminiferous tubules within each testis in which they are produced. Once they are formed, they travel toward and assemble in epididymal ducts that run alongside each testicle. The sperm maturation process occurs throughout these ducts, and after completion of their development process (approximately 2-4 weeks), they enter vas deferens tubes – muscular ducts that extend from each epididymis region toward the pelvic region where they join with seminal vesicles to form ejaculatory ducts leading towards the prostate gland (where fluids are added before ejaculation) and finally out of the body via urethra tube during ejaculation.

2. What is happening in epididymal duct as sperm mature there?

During this time, sperms gain new features like improved motility – moving more efficiently due to an extra flagellum or even mosaics(carrying two different morphologies stuck together). Additionally, sperms increase glandular secretions inside themselves – special proteins serving at various points along their path through both female reproductive tract environments without being destroyed completely by hostile immune cells intended to protect against infections thereby increasing chances of fertilization once inside a compatible egg,

3. Why do sperm move forward in only one direction?

Aided by peristaltic waves generated inside their vas deferens tissue walls but also needing help crossing areas with opposing directional forces like anti-peristaltic waves (which can result with regurgitation out of urethra tube) or uterine contractions in females. Sperm and their outputs are designed to swim purposefully towards an egg, but also cover a lot of ground faster than potential competitors.

4. Is it possible for sperm to make its way to the egg if they were not ejaculated?

It is highly unlikely, and chances are next to none. Ejaculation delivers millions of sperms into the female reproductive tract at once, stacking the odds in favor of fertilization significantly. A single man’s semen contains only about 200-300 million healthy forward motile sperm cells per milliliter during ejaculation, while typically a woman releases one egg each month approximately on day 14 of her menstrual cycle within an interval window that lasts for up to 24 hours.

5. Can a man lose his ability to produce sperm?

Men who have undergone cancer treatment can experience damage to testes which severely impacts producing enough amount or quality of spermatogenesis capacity levels concerned with male fertility (e.g.,

Let’s begin by understanding how sperm is produced. The production of sperm begins in the testes and continues through a five-stage process called spermatogenesis. Throughout this process, several chemical messengers or hormones play crucial roles that impact how and where sperm travel before reaching their final destination at the urethra.

The pituitary gland located in the brain secretes two essential hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH promotes spermatogenesis and stimulates cells in the testes called Sertoli cells that are necessary for generating viable sperm. These cells are ultimately responsible for pushing mature sperm towards the ducts in the testis’ epididymis.

Next, LH stimulates testosterone production in cells known as Leydig cells found near seminiferous tubules where spermatogenesis occurs. Testosterone is a key male sex hormone responsible for regulating various processes such as fertility, sexual function, body hair growth, muscle mass development among others.

Now here comes an interesting part: After they leave either side of each testicle via many tiny tubes called tubules, immature but fully formed sperm move into larger tubes known as epididymis where they would mature further during storage until ejaculation occurs during sexual intercourse or masturbation.

During ejaculation process, smooth muscles surrounding vas deferens – a tube connecting epididymis with urethra – will contract due to messages from sympathetic neurons closely associated with ejaculatory center located within spinal cord. This results in movement of fluid carrying millions of live sperms into prostatic urethra while dispatching secretions from three different glands namely prostate gland bulbourethral gland, and seminal vesicles. Finally those fluids mix together with sperms and travel to distal urethra.

In conclusion, for sperm to reach their final destination at the urethra, a complex series of events must occur continuously as they travel from the testes through epididymis to vas deferens before joining secretions from accessory glands near their entry point into urethra. The delicate hormonal balance involving FSH, LH along with testosterone levels is what ensures that the pathway stays clear through various anatomical structures so that mature viable sperms can be released during ejaculation.

As we all know, sperm is an essential component of a man’s reproductive system. Without it, fertilization cannot occur, and the dream of starting a family remains out of reach for many couples. Unfortunately, there are several problems that can impact the pathway of sperm from testes to urethra. In this blog post, we will be discussing some common issues associated with this complex process and how they can be addressed for increased male fertility.

One problem that affects sperm transportation is blockage or obstruction in the epididymis, one of the collecting ducts within the testicles where sperm mature and are stored before ejaculation. Blockages may result from infections like sexually transmitted diseases (STDs) or scar tissue formation due to previous surgeries such as vasectomy or hernia repair. When such a blockage occurs, it prevents semen containing the sperm from being ejaculated out through the urethra during orgasm.

Varicocele is another condition that can adversely impact male fertility by restricting blood flow to the testicles’ veins which further results in reducing sperm production as well as motility leading to poor quality sperms.. These swollen veins cause an increase in testicular temperature significant enough to impair normal sperm development .

Other factors including hormonal imbalances affecting spermatogenesis could negatively influence fertility likewise certain treatments like chemotherapy may also cause damage to healthy sperms leading tot loss or reduction in quality/quantity . Similarly prolonged use/addiction of drugs/substances such as tobacco/alcohol supplements would forbid optimal results when seeking conception.

To address these issues and enhance your chances of starting a family, men facing infertility should consider consulting with their healthcare provider regarding potential medical interventions available for their conditions. For instance surgical corrections may help unclog blocked ducts this could include vasectomy reversal surgery , artificial insemination ,medications/drugs ,or even natural remedies /therapies better nutritional supplements like Zinc,Magnesium,selenium have been linked to healthy sperm production.

In conclusion, the pathway of sperm from testes to urethra is prone to several glitches that can significantly impact male fertility. However, by identifying and treating these issues promptly upon diagnosis or decline of fertility, men can enhance their chances for conception and ultimately build the family they have always desired.

The process of impregnation is a fascinating one, and one that involves many intricate biological processes including the movement of sperm along the pathway towards the egg. One important factor in this journey is ejaculation. It has been long touted that ejaculation plays a major role in assisting the movement of sperm towards fertilization, but how exactly does it do this? Let’s examine the role of ejaculation in facilitating the movement of sperm along the pathway.

Firstly, it’s important to understand what ejaculation actually is. Ejaculation is the forceful expulsion of semen from the male reproductive system during sexual activity. Semen consists of various components including spermatozoa (sperm), seminal fluid, and prostatic fluid which all play different roles in facilitating successful impregnation.

So how does ejaculation assist in getting sperm to their final destination? During ejaculation, semen is propelled outwards through contractions of various muscles surrounding the male reproductive organs. This forceful release can provide initial momentum for sperm to begin their journey along the female reproductive tract towards fertilization.

Furthermore, semen contains substances such as fructose (a type of sugar) which provides energy for sperm to move forward. The viscosity (thickness) and pH level (acidity) also help create an environment conducive to sperm survival and motility.

In addition to these physical factors, there are also psychological factors at play during ejaculation that can impact fertility rates. Orgasm and arousal cause a release of hormones such as oxytocin which contribute to feelings of bonding between partners and improve chances for conception through increased sexual frequency.

It’s clear that ejaculation plays a significant role in facilitating the movement of sperm along the pathway towards fertilization. From providing initial propulsion force to nourishing sustenance necessary for survival while traveling through unfamiliar terrain, this process offers crucial support from start-to-finish in order for successfully conception to occur. So next time you go about your intimate activities with your partner(s), you’ll appreciate the powerful role of ejaculation during reproduction, perhaps in ways that you never imagined before.

Chapter 14: The Urinary System and The Reproductive System

Anatomy and physiology of the male reproductive system, learning objectives.

By the end of this section, you will be able to:

• Describe the structure and function of the organs of the male reproductive system
• Describe the structure and function of the sperm cell
• Explain the events during spermatogenesis that produce haploid sperm from diploid cells
• Identify the importance of testosterone in male reproductive function

Unique for its role in human reproduction, a gamete is a specialized sex cell carrying 23 chromosomes—one half the number in body cells. At fertilization, the chromosomes in one male gamete, called a sperm (or spermatozoon), combine with the chromosomes in one female gamete, called an oocyte. The function of the male reproductive system is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and androgens, the hormones that support male reproductive physiology. In humans, the most important male androgen is testosterone. Several accessory organs and ducts aid the process of sperm maturation and transport the sperm and other seminal components to the penis, which delivers sperm to the female reproductive tract. In this section, we examine each of these different structures, and discuss the process of sperm production and transport.

Figure 1. Click for a larger image. The structures of the male reproductive system include the testes, the epididymides, the penis, and the ducts and glands that produce and carry semen. Sperm exit the scrotum through the ductus deferens, which is bundled in the spermatic cord. The seminal vesicles and prostate gland add fluids to the sperm to create semen.

MALE REPRODUCTIVE SYSTEM

• Testes – male gonads
• Spermatogenesis – production of sperm cells by the seminiferous tubules; begins at puberty and is controlled by FSH and LH (also known as interstitial cell-stimulating hormone – ICSH)
• Stages of sperm development:
• Spermatogonia – outermost cells of the seminiferous tubule that divides by mitosis to produce more cells
• Primary spermatocyte – inward spermatogonia that undergoes meiosis to produce sperm
• Secondary spermatocytes – 2 cells produced through the first meiotic division
• Spermatids – 4 cells produced through the second meiotic division that mature into a sperm cell
• Epididymis – caps the superior part of the testis and forms a temporary storage site for sperm to mature; takes 10-14 days for sperm to mature in the epididymis
• Vas Deferens – carries sperm from the epididymus to the ejaculatory duct
• Ejaculatory duct – passes through the prostate gland to merge with the urethra; during ejaculation the thick walls of smooth muscle create peristaltic waves that rapidly squeeze the sperm forward
• Urethra – carries both urine and sperm; sperm enters from ejaculatory duct, urine is coming from the bladder
• Accessory glands
• Seminal vesicle – produces 60% of seminal fluid; thick yellowish secretion that contains fructose, prostaglandins, and other substances to nourish and activate sperm; pH is slightly alkaline (basic)
• Prostate gland –doughnut-shaped gland that secretes a milky fluid that helps to activate the swimming movements of sperm
• Bulbourethral gland – produces thick clear mucus that cleanses the urethra of acidic urine and lubricates the end of the penis in response to sexual stimulation
• Semen – mixture of sperm and accessory gland secretions
• External genitalia
• Scrotum – sac of skin that hangs outside the abdominal cavity and encloses the testes; keeps testes about 3º lower than body temperature (necessary for healthy sperm)
• Penis – delivers sperm into female reproductive tract
• The most important hormone of the testes is testosterone.
• Functions of testosterone:
• Stimulate reproductive organ development
• Promotes sex drive
• Causes development of the secondary sex characteristics – deepening of voice, increased hair growth, enlargement of skeletal muscles, thickening of bones

The testes are located in a skin-covered, highly pigmented, muscular sack called the scrotum that extends from the body behind the penis. This location is important in sperm production, which occurs within the testes, and proceeds more efficiently when the testes are kept 2 to 4°C below core body temperature.

The dartos muscle makes up the subcutaneous muscle layer of the scrotum. It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each housing one testis. Descending from the internal oblique muscle of the abdominal wall are the two cremaster muscles, which cover each testis like a muscular net. By contracting simultaneously, the dartos and cremaster muscles can elevate the testes in cold weather (or water), moving the testes closer to the body and decreasing the surface area of the scrotum to retain heat. Alternatively, as the environmental temperature increases, the scrotum relaxes, moving the testes farther from the body core and increasing scrotal surface area, which promotes heat loss. Externally, the scrotum has a raised medial thickening on the surface called the raphae.

Figure 2. This anterior view shows the structures of the scrotum and testes.

The testes (singular = testis) are the male gonads —that is, the male reproductive organs. They produce both sperm and androgens, such as testosterone, and are active throughout the reproductive lifespan of the male.

Figure 3. This sagittal view shows the seminiferous tubules, the site of sperm production. Formed sperm are transferred to the epididymis, where they mature. They leave the epididymis during an ejaculation via the ductus deferens.

Paired ovals, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum. They are surrounded by two distinct layers of protective connective tissue. The outer tunica vaginalis is a serous membrane that has both a parietal and a thin visceral layer. Beneath the tunica vaginalis is the tunica albuginea, a tough, white, dense connective tissue layer covering the testis itself. Not only does the tunica albuginea cover the outside of the testis, it also invaginates to form septa that divide the testis into 300 to 400 structures called lobules. Within the lobules, sperm develop in structures called seminiferous tubules. During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. This is called the “descent of the testis.” Cryptorchidism is the clinical term used when one or both of the testes fail to descend into the scrotum prior to birth.

The tightly coiled seminiferous tubules form the bulk of each testis. They are composed of developing sperm cells surrounding a lumen, the hollow center of the tubule, where formed sperm are released into the duct system of the testis. Specifically, from the lumens of the seminiferous tubules, sperm move into the straight tubules (or tubuli recti), and from there into a fine meshwork of tubules called the rete testes. Sperm leave the rete testes, and the testis itself, through the 15 to 20 efferent ductules that cross the tunica albuginea.

Inside the seminiferous tubules are six different cell types. These include supporting cells called sustentacular cells, as well as five types of developing sperm cells called germ cells. Germ cell development progresses from the basement membrane—at the perimeter of the tubule—toward the lumen. Let’s look more closely at these cell types.

Sertoli Cells

Surrounding all stages of the developing sperm cells are elongate, branching Sertoli cells . Sertoli cells are a type of supporting cell called a sustentacular cell, or sustenocyte, that are typically found in epithelial tissue. Sertoli cells secrete signaling molecules that promote sperm production and can control whether germ cells live or die. They extend physically around the germ cells from the peripheral basement membrane of the seminiferous tubules to the lumen. Tight junctions between these sustentacular cells create the blood–testis barrier , which keeps bloodborne substances from reaching the germ cells and, at the same time, keeps surface antigens on developing germ cells from escaping into the bloodstream and prompting an autoimmune response.

The least mature cells, the spermatogonia (singular = spermatogonium), line the basement membrane inside the tubule. Spermatogonia are the stem cells of the testis, which means that they are still able to differentiate into a variety of different cell types throughout adulthood. Spermatogonia divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. The process that begins with spermatogonia and concludes with the production of sperm is called spermatogenesis .

Spermatogenesis

As just noted, spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis. The process begins at puberty, after which time sperm are produced constantly throughout a man’s life. One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules. Sperm counts—the total number of sperm a man produces—slowly decline after age 35, and some studies suggest that smoking can lower sperm counts irrespective of age.

The process of spermatogenesis begins with mitosis of the diploid spermatogonia. Because these cells are diploid (2 n ), they each have a complete copy of the father’s genetic material, or 46 chromosomes. However, mature gametes are haploid (1 n ), containing 23 chromosomes—meaning that daughter cells of spermatogonia must undergo a second cellular division through the process of meiosis.

Figure 4. (a) Mitosis of a spermatogonial stem cell involves a single cell division that results in two identical, diploid daughter cells (spermatogonia to primary spermatocyte). Meiosis has two rounds of cell division: primary spermatocyte to secondary spermatocyte, and then secondary spermatocyte to spermatid. This produces four haploid daughter cells (spermatids). (b) In this electron micrograph of a cross-section of a seminiferous tubule from a rat, the lumen is the light-shaded area in the center of the image. The location of the primary spermatocytes is near the basement membrane, and the early spermatids are approaching the lumen (tissue source: rat). EM × 900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Two identical diploid cells result from spermatogonia mitosis. One of these cells remains a spermatogonium, and the other becomes a primary spermatocyte , the next stage in the process of spermatogenesis. As in mitosis, DNA is replicated in a primary spermatocyte, and the cell undergoes cell division to produce two cells with identical chromosomes. Each of these is a secondary spermatocyte. Now a second round of cell division occurs in both of the secondary spermatocytes, separating the chromosome pairs. This second meiotic division results in a total of four cells with only half of the number of chromosomes. Each of these new cells is a spermatid . Although haploid, early spermatids look very similar to cells in the earlier stages of spermatogenesis, with a round shape, central nucleus, and large amount of cytoplasm. A process called spermiogenesis transforms these early spermatids, reducing the cytoplasm, and beginning the formation of the parts of a true sperm. The fifth stage of germ cell formation—spermatozoa, or formed sperm—is the end result of this process, which occurs in the portion of the tubule nearest the lumen. Eventually, the sperm are released into the lumen and are moved along a series of ducts in the testis toward a structure called the epididymis for the next step of sperm maturation.

Structure of Formed Sperm

Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas women typically ovulate only one oocyte per month as is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, mid-piece, and tail region. The head of the sperm contains the extremely compact haploid nucleus with very little cytoplasm. These qualities contribute to the overall small size of the sperm (the head is only 5 μ m long). A structure called the acrosome covers most of the head of the sperm cell as a “cap” that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. Tightly packed mitochondria fill the mid-piece of the sperm. ATP produced by these mitochondria will power the flagellum, which extends from the neck and the mid-piece through the tail of the sperm, enabling it to move the entire sperm cell. The central strand of the flagellum, the axial filament, is formed from one centriole inside the maturing sperm cell during the final stages of spermatogenesis.

Figure 5. Sperm cells are divided into a head, containing DNA; a mid-piece, containing mitochondria; and a tail, providing motility. The acrosome is oval and somewhat flattened.

Sperm Transport

To fertilize an egg, sperm must be moved from the seminiferous tubules in the testes, through the epididymis, and—later during ejaculation—along the length of the penis and out into the female reproductive tract.

Role of the Epididymis

From the lumen of the seminiferous tubules, the immotile sperm are surrounded by testicular fluid and moved to the epididymis (plural = epididymides), a coiled tube attached to the testis where newly formed sperm continue to mature. Though the epididymis does not take up much room in its tightly coiled state, it would be approximately 6 m (20 feet) long if straightened. It takes an average of 12 days for sperm to move through the coils of the epididymis, with the shortest recorded transit time in humans being one day. Sperm enter the head of the epididymis and are moved along predominantly by the contraction of smooth muscles lining the epididymal tubes. As they are moved along the length of the epididymis, the sperm further mature and acquire the ability to move under their own power. Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg. The more mature sperm are then stored in the tail of the epididymis (the final section) until ejaculation occurs.

Duct System

During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the ductus deferens (also called the vas deferens). The ductus deferens is a thick, muscular tube that is bundled together inside the scrotum with connective tissue, blood vessels, and nerves into a structure called the spermatic cord . Because the ductus deferens is physically accessible within the scrotum, surgical sterilization to interrupt sperm delivery can be performed by cutting and sealing a small section of the ductus (vas) deferens. This procedure is called a vasectomy, and it is an effective form of male birth control. Although it may be possible to reverse a vasectomy, clinicians consider the procedure permanent, and advise men to undergo it only if they are certain they no longer wish to father children.

Practice Question

Watch this video to learn about a vasectomy. As described in this video, a vasectomy is a procedure in which a small section of the ductus (vas) deferens is removed from the scrotum. This interrupts the path taken by sperm through the ductus deferens. If sperm do not exit through the vas, either because the man has had a vasectomy or has not ejaculated, in what region of the testis do they remain?

From each epididymis, each ductus deferens extends superiorly into the abdominal cavity through the inguinal canal in the abdominal wall. From here, the ductus deferens continues posteriorly to the pelvic cavity, ending posterior to the bladder where it dilates in a region called the ampulla (meaning “flask”).

Sperm make up only 5 percent of the final volume of semen , the thick, milky fluid that the male ejaculates. The bulk of semen is produced by three critical accessory glands of the male reproductive system: the seminal vesicles, the prostate, and the bulbourethral glands.

Seminal Vesicles

As sperm pass through the ampulla of the ductus deferens at ejaculation, they mix with fluid from the associated seminal vesicle . The paired seminal vesicles are glands that contribute approximately 60 percent of the semen volume. Seminal vesicle fluid contains large amounts of fructose, which is used by the sperm mitochondria to generate ATP to allow movement through the female reproductive tract.

The fluid, now containing both sperm and seminal vesicle secretions, next moves into the associated ejaculatory duct , a short structure formed from the ampulla of the ductus deferens and the duct of the seminal vesicle. The paired ejaculatory ducts transport the seminal fluid into the next structure, the prostate gland.

Prostate Gland

As shown in Figure 1, the centrally located prostate gland sits anterior to the rectum at the base of the bladder surrounding the prostatic urethra (the portion of the urethra that runs within the prostate). About the size of a walnut, the prostate is formed of both muscular and glandular tissues. It excretes an alkaline, milky fluid to the passing seminal fluid—now called semen—that is critical to first coagulate and then decoagulate the semen following ejaculation. The temporary thickening of semen helps retain it within the female reproductive tract, providing time for sperm to utilize the fructose provided by seminal vesicle secretions. When the semen regains its fluid state, sperm can then pass farther into the female reproductive tract.

The prostate normally doubles in size during puberty. At approximately age 25, it gradually begins to enlarge again. This enlargement does not usually cause problems; however, abnormal growth of the prostate, or benign prostatic hyperplasia (BPH), can cause constriction of the urethra as it passes through the middle of the prostate gland, leading to a number of lower urinary tract symptoms, such as a frequent and intense urge to urinate, a weak stream, and a sensation that the bladder has not emptied completely. By age 60, approximately 40 percent of men have some degree of BPH. By age 80, the number of affected individuals has jumped to as many as 80 percent. Treatments for BPH attempt to relieve the pressure on the urethra so that urine can flow more normally. Mild to moderate symptoms are treated with medication, whereas severe enlargement of the prostate is treated by surgery in which a portion of the prostate tissue is removed.

Another common disorder involving the prostate is prostate cancer. According to the Centers for Disease Control and Prevention (CDC), prostate cancer is the second most common cancer in men. However, some forms of prostate cancer grow very slowly and thus may not ever require treatment. Aggressive forms of prostate cancer, in contrast, involve metastasis to vulnerable organs like the lungs and brain. There is no link between BPH and prostate cancer, but the symptoms are similar. Prostate cancer is detected by a medical history, a blood test, and a rectal exam that allows physicians to palpate the prostate and check for unusual masses. If a mass is detected, the cancer diagnosis is confirmed by biopsy of the cells.

Bulbourethral Glands

The final addition to semen is made by two bulbourethral glands (or Cowper’s glands) that release a thick, salty fluid that lubricates the end of the urethra and the vagina, and helps to clean urine residues from the penile urethra. The fluid from these accessory glands is released after the male becomes sexually aroused, and shortly before the release of the semen. It is therefore sometimes called pre-ejaculate. It is important to note that, in addition to the lubricating proteins, it is possible for bulbourethral fluid to pick up sperm already present in the urethra, and therefore it may be able to cause pregnancy.

Practice Question

Watch this video to explore the structures of the male reproductive system and the path of sperm , which starts in the testes and ends as the sperm leave the penis through the urethra. Where are sperm deposited after they leave the ejaculatory duct?

The penis is the male organ of copulation (sexual intercourse). It is flaccid for non-sexual actions, such as urination, and turgid and rod-like with sexual arousal. When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract.

Figure 6. Three columns of erectile tissue make up most of the volume of the penis.

The shaft of the penis surrounds the urethra. The shaft is composed of three column-like chambers of erectile tissue that span the length of the shaft. Each of the two larger lateral chambers is called a corpus cavernosum (plural = corpora cavernosa). Together, these make up the bulk of the penis. The corpus spongiosum , which can be felt as a raised ridge on the erect penis, is a smaller chamber that surrounds the spongy, or penile, urethra. The end of the penis, called the glans penis , has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation. The skin from the shaft extends down over the glans and forms a collar called the prepuce (or foreskin). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. A surgical procedure called circumcision, often performed for religious or social reasons, removes the prepuce, typically within days of birth.

Both sexual arousal and REM sleep (during which dreaming occurs) can induce an erection. Penile erections are the result of vasocongestion, or engorgement of the tissues because of more arterial blood flowing into the penis than is leaving in the veins. During sexual arousal, nitric oxide (NO) is released from nerve endings near blood vessels within the corpora cavernosa and spongiosum. Release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis and induces the endothelial cells in the penile arterial walls to also secrete NO and perpetuate the vasodilation. The rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the penis. The result of this increased blood flow to the penis and reduced blood return from the penis is erection. Depending on the flaccid dimensions of a penis, it can increase in size slightly or greatly during erection, with the average length of an erect penis measuring approximately 15 cm.

Disorders of the Male Reproductive System: Erectile dysfunction (ED)

Erectile dysfunction (ED) is a condition in which a man has difficulty either initiating or maintaining an erection. The combined prevalence of minimal, moderate, and complete ED is approximately 40 percent in men at age 40, and reaches nearly 70 percent by 70 years of age. In addition to aging, ED is associated with diabetes, vascular disease, psychiatric disorders, prostate disorders, the use of some drugs such as certain antidepressants, and problems with the testes resulting in low testosterone concentrations. These physical and emotional conditions can lead to interruptions in the vasodilation pathway and result in an inability to achieve an erection.

Recall that the release of NO induces relaxation of the smooth muscles that surround the penile arteries, leading to the vasodilation necessary to achieve an erection. To reverse the process of vasodilation, an enzyme called phosphodiesterase (PDE) degrades a key component of the NO signaling pathway called cGMP. There are several different forms of this enzyme, and PDE type 5 is the type of PDE found in the tissues of the penis. Scientists discovered that inhibiting PDE5 increases blood flow, and allows vasodilation of the penis to occur.

PDEs and the vasodilation signaling pathway are found in the vasculature in other parts of the body. In the 1990s, clinical trials of a PDE5 inhibitor called sildenafil were initiated to treat hypertension and angina pectoris (chest pain caused by poor blood flow through the heart). The trial showed that the drug was not effective at treating heart conditions, but many men experienced erection and priapism (erection lasting longer than 4 hours). Because of this, a clinical trial was started to investigate the ability of sildenafil to promote erections in men suffering from ED. In 1998, the FDA approved the drug, marketed as Viagra ® . Since approval of the drug, sildenafil and similar PDE inhibitors now generate over a billion dollars a year in sales, and are reported to be effective in treating approximately 70 to 85 percent of cases of ED. Importantly, men with health problems—especially those with cardiac disease taking nitrates—should avoid Viagra or talk to their physician to find out if they are a candidate for the use of this drug, as deaths have been reported for at-risk users.

Testosterone

Testosterone, an androgen, is a steroid hormone produced by Leydig cells . The alternate term for Leydig cells, interstitial cells, reflects their location between the seminiferous tubules in the testes. In male embryos, testosterone is secreted by Leydig cells by the seventh week of development, with peak concentrations reached in the second trimester. This early release of testosterone results in the anatomical differentiation of the male sexual organs. In childhood, testosterone concentrations are low. They increase during puberty, activating characteristic physical changes and initiating spermatogenesis.

Functions of Testosterone

The continued presence of testosterone is necessary to keep the male reproductive system working properly, and Leydig cells produce approximately 6 to 7 mg of testosterone per day. Testicular steroidogenesis (the manufacture of androgens, including testosterone) results in testosterone concentrations that are 100 times higher in the testes than in the circulation. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility. In addition to intratesticular secretion, testosterone is also released into the systemic circulation and plays an important role in muscle development, bone growth, the development of secondary sex characteristics, and maintaining libido (sex drive) in both males and females. In females, the ovaries secrete small amounts of testosterone, although most is converted to estradiol. A small amount of testosterone is also secreted by the adrenal glands in both sexes.

Control of Testosterone

The regulation of testosterone concentrations throughout the body is critical for male reproductive function. The intricate interplay between the endocrine system and the reproductive system is shown in Figure 7.

Figure 7. The hypothalamus and pituitary gland regulate the production of testosterone and the cells that assist in spermatogenesis. GnRH activates the anterior pituitary to produce LH and FSH, which in turn stimulate Leydig cells and Sertoli cells, respectively. The system is a negative feedback loop because the end products of the pathway, testosterone and inhibin, interact with the activity of GnRH to inhibit their own production.

The regulation of Leydig cell production of testosterone begins outside of the testes. The hypothalamus and the pituitary gland in the brain integrate external and internal signals to control testosterone synthesis and secretion. The regulation begins in the hypothalamus. Pulsatile release of a hormone called gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the endocrine release of hormones from the pituitary gland. Binding of GnRH to its receptors on the anterior pituitary gland stimulates release of the two gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These two hormones are critical for reproductive function in both men and women. In men, FSH binds predominantly to the Sertoli cells within the seminiferous tubules to promote spermatogenesis. FSH also stimulates the Sertoli cells to produce hormones called inhibins, which function to inhibit FSH release from the pituitary, thus reducing testosterone secretion. These polypeptide hormones correlate directly with Sertoli cell function and sperm number; inhibin B can be used as a marker of spermatogenic activity. In men, LH binds to receptors on Leydig cells in the testes and upregulates the production of testosterone.

A negative feedback loop predominantly controls the synthesis and secretion of both FSH and LH. Low blood concentrations of testosterone stimulate the hypothalamic release of GnRH. GnRH then stimulates the anterior pituitary to secrete LH into the bloodstream. In the testis, LH binds to LH receptors on Leydig cells and stimulates the release of testosterone. When concentrations of testosterone in the blood reach a critical threshold, testosterone itself will bind to androgen receptors on both the hypothalamus and the anterior pituitary, inhibiting the synthesis and secretion of GnRH and LH, respectively. When the blood concentrations of testosterone once again decline, testosterone no longer interacts with the receptors to the same degree and GnRH and LH are once again secreted, stimulating more testosterone production. This same process occurs with FSH and inhibin to control spermatogenesis.

Aging and the Male Reproductive System

Declines in Leydig cell activity can occur in men beginning at 40 to 50 years of age. The resulting reduction in circulating testosterone concentrations can lead to symptoms of andropause, also known as male menopause. While the reduction in sex steroids in men is akin to female menopause, there is no clear sign—such as a lack of a menstrual period—to denote the initiation of andropause. Instead, men report feelings of fatigue, reduced muscle mass, depression, anxiety, irritability, loss of libido, and insomnia. A reduction in spermatogenesis resulting in lowered fertility is also reported, and sexual dysfunction can also be associated with andropausal symptoms.

Whereas some researchers believe that certain aspects of andropause are difficult to tease apart from aging in general, testosterone replacement is sometimes prescribed to alleviate some symptoms. Recent studies have shown a benefit from androgen replacement therapy on the new onset of depression in elderly men; however, other studies caution against testosterone replacement for long-term treatment of andropause symptoms, showing that high doses can sharply increase the risk of both heart disease and prostate cancer.

Chapter Review

Gametes are the reproductive cells that combine to form offspring. Organs called gonads produce the gametes, along with the hormones that regulate human reproduction. The male gametes are called sperm. Spermatogenesis, the production of sperm, occurs within the seminiferous tubules that make up most of the testis. The scrotum is the muscular sac that holds the testes outside of the body cavity.

Spermatogenesis begins with mitotic division of spermatogonia (stem cells) to produce primary spermatocytes that undergo the two divisions of meiosis to become secondary spermatocytes, then the haploid spermatids. During spermiogenesis, spermatids are transformed into spermatozoa (formed sperm). Upon release from the seminiferous tubules, sperm are moved to the epididymis where they continue to mature. During ejaculation, sperm exit the epididymis through the ductus deferens, a duct in the spermatic cord that leaves the scrotum. The ampulla of the ductus deferens meets the seminal vesicle, a gland that contributes fructose and proteins, at the ejaculatory duct. The fluid continues through the prostatic urethra, where secretions from the prostate are added to form semen. These secretions help the sperm to travel through the urethra and into the female reproductive tract. Secretions from the bulbourethral glands protect sperm and cleanse and lubricate the penile (spongy) urethra.

The penis is the male organ of copulation. Columns of erectile tissue called the corpora cavernosa and corpus spongiosum fill with blood when sexual arousal activates vasodilatation in the blood vessels of the penis. Testosterone regulates and maintains the sex organs and sex drive, and induces the physical changes of puberty. Interplay between the testes and the endocrine system precisely control the production of testosterone with a negative feedback loop.

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

• Briefly explain why mature gametes carry only one set of chromosomes.
• What special features are evident in sperm cells but not in somatic cells, and how do these specializations function?
• What do each of the three male accessory glands contribute to the semen?
• Describe how penile erection occurs.
• While anabolic steroids (synthetic testosterone) bulk up muscles, they can also affect testosterone production in the testis. Using what you know about negative feedback, describe what would happen to testosterone production in the testis if a male takes large amounts of synthetic testosterone.
• A single gamete must combine with a gamete from an individual of the opposite sex to produce a fertilized egg, which has a complete set of chromosomes and is the first cell of a new individual.
• Unlike somatic cells, sperm are haploid. They also have very little cytoplasm. They have a head with a compact nucleus covered by an acrosome filled with enzymes, and a mid-piece filled with mitochondria that power their movement. They are motile because of their tail, a structure containing a flagellum, which is specialized for movement.
• The three accessory glands make the following contributions to semen: the seminal vesicle contributes about 60 percent of the semen volume, with fluid that contains large amounts of fructose to power the movement of sperm; the prostate gland contributes substances critical to sperm maturation; and the bulbourethral glands contribute a thick fluid that lubricates the ends of the urethra and the vagina and helps to clean urine residues from the urethra.
• During sexual arousal, nitric oxide (NO) is released from nerve endings near blood vessels within the corpora cavernosa and corpus spongiosum. The release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis, and induces the endothelial cells in the penile arterial walls to secrete NO, perpetuating the vasodilation. The rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the penis. An erection is the result of this increased blood flow to the penis and reduced blood return from the penis.
• Testosterone production by the body would be reduced if a male were taking anabolic steroids. This is because the hypothalamus responds to rising testosterone levels by reducing its secretion of GnRH, which would in turn reduce the anterior pituitary’s release of LH, finally reducing the manufacture of testosterone in the testes.

blood–testis barrier: tight junctions between Sertoli cells that prevent bloodborne pathogens from gaining access to later stages of spermatogenesis and prevent the potential for an autoimmune reaction to haploid sperm

bulbourethral glands: (also, Cowper’s glands) glands that secrete a lubricating mucus that cleans and lubricates the urethra prior to and during ejaculation

corpus cavernosum: either of two columns of erectile tissue in the penis that fill with blood during an erection

corpus spongiosum: (plural = corpora cavernosa) column of erectile tissue in the penis that fills with blood during an erection and surrounds the penile urethra on the ventral portion of the penis

ductus deferens: (also, vas deferens) duct that transports sperm from the epididymis through the spermatic cord and into the ejaculatory duct; also referred as the vas deferens

ejaculatory duct: duct that connects the ampulla of the ductus deferens with the duct of the seminal vesicle at the prostatic urethra

epididymis: (plural = epididymides) coiled tubular structure in which sperm start to mature and are stored until ejaculation

gamete: haploid reproductive cell that contributes genetic material to form an offspring

glans penis: bulbous end of the penis that contains a large number of nerve endings

gonadotropin-releasing hormone (GnRH): hormone released by the hypothalamus that regulates the production of follicle-stimulating hormone and luteinizing hormone from the pituitary gland

gonads: reproductive organs (testes in men and ovaries in women) that produce gametes and reproductive hormones

inguinal canal: opening in abdominal wall that connects the testes to the abdominal cavity

Leydig cells: cells between the seminiferous tubules of the testes that produce testosterone; a type of interstitial cell

penis: male organ of copulation

prepuce: (also, foreskin) flap of skin that forms a collar around, and thus protects and lubricates, the glans penis; also referred as the foreskin

prostate gland: doughnut-shaped gland at the base of the bladder surrounding the urethra and contributing fluid to semen during ejaculation

scrotum: external pouch of skin and muscle that houses the testes

semen: ejaculatory fluid composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands

seminal vesicle: gland that produces seminal fluid, which contributes to semen

seminiferous tubules: tube structures within the testes where spermatogenesis occurs

Sertoli cells: cells that support germ cells through the process of spermatogenesis; a type of sustentacular cell

sperm: (also, spermatozoon) male gamete

spermatic cord: bundle of nerves and blood vessels that supplies the testes; contains ductus deferens

spermatid: immature sperm cells produced by meiosis II of secondary spermatocytes

spermatocyte: cell that results from the division of spermatogonium and undergoes meiosis I and meiosis II to form spermatids

spermatogenesis: formation of new sperm, occurs in the seminiferous tubules of the testes

spermatogonia: (singular = spermatogonium) diploid precursor cells that become sperm

spermiogenesis: transformation of spermatids to spermatozoa during spermatogenesis

testes: (singular = testis) male gonads

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