Anatomy of the Male Reproductive System

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Chapter: Anatomy and Physiology for Health Professionals: Reproductive System

Anatomy and Structure of Scrotum, Testes, Penis, Male Duct System, Male Accessory Glands, Semen

Anatomy of the Male Reproductive System

Anatomy of the Male Reproductive System

The structures of the male reproductive system include two epididymides, two ductus deferentia, two ejacula-tory ducts, the urethra, two seminal vesicles, the pros-tate gland, and two bulbourethral glands. Sperm cells are produced and maintained by the male reproduc-tive organs, which also transport these cells outside the body and secrete male sex hormones. The primary sex organs or gonads of the male consist of the two tes-tes, in which sperm cells and male sex hormones are formed. The accessory sex organs are the internal and external reproductive organs (FIGURE 25-1).



Scrotum

The male external reproductive organs consist of the scrotum and the penis. The scrotum consists of a flesh pouch of skin and subcutaneous tissue suspended below the perineum and anterior to the anus. The scro-tum encloses the testes. Sparse hairs cover the scrotum externally. Internally, the medial septum or raphe sub-divides it into two chambers, each enclosing a testis. The scrotum protects and controls the temperature of the testes, which is important for sex cell production.

When environmental temperatures are cold, the scrotum contracts and wrinkles, moving the testes closer to the pelvic cavity to absorb heat. When it is warmer outside, the scrotum relaxes and hangs loosely to ensure the testes are about 3°C lower than body temperature. This is better for the sperm cells to be produced and to survive. Viable sperm cannot be produced at normal core body temperature, which is 98.6°F (37°C).

The scrotum contains two sets of muscles that respond to temperature changes. Each dartos muscle is a smooth muscle layer in the superficial fascia that acts to wrinkle the scrotal skin. Bands of skeletal mus-cle arising from the internal oblique muscles of the body’s trunk are known as the cremaster muscles, which act to elevate the testes toward the body, to con-trol scrotal temperature.


Testes

The testes are oval-shaped structures, about 4 cm (1.5 inches) long and 2.5 cm (1 inch) wide, located within the cavity of the scrotum. Each testis is also enclosed in two tunics. The outer is the tunica vaginalis, which has two serous layers and is formed from a peritoneal outpocket. The inner tunic is the tunica albuginea and is a fibrous capsule. Thin septa extend inward from the tunica albuginea and divide the testis into approximately 250 lobules. Each wedge-shaped lobule contains one to four highly coiled seminiferous­ tubules that, when uncoiled, may reach 80 cm in length. It is here where sperm are actually formed. The seminiferous tubules are made up of a thickened stratified epithelium that surrounds a central lumen filled with fluid. Spermatogenic cells are found in larger columnar cells known as sustenocytes­. These cells play a variety of roles in sperm ­formation. Sperm are ­generated ­continuously. The sustenocytes maintain the blood-testis barrier, support ­spermiogenesis, secrete inhibin hormone, and secrete androgen-­binding protein.

A normal testis contains nearly one-half of a mile of seminiferous tubules. Each of these tubules forms a loop connected to a network of passageways known as the rete testis. Fifteen to 20 large efferent ductules connect the rete testis to the epididymis. This tube coils on the outer surface of the testis and becomes the ductus deferens (FIGURE 25-2). Three to five layers of myoid cells, which resemble smooth muscle cells, surround each seminiferous tubule. Rhythmic con-tractions of the myoid cells aid in squeezing sperm and testicular fluids through the seminiferous tubules and out of the testes. The rete testis receives sperm through a straight tubule formed by the seminiferous tubules of each lobule, which leads into the epididymis for the maturation of sperm. The epididymis wraps around the posterior external surface of each testis. Immature sperm pass through the head and body of the epidid-ymis to be stored in its “tail” portion until ejaculation.


Interstitial endocrine cells, also known as ­Leydig cells, lie inside the soft connective tissue that surrounds the seminiferous tubules. They produce testosterone and less important types of androgens. These substances are secreted into the surround-ing interstitial fluid. The testes are supplied by long ­testicular arteries that branch from the abdominal aorta, superior to the pelvis. The testes are drained by the testicular veins, which arise from a network known as the pampiniform venous plexus. This network surrounds each testicular artery inside the scrotum, winding around it. In each pampiniform plexus, cooler venous blood absorbs heat from arterial blood. Therefore, this blood becomes cooler before entering the testes, which helps to keep the testes at their normal, cool, homeostatic temperature.

The testes are served by the sympathetic and para-sympathetic divisions of the autonomic nervous sys-tem. Forceful trauma to the testes transmits impulses, causing intense pain and nausea. In the testes, blood vessels, nerve fibers, lymphatic vessels, and the ductus deferens are enclosed by a connective tissue sheath. Together, these structures comprise the spermatic cord passing through the inguinal canal.

1. Explain the role of the scrotum in protection of the testes.

2. Describe the two major functions of the testes.

3. Identify the structures in which sperm are actually formed.


Penis

The penis is cylindrical in shape and conveys urine and semen through the urethra. When erect, it stiff-ens and enlarges, enabling insertion into the vagina during sexual intercourse. The penis is divided into three regions: the root, body, and glans or glans penis. The root of the penis is the fixed portion that attaches the penis to the body wall. At birth, skin that covers the penis is loose. It slides distally over the head, forming the foreskin or prepuce­ around the glans. The prepuce is often removed surgically soon after birth in a procedure called a circumcision. This practice is more common in the United States, where at least 65% of males are circumcised. In the rest of the world, about 30% of males experience this procedure. Many cultures are not familiar with circumcision, including some Hispanic cultures and many European and Asian cultures. Most males from Muslim and Jew-ish cultures are circumcised. It is widely believed that circumcision reduces risk of acquiring HIV or other reproductive system infections.


The dorsal and ventral surfaces of the penis are actually named in relation to the penis being erect, not flaccid. The shaft or body of the penis is the tubular, movable portion of the organ. It contains three ­columns of erectile tissue. It has two dorsal corpora cavernosa and one ventral corpus spongiosum (FIGURE 25 -3). The corpora cavernosa is the erectile tis-sue that is located on the anterior surface of the penis. The urethra is surrounded by the corpus spongiosum. Dense connective tissue surrounds each column in a capsule. The penis is enclosed by a layer of connective tissue, a thin layer of subcutaneous tissue, and skin. The erectile tissue of the penis contains many vascular spaces. These spaces fill with blood during sexual stimulation.


Male Duct System

When sperm are produced by the testes, they move out of the body via a system of ducts. These ducts are the epididymis, ductus deferens, ejaculatory duct, and urethra.

Epididymis

The cup-shaped epididymis can be felt through the skin of the scrotum. It is coiled and twisted to take up only a small amount of space, about 3.8 cm or 1.5 inches. The head of the epididymis contains effer-ent ductules and lies above the superior aspect of each testis. The body and tail of the epididymis are found on the posterolateral area of each testis.

The epididymis controls the composition of the fluid produced by the seminiferous tubules. It also absorbs and recycles damaged spermatozoa and absorbs cellular debris. The products of the break-down of enzymes are released into the surrounding interstitial fluids for pickup by the epididymal blood vessels. The epididymis also stores and protects sper-matozoa and facilitates their functional maturation.

Most epididymides are tightly coiled tubes about 6 m or 20 feet in length, connected to the posterior border of the testes. These tubes are also described as the ducts of the epididymides. They course upward to become the ductus deferens. In the duct mucosa, certain pseudostratified epithelial cells have stereocilia, which are long, nonmoving microvilli. Having a large surface area, the stereocilia can absorb extra testicular fluid and pass nutrients to the millions of sperm cells temporarily stored in the lumen.

Immature sperm cells are nonmotile when they reach the epididymis; therefore, rhythmic peristal-tic contractions move them through the duct as they mature. The surrounding fluid contains antimicrobial proteins such as defensins. The transfer of sperm from the testes and through the epididymides takes approx-imately 20 days. Once mature, sperm cells can move independently to fertilize egg cells, but usually do not actually “swim” until after ejaculation. This occurs from the epididymides and not the testes. The secretions of the seminal vesicles are discharged into the ejaculatory duct at emission. This is when peristaltic contractions are occurring in the ductus deferens, seminal vesicles, and prostate gland. These contractions are controlled by the sympathetic nervous ­system. Although sperm are normally stored in the epididymides for several months, longer storage results in them being phagocy-tized by the epithelial cells there.

Ductus Deferens and Ejaculatory Duct

The ductus deferentia, also called the vasa deferentia, are muscular tubes approximately 45 cm (18 inches) in length. Singularly, each ductus defer-ens is called a vas deferens. They each pass upward, as part of the spermatic cord, along the medial side of a testis, through the inguinal canal in the lower abdominal wall to enter the pelvic cavity. They end behind the urinary bladder, uniting just outside the prostate gland with the duct of a seminal vesicle. This forms an ejaculatory duct, which is a short structure passing through the prostate gland to empty into the urethra. The vas deferens is the duct that is altered when a male undergoes a vasectomy.

Each ductus deferens can easily be felt as it passes anterior to the pubic bone, looping medially over the ureter and descending along the posterior wall of the bladder. The terminus portion expands to form an ampulla, joining the duct of a seminal vesicle. The mucosa of the ductus deferens is, like the epidid-ymis, pseudostratified epithelium. It differs in that its muscular layer is very thick. During ejaculation, the smooth muscle of its walls creates peristaltic waves, quickly squeezing sperm forward into the urethra.


Male Accessory Glands

The male accessory glands consist of a pair of seminal glands and bulbourethral glands, plus a single prostate gland.

Seminal Glands

The seminal glands are also called seminal vesicles and are sac-like structures lying on the posterior bladder surface. They are approximately 5 cm long, attached to the ductus deferens near the base of the bladder. Each seminal vesicle is a tubular gland having a total uncoiled length of about 15 cm. However, these glands are normally coiled back on themselves, mak-ing their coiled size only 5–7 cm. During ejaculation, they are emptied by a thick layer of smooth muscle that contracts inside their fibrous capsules. They have glandular tissue linings that contribute nearly 60% of semen volume.

The seminal vesicles secrete a slightly alkaline fluid that is yellowish in color and viscous. This fluid helps to regulate the pH of the tubular contents as sperm cells travel to outside the body. The yellow color of seminal fluid comes from a pigment that becomes flu-orescent under ultraviolet light, a fact that is used for the investigation of certain crimes. Seminal fluid con-tains fructose, a monosaccharide that provides energy for sperm cells, as well as prostaglandins that stimu-late muscular contractions within the female repro-ductive organs. These contractions aid the movement of sperm cells toward the egg cell. The fluid from the seminal glands also contains citric acid and a coagu-lating enzyme known as vesiculase.

Remember that the duct of each seminal gland joins the duct of the ductus deferens on the same side, forming the ejaculatory duct. Here, seminal fluid mixes with sperm, entering the prostatic urethra simultaneously during ejaculation. Semen, therefore, is 70% made up by seminal gland secretions.

Prostate

The prostate gland surrounds the proximal portion of the urethra, slightly inferior to the urinary bladder. It is a chestnut- or doughnut-shaped, muscular struc-ture that is approximately 4-cm wide and 3-cm thick. It is surrounded by a thick connective tissue capsule and made up of 20–30 branched tubular glands with ducts that open into the urethra. These glands are embedded in a stroma, which is a mass of dense con-nective tissue and smooth muscle.

The prostatic smooth muscle contracts during ejaculation. Prostatic secretions are squeezed into the prostatic urethra through several ducts. The secretions consist of a milky fluid that is slightly acidic. Prostatic fluid enhances the motility of the sperm cells and helps neutralize the vagina’s highly acidic secretions. It makes up to one-third of the volume of the semen. Prostatic fluid contains citrate, which provides nutri-ents, prostate-specific antigen, and enzymes such as fibrinolysin, acid phosphatase, and hyaluronidase.

Bulbourethral Glands

The bulbourethral glands, also known as Cowper’s glands, are about 1 cm in diameter and lie inferior to the prostate gland surrounded by the external urethral sphincter muscle’s fibers. These glands have tubes with epithelial linings secreting a thick, clear mucous-like fluid as a response to sexual stimulation. The fluid lubricates the end of the penis to prepare for sexual intercourse, even though females secrete most of the lubricating fluid needed for sexual intercourse. The fluid from the bulbourethral glands also neutralizes any urine, which is acidic.


Semen

The milky white, slightly sticky fluid the male ure-thra conveys to outside of the body during ejacula-tion is known as semen. It is made up of sperm cells from the testes and secretions of the seminal vesicles, prostate gland, and bulbourethral glands. Semen has an alkaline pH of between 7.2 and 8.0, and includes prostaglandins and nutrients. It helps to neutralize the acidic environment of the male urethra and the female vagina. Under acidic conditions, the sperm “swim” more slowly than normal.

Between 2 and 5 mL of semen are released at one time, with between 20 and 150 million sperm/mL. How-ever, sperm only make up about 10% of the semen. Sperm cells begin to swim as they mix with accessory gland secretions. They acquire the ability to fertilize a female egg cell once they are inside the female reproductive tract in a process called capacitation, which is due to the weakening of the sperm cells’ acrosomal membranes.

Mature sperm do not contain significant amounts of stored nutrients or cytoplasm. Nearly all energy needed for sperm adenosine triphosphate synthe-sis is provided by the catabolism of fructose in sem-inal gland secretions. The prostaglandins in semen decrease viscosity in the femalecervix, stimulating reverse peristalsis. This speeds up the movement of sperm through the female reproductive tract. Semen also contains the hormone relaxin, various enzymes, ingredients that suppress the immune response in the female reproductive tract, antibiotics that destroy certain bacteria, and clotting factors. Just after ejacu-lation, the clotting factors coagulate the semen, which causes the sperm to stick to the vaginal walls of the female so they do not drain out of the vagina. Fibri-nolysin then liquefies the sticky mass, allowing the sperm to swim along their journey to the ovum.

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