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2.3: Anatomy and Physiology of the Male Reproductive System - Biology

2.3: Anatomy and Physiology of the Male Reproductive System - Biology


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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.

Scrotum

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.

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.

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.

Germ Cells

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 (2n), they each have a complete copy of the father’s genetic material, or 46 chromosomes. However, mature gametes are haploid (1n), 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. 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 μm 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. 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?
[reveal-answer q=”820110″]Show Answer[/reveal-answer]
[hidden-answer a=”820110″]Sperm remain in the epididymis until they degenerate.[/hidden-answer]

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.

The Penis

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. 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.

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.

Self Check

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

Critical Thinking Questions

  1. Briefly explain why mature gametes carry only one set of chromosomes.
  2. What special features are evident in sperm cells but not in somatic cells, and how do these specializations function?
  3. What do each of the three male accessory glands contribute to the semen?
  4. Describe how penile erection occurs.
  5. 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.

[reveal-answer q=”209176″]Show Answers[/reveal-answer]
[hidden-answer a=”209176″]

  1. 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.
  2. 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.
  3. 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.
  4. 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. An erection is the result of this increased blood flow to the penis and reduced blood return from the penis.
  5. 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.

[/hidden-answer]

Glossary

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


2.3: Anatomy and Physiology of the Male Reproductive System - Biology

The male reproductive system includes external (penis, scrotum, epididymus, and testes) and internal (accessory) organs.

Learning Objectives

Distinguish among the parts and functions of the male reproductive system

Key Takeaways

Key Points

  • The functions of the male reproductive system include producing and transporting sperm, ejaculating sperm into the female reproductive tract, and producing and secreting male hormones.
  • Most of the male reproductive system is located outside of the body. These external structures are the penis, scrotum, epididymis, and testes.
  • The internal organs of the male reproductive system are called accessory organs. They include the vas deferens, seminal vesicles, prostate gland, and bulbourethral glands.

Key Terms

  • semen: Contains spermatozoa, proteolytic and other enzymes, and
    fructose that promotes spermatozoa survival. It also provides a medium for sperm
    motility.
  • spermatogenesis: The process of sperm production within the seminiferous tubules in the testes.
  • testosterone: Steroid hormone produced primarily in the male testes and responsible for the development of male secondary sex characteristics.

The organs of the male reproductive system are specialized for three primary functions:

  1. To produce, maintain, transport, and nourish sperm (the male reproductive cells), and protective fluid ( semen ).
  2. To discharge sperm within the female reproductive tract.
  3. To produce and secrete male sex hormones.

External Male Sex Organs

Most of the male reproductive system is located outside of the man’s body. These external structures are the penis, scrotum, epididymis, and testes.

Male Reproductive System: Lateral view of male reproductive system with organs labeled.

The penis is the male organ for sexual intercourse and urination. Semen and urine leave the penis through the urethra. The scrotum is a loose, pouch-like sack of skin that hangs behind the penis, containing the testes.

The scrotum has a protective function, including the maintenance of optimal temperatures for sperm survival and function. For sperm development, the testes must maintain a temperature slightly cooler than normal body temperature. Special muscles in the wall of the scrotum contract and relax in order to move the testes near the body.

The epididymus is located at the back of the testis and connects it to the vas deferens. Its function is to store and carry sperm. The testis is the location for testosterone production. The coiled collection of tubes within the testes are the seminiferous tubules. Within these tubules, spermatogenesis takes place.

Accessory Sex Organs

The internal organs of the male reproductive system are called accessory organs. They include the vas deferens, seminal vesicles, prostate gland, and bulbourethral (Cowper’s) glands.

  • Vas deferens: Transports mature sperm to the urethra in preparation for ejaculation.
  • Seminal vesicles: Sac-like pouches that attach to the vas deferens near the base of the bladder. The vesicles produce molecules such as fructose that serve as energy sources for sperm. The seminal vesicle fluid makes up most of the volume of a man’s ejaculate.
  • Prostate gland: A walnut-sized structure located below the urinary bladder in front of the rectum. It contributes additional fluid to the ejaculate that serves as nourishment for sperm.
  • Bulbourethral (Cowper’s) glands: Pea-sized structures located on the sides of the urethra just below the prostate gland. These glands produce a clear, slippery fluid that empties directly into the urethra. Fluid produced by these glands lubricates the urethra and neutralizes acidity associated with residual urine.

Anatomy & Physiology : Reproductive System Puzzle

Puzzles are a fun, interactive, and rigorous way to review key concepts, units, and essential questions.

Differentiation is a snap when you have “easy” and “expert” level puzzles. The students don’t even know they have different puzzles.

The students are engaged bell to bell! They are so engrossed in their puzzles they don’t even realize they’re learning!!

Reproductive System Puzzle cover concepts/vocabulary words such as:

  • Parts of the male reproductive system
  • parts of the female reproductive system
  • Hormones required for reproduction
  • Essential vocabulary
  • Oogenesis/Spermatogenesis

Perfect for medical skills, anatomy and physiology class, or medical terminology class.

I use these Puzzles in my classroom as a:

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Three different glands may be involved in producing the secretions in which sperm are suspended, although the number and type of glands varies from species to species.

Seminal vesicles are important in rats, bulls, boars and stallions but are absent in cats and dogs. When present they produce secretions that make up much of the volume of the semen, and transport and provide nutrients for the sperm.

The prostate gland is important in dogs and humans. It produces an alkaline secretion that neutralizes the acidity of the male urethra and female vagina.

Cowper&rsquos glands (bulbourethral glands) have various functions in different species. The secretions may lubricate, flush out urine or form a gelatinous plug that traps the semen in the female reproductive system after copulation and prevents other males of the same species fertilizing an already mated female. Cowper&rsquos glands are absent in bears and aquatic mammals.


Education

Employing research-validated instructional design and based on content presented in Pathways to Pregnancy and Parturition (3rd Edition, by PL Senger), the intent of Reproductive Science & Health (RSH) on-line is to increase educational flexibility and extend the reach of this discipline globally to all with an interest in teaching and learning about the function of the reproductive system in animals and humans.

Upon completion of the entire suite of RSH content units, students will understand: vocabulary specific to reproductive science anatomy and function of male and female reproductive organs and tissues endocrine and neuroendocrine regulation of reproduction mechanisms regulating reproductive behavior, establishment of pregnancy, gestation, and parturition and principles of modern assisted reproductive technologies (ARTs).

Content and Design

This on-line resource for teaching and learning consists of a suite of 16 Content Units. Some content units contain a brief “Human Health Highlight” and/or “Animal Health Highlight”. Health highlights describe important health components that are linked to concepts explained in content units.

Content Unit 1 – Overview and Importance of Reproductive Science.

Content Unit 2– The Female Reproductive System-Structure and Function

Content Unit 3– The Male Reproductive System-Structure and Function

Content Unit 4– Embryogenesis of the Pituitary and the Male and Female Reproductive Systems

Content Unit 5– Regulation of Reproduction: Nerves, Hormones and Target Tissues

Content Unit 6– The Acquisition of Puberty

Content Unit 7– Reproductive Cyclicity: Terminology and Basic Concepts

Content Unit 8– Reproductive Cyclicity: The Follicular Phase

Content Unit 9– Reproductive Cyclicity: The Luteal Phase

Content Unit 10– Endocrinology of the Male and Spermatogenesis

Content Unit 11– Reproductive Behavior

Content Unit 12– Spermatozoa in the Female Tract-Transport, Capacitation and Fertilization

Content Unit 13– Early Embryogenesis and Maternal Recognition of Pregnancy

Content Unit 14– Placentation, Gestation and Parturition

Content Unit 15– The Puerperium and Lactation

Content Unit 16– Reproductive Physiology: The Human Factor

  • Highly Visual.The course includes detailed specimen photographs, animated 2-D graphics, animated 3-D anatomical reconstructions, and real-time videos illustrating reproductive behavior, parturition, in vitro fertilization, etc.
  • Narrated Content.Narrations are carefully synchronized with animations to maximize learning.
  • Concise.Each unit is 30-60 min. divided into 2-3 “mini units” for self-paced learning.
  • Knowledge Practice.Each content unit includes several practice quizzes consisting of 10-20 items. All quizzes are “auto-graded” for immediate feedback.

Options

License as a complete course by Universities/Organizations. Under this agreement, the home institution or organization can use the content as either a stand-alone course, a course supplement, or as a resource with which to “flip” the course so that advanced problem-solving can take place after basic information has been learned. See the Academic Agreement link for fee structure.

Certificate credit for any individual wishing to expand, advance or refresh their understanding of reproductive science. Performance of 70 percent or better is required to receive a certificate of completion. Actual performance will be noted on the certificate. The cost of certificate enrollment is $825.

Academic credit (3 semester credit hours) for students seeking to accelerate their academic progress, or searching for a state-of-the-art elective course in the life sciences. This course is approved for award of academic credit by the Auburn University Curriculum Committee. Students pursuing this option must enroll on-line at Auburn University. Standard tuition rates (in-state or out-of-state as appropriate) apply. Academic credit from Auburn University can be transferred to other institutions of higher education contingent upon their admission guidelines and standards.

This online program has been approved for continuing education (CE) credit by the Alabama State Board of Veterinary Medical Examiners (ASBVME). Licensed practitioners may earn one (1) hour of CE credit for each of 15 content units. A certificate of completion will be issued for each unit upon successful completion. The ASBVME accepts a maximum of five (5) hours of approved CE credit for verifiable audio, video, compendium journal and computer review per year. Practitioners maintaining licenses outside the state of Alabama should verify online CE requirements with their state licensing board. Auburn University College of Veterinary Medicine is accredited by the American Veterinary Medical Association. The cost of CE credit is $55/hour.
To achieve one CE credit, the enrollee must successfully complete the practice quizzes within each content unit that can be taken multiple times to achieve a score of 70%.

Leadership Team:

Dr. P. L. (Phil) Senger is Professor Emeritus at Washington State University, Affiliate Professor in the Department of Anatomy, Physiology and Pharmacology at the Auburn University College of Veterinary Medicine, and President of Current Conceptions, Inc. The author of Pathways to Pregnancy and Parturition, Dr. Senger has over 30 years of experience teaching systemic physiology and reproductive physiology at both Washington State University and Pennsylvania State University. Email: pls0012@auburn.edu

Dr. Frank F. (Skip) Bartol is Alumni Professor of Anatomy, Physiology and Pharmacology and Associate Dean for Research and Graduate Studies at the Auburn University College of Veterinary Medicine. He conducts research in reproductive/developmental biology focused on economically important domestic animals and has taught reproductive physiology at undergraduate and graduate levels for more than 30 years. Email: bartol@aݛurnˮdu

Consulting Faculty:

Faculty members at the Auburn University College of Veterinary Medicine with expertise in the reproductive sciences, theriogenology, endocrinology and physiology have and will continue to contribute to Reproductive Science and Health through review and development of content presented in this on-line course. These individuals are listed below:

Test Drive

Instructors seeking to evaluate Reproductive Science and Health, the on-line course, are invited to request a ‘test drive.’ Send your request to Ms. Amelia Pendleton (cvmrsh@auburn.edu).

Acknowledgements

Reproductive Science and Health did not just happen!!

Developed through a partnership between the AU College of Veterinary Medicine and Current Conceptions, Inc., this on-line course is the result of over six years of research and development, field testing and immeasurable input provided by reproductive scientists from around the world. Instructional design for the course was validated experimentally through controlled studies involving reproductive physiology courses at seven Land Grant universities throughout the U.S. Current Conceptions, Inc., received a Small Business Innovation Research (SBIR) contract from the U.S. Department of Education through the Institute of Educational Sciences to fund this R&D effort (contract # ED-08-CO-0050).

Exam items used in generating achievement scores for academic and certificate credit were reviewed and critiqued by a team of eight professors of reproductive science.

Portions of Reproductive Science and Health were “test driven” on-line by students at four additional Land Grant universities.

Collectively, R&D and field testing associated with production of the course involved over 1,400 university students. Their input was invaluable.

University faculty members identified below contributed to the R&D effort that produced this course. We thank them for their dedicated contributions aimed at advancing education in the reproductive sciences.

  • C. A. Bagnell (Rutgers University)
  • J. G. Berardinelli (Montana State University)
  • J. A. Clapper (South Dakota State University)
  • D. A. Coleman (Auburn University)
  • J. L. Edwards (University of Tennessee, Knoxville)
  • C. E. Farin (North Carolina State University)
  • M. J. Fields (University of Florida)
  • D. R. Hagen (Penn State University)
  • J. W. Knight (Virginia Tech)
  • D. J. McLean (Washington State University)
  • T. L. Ott (Penn State University)
  • J. S. Ottobre (Ohio State University)
  • J. J. Parrish (University of Wisconsin)
  • D. H. Poole (North Carolina State University)
  • M. F. Smith (University of Missouri)
  • C. S. Whisnant (North Carolina State University)
  • J. V. Yelich (University of Florida)
  • C.R. Youngs (Iowa State University)

Course content for RSH is based upon information presented in:

Senger, PL. 2012. Pathways to pregnancy and parturition. 3rd edition. Current Conceptions, Inc., Redmond, OR, USA. ISBN 0-9657648-3-4

Refereed publications supportive of instructional design include:

Oki AC, Berardinelli JG, Clapper JA, Poole DH, Senger PL. 2014. Enhancing the learning experience of students in reproductive science with multimedia platforms. Clinical Theriogenology 6(Suppl. 3):147-153.

Oki AC, Senger PL, Bartol FF. 2015. Multimedia and global communication of scientific concepts: An example using animal reproduction. Animal Frontiers. 5 (3). doi: 10.2527/af.2015-0034.

Senger PL, Oki AC, Trevisan MS, McLean DJ. 2012. Exploiting multimedia in reproductive science education: research findings. Reproduction in Domestic Animals 47(Suppl. 4):38-45.

Trevisan MS, Oki AC, Senger PL. 2010. An exploratory study of the effects of time compressed animation delivery multimedia technology on student learning in reproductive physiology. Journal of Science Education and Technology 19:293-302.


Professionals, researchers, and academics in endocrinology and physiology

Section I: Gametes, Fertilization, and Embryogenesis
The Spermatozoon
The Ovum
Gamete and Zygote Transport
Implantation
Anatomy and Genesis of the Placenta
Sex Determination and Differentiation
Early Embryogenesis

Section II: Female Reproductive System
Embryology and Genetics of the Mammalian Gonads and Ducts
Cyclic changes in the Primate Oviduct and Endometrium
Hormonal Control of Follicular Development: Mouse, Primate, and Equine Models
Physiology and Molecular Biology of Ovulation
Growth Factors, IGF-1 and Ovarian Function
Gonadotropin Receptors in the Ovary: Physiology and Molecular Biology
Steroid Receptors in the Ovary and Uterus
Relaxin and Related Hormones: Physiology and Molecular Biology

Section III: Male Reproductive System
Anatomy, Vasculature, and Innervation of the Male Reproductive Tract
Spermatogenesis and Its Intrinsic Control
The Sertoli Cell
Physiology of Testicular Steroidogenesis
Regulation of Spermatogenesis
Epididymis
The Physiology of the Male Sex Accessory Tissues
Male Sexual Function: Erection, Emission, and Ejaculation
Reproductive Immunology

Section IV: Pituitary and Hypothalamus
Perspectives and Overview
Gonadotropin-Releasing Hormone (GnRH) Neuronal Systems
Gonadotropes and Lactotropes
Gonadotropins: Chemistry and Biosynthesis
GnRH Regulation of Gonadotropin Biosynthesis and Secretion
Prolactin: Structure, Function, and Regulation of Secretion

Section V: Reproduction Behavior & Its Control
Neurobiology of Male Sexual Behaviors
Neural and Genetic Influences on Female Reproductive Behaviors
Maternal Behaviors
Communicative Behaviors
Pheromones and Mammalian Reproduction

Section VI: Reproductive Processes and Their Control
Puberty in the Rat
Puberty in the Sheep
Seasonal Regulation of Reproduction in Mammals
Neuroendocrine Control of the Ovarian Cycle in Reflex Ovulators
Neuroendocrine Control of the Rat Ovarian Cycle
Neuroendocrine Control of the Ovarian Cycle of the Sheep
Control of the Menstrual Cycle and the Consequences of Fertilization on the Life of the Corpus Luteum
Suckling and the Control of Gonadotropin Secretion
Metabolic Regulation of Reproduction
The Influence of Stress on Reproduction
Aging in the Hypothalamic-Pituitary-Testicular Axis
Aging in the Hypothalamic-Pituitary-Ovarian Axis

Section VII: Pregnancy
Immunobiology of Early Pregnancy
Placental Transport
Placental Endocrine Function
Maternal Adaptation to Pregnancy
Parturition
Developmental Origins of Health and Disease

Section VIII: Lactation
Mammary Gland Growth and Differentiation
Lactation and Its Hormonal Control
Oxytocin: Synthesis, Secretion, and Behavioral Effects
Milk Ejection and Its Control


2.3: Anatomy and Physiology of the Male Reproductive System - Biology

  1. Sexual Reproduction
  2. Development of the reproductive system
    1. Formation of Testis and Ovaries
    2. Accessory Sex Organs
    3. External Genitalia
    1. Testis
    2. Spermatogenesis
    3. Male Reproductive Tract
    4. Semen
    1. Anatomy
    2. Oocyte and Follicle Development
    3. Menstrual Cycle
    4. Contraceptive Methods
    5. Fertilization
    1. Sexual Reproduction
    • In sexual reproduction, genes from two individuals are combined in random and novel ways. This generates diversity within the species.
    • Normally each cell in the adult has 23 pairs of chromosomes or 46 total chromosomes.
    • 22 pairs are called autosomal chromosomes
    • 1 pair is called the sex chromosomes
      • XX sex chromosome is female
      • XY sex chromosome is male.

      At puberty cells in the gonads (testis or ovaries) undergo meiosis. 23 pairs of homologous chromosomes become 23 chromosomes. The germ cell from the male (sperm) will then fuse with the germ cell of the female (ovum) during reproduction to reform a cell with 23 pairs of homologous chromosomes.

      The sex of the zygote is determined by the sex chromosome of the fertilizing sperm.

      After conception the embryonic gonads of males and females are similar (for about the first 40 days). Therefore the embryo can form either testes or ovaries. The presence or absence of the Y chromosome determines what happens. SRY (sex determining region of the Y chromosome) on the Y chromosome ® male. SRY gene encodes the testi-determining factor.

      For the first 40 days the reproductive system of the embryo is undifferentiated and has accessory organs characteristic of either sex.

      Male: Wolffian ducts ® epididymis, ductus (vas deferens), seminal vesicles, ejaculatory duct.

      • Sertoli cells: Mullerian inhibitory factor (MIF): regression of the Mullerian ducts
      • Leydig cells: Testosterone: epididymis, ductus (vas) deferens, seminal vesicles, ejaculatory duct.

      Female: Mullerian ducts ® uterus, fallopian tubes

      External genitalia of males and females are identical for the first 60 days.

      The testis contain two compartments:

      1. Seminiferous tubules (90% of weight): Sertoli cells, spermatogenesis, stimulated by FSH
      2. Interstitial compartment: Leydig cells, stimulated by LH

      From spermatogonia (original stem cell in gonad) to spermatozoa. Cells migrate from the embryonic yolk sac to the testes. In the seminiferous tubules they become spermatogonia and then through a process called spermatogenesis the spermatogonia become spermatids and then mature spermatozoa. The process from spermatids to spermatozoa is called spermiogenesis.

      • Testosterone required for spermiogenesis in adult.
      • Later stages of spermatogenesis require FSH. Testosterone and FSH act on Sertoli cells which probably release paracrine substances that stimulate spermatogenesis
      • spermatozoa are non-motile in testes. They become motile and undergo other changes outside of the testes.
      1. oval-shaped head: contains the DNA
      2. midpiece or body: contains mitochondria for energy
      3. tail: for swimming
      1. Male Reproductive Tract

      Testis ® epididymis ® Vas deferens ® ejaculatory duct ® urethra

      • Testes : formation of sperm
      • Epididymis : sperm maturation and sperm storage
      • Vas deferens : duct to transport spermatozoa towards the seminal vesicles
      • Seminal vesicles : add secretions to form semen
      • Prostate gland : add secretions to semen
      1. Semen
      1. seminal vesicles: 60% of semen volume comes from seminal vesicles — contains fructose
      2. prostate gland: citric acid, calcium, coagulation proteins
      1. Female Reproductive System
      1. Anatomy
      • Ovaries: contain follicles which contain ova.
      • Accessory Sex Organs
      • Uterine or Fallopian tubes: ducts directly connected to uterus
      • Uterus: 3 layers perimetrium (connective tissue), myometrium (smooth muscle), endometrium (epithelium)
      1. Oocyte and Follicle Development

      1-2 million (primary oocytes)

      At 5 months of gestation there is a peak of about 6-7 million oogonia. After 5-6 months production of oogonia stops and never resumes. These oogonia become primary oocytes by the end of gestation. At puberty there are only about 400,000 primary oocytes. Only about 400 of these oocytes will actually ovulate during a woman's lifetime.

      • Oocyte Development : oogonia (46 chromosomes) ® primary oocyte (46 chromosomes) ® secondary oocyte (23 chromosomes) ® zygote (if fertilized)
      • Follicular Development : primary follicles ® secondary follicles ® graafian follicle ® corpus luteum (after ovulation)
      • Primary follicle : immature: primary oocyte + a single layer of follicular cells mature: primary oocyte + a number of layers of follicular cells.
      • Secondary follicle : primary oocyte or secondary oocyte plus numerous layers of granulosa cells and fluid filled vesicular cavities
      • Graafian follicle : secondary oocyte, arrested before second meiotic division, plus layers of granulosa cells and a single large fluid filled cavity (antrum).
      1. Menstrual Cycle
      • Menstruation (Day 1 to Day 4or 5): steroid hormones lowest, ovaries contain primary follicles
      • Follicular Phase (Day 1 to Day 13, highly variable): FSH ® growth of follicles, one becomes mature graafian follicle, granulosa cells secrete estradiol

      Positive feedback loop: (LH surge): estradiol ® ­ GnRH ® ­ LH secretion ® ­ estradiol by ovaries

      • Ovulation: FSH, followed by LH surge causes rupture of graafian follicle, expulsion of secondary oocyte into uterine tubes. Occurs about 24 hours after beginning of LH surge
      • Luteal Phase : empty follicle becomes corpus luteum, secretes estradiol and progesterone.

      Negative feedback loop : progesterone, estradiol ® ¯ FSH, LH secretion ® ¯ development of follicles.

      Changes in the endometrium

      • Proliferative Phase : proliferation of endometrium and increase in blood vessels (spiral arteries)
      • Secretory Phase : development of endometrium — thick, vascular, spongy — in preparation for the embryo.
      • Menstrual Phase : constriction of spiral arteries ® cell death ® sloughing of layer of endometrium. Bleeding phase.

      Conception is most likely to occur when intercourse takes place 1-2 days prior to ovulation.

      • Contraceptive pill: contains estrogen and progesterone, maintains negative feedback throughout cycle, so no LH surge and therefore no ovulation.
      • Rhythm method: measure slight variations in temperature that normally occur just prior to ovulation. Often hard to detect temperature changes.
      1. Fertilization

      Fertilization occurs in the uterine tubes.

      1. Sperm capacitation, a series of changes which makes sperm fertile, occurs in the female tract. Sperm can last up to 3 days in female reproductive tract.
      2. Sperm fuses with ovulated oocyte in the uterine tube
      3. Fusion of one sperm prevents other sperm from fertilizing oocyte
      4. Zygote (diploid, 46 chromosomes) forms 12 hours after fertilization
      5. The zygote begins dividing (cleavage)
      6. Unfertilized oocyte will degenerate 12-24 hours after ovulation.

      Fertilization cannot take place more than 1 day after ovulation. Since sperm live for 3 days in the female reproductive tract, fertilization can occur up to 3 days prior to ovulation.

      • 3 days after ovulation the embryo (8 cells at this point) enters the uterus
      • 2 days later blastocyst forms
      • 7 days after fertilization embryo is implanted in the uterine wall (endometrium)

      Human chorionic gonadotropin

      The embryo secretes chorionic gonadotropin (hCG). This maintains the corpus luteum and thus estradiol and progesterone secretion remain elevated. Since estradiol and progesterone levels remain elevated the endometrium is maintained, menstruation is prevented, and the embryo continues to grow.


      Mammary Glands

      A mammary gland is an organ in female mammals that produces milk to feed young offspring.

      Learning Objectives

      Describe the function and structure of mammary glands

      Key Takeaways

      Key Points

      • Mammary glands are not associated with the female reproductive tract, but develop as secondary sex characteristics in reproductive-age females.
      • The basic components of a mature mammary gland are the alveoli, hollow cavities, a few millimeters large lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells.
      • Alveoli join up to form groups known as lobules, and each of which has a lactiferous duct that drains into openings in the nipple.
      • Secretory alveoli develop mainly in pregnancy, when rising levels of prolactin, estrogen, and progesterone cause further branching, together with an increase in adipose tissue and a richer blood flow.

      Key Terms

      • Wnts: Morphogenic signaling proteins that regulate cell-cell interactions.
      • beta-1 integrin: One of the regulators of mammary epithelial cell growth and
        differentiation.
      • mammary gland: A gland that secretes milk for suckling an infant or offspring.
      • lactiferous duct: The components that form a branched system connecting the lobules of the mammary gland to the tip of the nipple.

      A mammary gland is an organ in female mammals that produces milk to feed young offspring.

      Anatomy of the Mammary Gland

      The basic components of a mature mammary gland are the alveoli, hollow cavities, a few millimeters large lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells can contract under the stimulation of oxytocin, excreting milk secreted from alveolar units into the lobule lumen toward the nipple where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) mediated “let-down reflex” ensues, and the mother’s milk is secreted into the baby’s mouth.

      Mammary Gland: Cross-section of the mammary-gland. 1. Chest wall 2. Pectoralis muscles 3. Lobules 4. Nipple 5. Areola 6. Milk duct 7. Fatty tissue 8. Skin EndFragment

      All the milk-secreting tissue leading to a single lactiferous duct is called a simple mammary gland a complex mammary gland is all the simple mammary glands serving one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10–20 simple glands. The presence of more than two nipples is known as polythelia, and the presence of more than two complex mammary glands as polymastia.

      Development of the Mammary Glands

      Mammary glands develop during different growth cycles. They exist in both sexes during the embryonic stage, forming only a rudimentary duct tree at birth. In this stage, mammary gland development depends on systemic (and maternal) hormones, but is also under the local regulation of paracrine communication between neighboring epithelial and mesenchymal cells by parathyroid hormone-related protein. This locally-secreted factor gives rise to a series of outside-in and inside-out positive feedback between these two types of cells, so that mammary bud epithelial cells can proliferate and sprout down into the mesenchymal layer until they reach the fat pad to begin the first round of branching.

      Lactiferous duct development occurs in females in response to circulating hormones, first during pre- and postnatal stages and later during puberty. Estrogen promotes branching differentiation, which is inhibited by testosterone in males. A mature duct tree reaching the limit of the fat pad of the mammary gland is formed by bifurcation of duct terminal end buds, secondary branches sprouting from primary ducts and proper duct lumen formation.

      The Process of Milk Production

      Secretory alveoli develop mainly in pregnancy, when rising levels of prolactin, estrogen, and progesterone cause further branching, together with an increase in adipose tissue and a richer blood flow. In gestation, serum progesterone remains at a high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce branching of neighboring cells. When the lactiferous duct tree is almost ready, alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted.

      Milk secretion (lactation) begins a few days after birth, caused by reduction in circulating progesterone and the presence of prolactin, which mediates further alveologenesis and milk protein production and regulates osmotic balance and tight junction function.
      The binding of laminin and collagen in the myoepithelial basement membrane with beta-1 integrin on the epithelial surface insures correct placement of prolactin receptors on basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts. Suckling of the baby causes release of hormone oxytocin which stimulates contraction of the myoepithelial cells. With combined control from the extracellular matrix (ECM) and systemic hormones, milk secretion can be reciprocally amplified to provide enough nutrition for the baby.

      During weaning, decreased prolactin, lack of mechanical stimulation through suckling, and changes in osmotic balance caused by milk stasis and leaking of tight junctions cause cessation of milk production. In some species there is complete or partial involution of alveolar structures after weaning however, in humans there is only partial involution, which widely varies among individuals. Shrinkage of the mammary duct tree and ECM remodeling by various proteinase is under the control of somatostatin and other growth-inhibiting hormones and local factors. This structure change leads loose fat tissue to fill the empty space. However, a functional lactiferous duct tree can be reformed when a female is pregnant again.


      Contents

      Penis Edit

      The penis is the male intromittent organ. It has a long shaft and an enlarged bulbous-shaped tip called the glans penis, which supports and is protected by the foreskin. When the male becomes sexually aroused, the penis becomes erect and ready for sexual activity. Erection occurs because sinuses within the erectile tissue of the penis become filled with blood. The arteries of the penis are dilated while the veins are compressed so that blood flows into the erectile cartilage under pressure. The penis is supplied by the pudendal artery.

      Scrotum Edit

      The scrotum is a pouch-like structure that hangs behind the penis. It holds and protects the testicles. It also contains numerous nerves and blood vessels. During times of lower temperatures, the Cremaster muscle contracts and pulls the scrotum closer to the body, while the Dartos muscle gives it a wrinkled appearance when the temperature increases, the Cremaster and Dartos muscles relax to bring down the scrotum away from the body and remove the wrinkles respectively.

      The scrotum remains connected with the abdomen or pelvic cavity by the inguinal canal. (The spermatic cord, formed from spermatic artery, vein and nerve bound together with connective tissue passes into the testis through inguinal canal.)

      Testis Edit

      Testis has two major functions: To produce sperm by meiotic division of germ cells within the seminiferous tubules, [1] and to synthesize and secrete androgens that regulate the male reproductive functions. The site of production of androgens is the Leydig cells that are located in the interstitium between seminoferous tubules. [1]

      Epididymis Edit

      The epididymis is a long whitish mass of tightly coiled tube. The sperm that are produced in the seminiferous tubules flow into the epididymis. During passage via the epididymis, the sperm undergo maturation and are concentrated by the action of ion channels located on the apical membrane of the epididymis. [2]

      Vas deferens Edit

      The vas deferens, which is also known as the sperm duct, is a thin tube approximately 30 centimetres (0.98 ft) long that starts from the epididymis to the pelvic cavity. It carries the spermatozoa from the epididymis to ejaculatory duct.

      Accessory glands Edit

      Three accessory glands provide fluids that lubricate the duct system and nourish the sperm cells. They are the seminal vesicles, the prostate gland, and the bulbourethral glands (Cowper glands).

      The embryonic and prenatal development of the male reproductive system is the process whereby the reproductive organs grow, mature and are established. It begins with a single fertilized egg and culminates 38 weeks later with birth of a male child. It is a part of the stages of sexual differentiation. The development of the male reproductive system coincides with the urinary system. The development of them can also be described together as the development of the urinary and reproductive organs.

      Sexual determination Edit

      Sexual identity is determined at fertilization when the genetic sex of the zygote has been initialized by a sperm cell containing either an X or Y chromosome. If this sperm cell contains an X chromosome it will coincide with the X chromosome of the ovum and a female child will develop. A sperm cell carrying a Y chromosome results in an XY combination, and a male child will develop. [3]

      Genetic sex determines whether the gonads will be testes or ovaries. In the developing embryo if the testes are developed, it will produce and secrete male sex hormones during late embryonic development and cause the secondary sex organs of the male to develop. [4] [ clarification needed ]

      Other embryonic reproductive structures Edit

      The structures are masculinized by secretions of the testes:

      The prostate gland derives from the urogenital sinus, and the other embryonic structures differentiate into the external genitalia. In the absence of testicular secretions, the female genitalia are formed. [7]

      External structures Edit

      At six weeks post conception, the differentiation of the external genitalia in the male and female has not taken place. At eight weeks, a distinct phallus is present during the indifferent stage. By the 10th-12th week, the genitalia are distinctly male or female being and derived from their homologous structures. At 16 weeks post conception, the genitalia are formed and distinct. [8] [9]

      The masculinization of the embryonic reproductive structures occurs as a result of testosterone secreted by the embryonic testes. Testosterone, however, is not the active agent within these organs. Once inside the target cells, testosterone is converted by means of an enzyme called 5α-reductase into the dihydrotestosterone (DHT). DHT mediates the androgen effect in these organs. [10]

      Testes Edit

      At nine weeks, male differentiation of the gonads and the testes is well underway. Internal changes include the formation of the tubular seminar Chris tubules in the Rete testis from the primary sex cord. Developing on the outside surface of each testis is a Phibro muscular cord called the gubernaculum. This structure attaches to the inferior portion of the testis and extends to the labial sacral fold of the same side at the same time, a portion of the embryonic mesonephric duct adjacent to the testis becomes attached and convoluted informs the epididymis. Another portion of the mesonephric duct becomes the ductus deferens. [10]

      The seminal vesicles form from lateral outgrowths of the caudal and of each mesonephric duct the prostate gland arises from an Indo dermal outgrowth of the urogenital sinus the bulbourethral glands develop from outgrowths in the membrane-like portion of the urethra. [10]

      The descent of the testes to its final location at the anterior abdominal wall, followed by the development of the gubernaculum, which subsequently pulls and translocates the testis down into the developing scrotum. Ultimately, the passageway closes behind the testis. A failure in this process can cause indirect inguinal hernia or an infantile hydrocoele. [ citation needed ] The testes descend into the scrotal sac between the sixth and 10th week. Dissent into this not occur until about the 28th week when compared and we know canals form and the abdominal wall to provide openings from the pelvic cavity to the scrotal sac. The process by which a testis to send is not well understood but it seems to be associated with the shortening of the gubernaculum, which is attached to the testis and extends to the inguinal canal to the wall of the scrotum as a testis to sense it passes to the side of the urinary bladder and anterior to the symphysis pubis. It carries with it the ductus deference, that is testicular vessels and nerves, a portion of the abdominal muscle, and lymph vessels. All of the structures remain attached to the testis and form what is known as the spermatic cord by the time the testis is in the scrotal sac, the gubernaculum is no more than a remnant of scar like tissue. [10]

      External genitalia Edit

      The external genitalia of the male is distinct from those of the female by the end of the ninth week. Prior to that, the genital tubercle in both sexes is a phallus. The urethral groove forms on the ventral surface of the phallus early in development during the differentiation of the external genitalia. This is caused by the androgens produced and secreted by the testes. Androgen induced development causes the elongation and differentiation of the phallus into a penis, a fusion of the urogenital folds surrounding the urethral groove along the ventral surface of the penis, and a midline closure of the labioscrotal folds. This closure forms the wall of the scrotum the external genitalia. The external genitalia are completely formed by the end of the 12th week. [10] [11]

      At birth, the development of the prepubertal male reproductive system is completed. During the second trimester of pregnancy, testosterone secretion in the male declines so that at birth the testes are inactive. [12] Gonadotropin secretion is low until the beginning of puberty. [13]

      Summary Edit

      The genetic sex is determined by whether a Y bearing or next bearing sperm fertilizes the open the presence or absence of a Y chromosome in turn determines whether the gonads of the embryo will be testes or ovaries and the presence or absence of testes, finally, determines whether the sex accessory organs and external genitalia will be male or female. This sequence is understandable in light of the fact that both male and female embryos develop within the maternal environment - high in estrogen secreted by the mother's ovaries and the placenta. If estrogen determined the gender, all embryos would become feminized. [10]

      Puberty Edit

      During puberty, increased gonadotropin secretion stimulates a rise in sex steroids creation from the testes. The increased secretion of testosterone from the testes during puberty causes the male secondary sexual characteristics to be manifested. [14]

      Male secondary sex characteristics include:

      • Growth of body hair, including underarm, abdominal, chest hair and pubic hair. [15][16]
      • Growth of facial hair. [16]
      • Enlargement of larynx (Adam's apple) and deepening of voice. [16][17]
      • Increased stature adult males are taller than adult females, on average. [16]
      • Heavier skull and bonestructure. [16]
      • Increased muscle mass and strength. [16]
      • Broadening of shoulders and chest shoulders wider than hips. [18]
      • Increased secretions of oil and sweat glands. [17]

      Secondary development includes the increased activity of the eccrine sweat glands and sebaceous glands along with the darkening of the skin in the scrotal region. [13]


      Comparative Reproductive Biology

      Written by renowned scientists in their respective fields, Comparative Reproductive Biology is a comprehensive reference on the reproductive systems of domestic species. The book offers both broad and specific knowledge in areas that have advanced the field in recent years, including advances in cell and molecular biology applied to reproduction, transgenic animal production, gender selection, artificial insemination, embryo transfer, cryobiology, animal cloning and many others. This seminal text includes topics in animal reproduction that are usually only found as part of other books in animal science such as anatomy, histology, physiology, radiology, ultrasonogrophy, and others.


      • Comprehensive reference of the reproductive systems of domestic species

      • Written by a team of top researchers

      • Richly illustrated throughout, including 12 pages of color images

      Reviews

      "Comparative Reproductive Biology fills a niche that has not been filled in many years. It provides broad as well as specific information for the field of reproduction this information is founded in anatomy and expanded to assisted reproductive technology." (Journal of the American Veterinary Medical Association, March 2009)

      “Fortunate collaboration of authors with complimentary backgrounds… The book contains numerous beautiful and highly detailed hand drawn illustrations of comparative anatomy and placentation. Despite [the] broad approach, there is an abundance of detailed information in many chapers that can be difficult to find in one place. an essential reference for veterinarians, graduate students, and scientists working or teaching in the field.” - Interface

      “It clearly acheives [its] goal, particularly with its chapters on the different types of assisted reproductive technology. Overall, [this book] will serve as a useful resource for veterinary institutions and research facilities.” - Journal of the American Association for Laboratory Animal Science

      Author Bios

      Gheorghe M. Constantinescu, DVM, PhD, Drhc, is Professor of Veterinary Anatomy and Medical Illustrator at the College of Veterinary Medicine of the University of Missouri, Columbia. He is a member of the American, European and World Associations of Veterinary Anatomists and also author of more than 380 publications, including Clinical Anatomy for Small Animal Practitioners (Blackwell, 2002) translated in three languages. During his career of over 50 years, he has been honored by numerous invited presentations, awards, diplomas and certificates of recognition.


      Watch the video: Lab #10: The Male Reproductive Tract (May 2022).


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