Anatomy and Physiology

The male reproductive system consists of the following structures, as shown in Figure 1:

• The scrotum is a sac consisting of skin and superficial fascia that hangs from the base of the penis. A vertical septum divides the scrotum into left and right compartments, each of which encloses a testis. The external scrotum positions the testes outside the body in an environment about 3°C below that of the body cavity, a condition necessary for the development and storage of sperm. The following two muscles help maintain this temperature if the external conditions get too cold:

  • The dartos muscle is located in the superficial fascia of the scrotum and septum. Contraction of this smooth muscle creates wrinkles in the scrotum skin. The wrinkling thickens the skin, reducing heat loss when external temperatures are too cold.

  • The cremaster muscles extend from the internal oblique muscle to the scrotum. Contraction of these skeletal muscles lifts the scrotum closer to the body when external temperatures are too cold.

• Each of the two testes (singular, testis) consists of the following structures:

  • The tunica vaginalis is a two‐layer outer serous membrane surrounding each testis.

  • The tunica albuginea lies inside the tunica vaginalis and protrudes inward, dividing each testis into compartments called lobules.

  • One to four tightly coiled tubes, the seminiferous tubules, lie inside each lobule. The seminiferous tubules are the sites of sperm production (spermatogenesis). The tubule is lined with spermatogenic cells, which form sperm, and sustentacular cells (Sertoli cells), which support the developing sperm. The coiled seminiferous tubules inside each lobule unite to form a straight tube, the tubulus rectus.

  • The rete testis is a network of tubes formed by the merging of the tubules recti from each lobule.

  • The efferent ducts transport sperm out of the testis (from the rete testis) to the epididymis.

  • Interstitial cells surrounding the seminiferous tubules secrete testosterone and other androgen hormones.

• The epididymis is a comma‐shaped organ that lies adjacent to each testis. Each of the two epididymides contains a tightly coiled tube, the ductus epididymis. Here, sperm complete their maturation and are stored until ejaculation. During ejaculation, smooth muscles encircling the epididymis contract, forcing mature sperm into the next tube, the ductus deferens. The walls of the ductus epididymis contain microvilli called stereocilia that nourish sperm.

• The ductus deferens (vas deferens) is the tube through which sperm travel when they leave the epididymis. Each of the two tubes enters the abdominal cavity, passes around the urinary bladder (refer to Figure 1), and together with the duct from the seminal vesicle, joins the ejaculatory duct. Before entering the ejaculatory duct, the ductus deferens enlarges, forming a region called the ampulla. Sperm are stored in the ductus deferens until peristaltic contractions of the smooth muscles surrounding the ductus force sperm forward during ejaculation.

• The ejaculatory ducts are short tubes that connect each ductus deferens to the urethra.

• The urethra is the passageway for urine and semen (sperm and associated secretions). Three regions of the urethra are distinguished:

  • The prostatic urethra passes through the prostate gland.

  • The membranous urethra passes through the urogenital diaphragm (muscles associated with the pelvic region).

  • The spongy (penile) urethra passes through the penis.

  The urethra ends at the external urethral orifice.

• The spermatic cord contains blood vessels, lymphatic vessels, nerves, the ductus deferens, and the cremaster muscle. It connects each testis to the body cavity, entering the abdominal wall through the inguinal canal.

• The accessory sex glands secrete substances into the passageways that transport sperm. These substances contribute to the liquid portion of the semen:

  • The seminal vesicles secrete into the vas deferens an alkaline fluid (which neutralizes the acid in the vagina), fructose (which provides energy for the sperm), and prostaglandins (which increase sperm viability and stimulate female uterine contractions that help sperm move into the uterus).

  • The prostate gland secretes a milky, slightly acidic fluid into the urethra. Various substances in the fluid increase sperm mobility and viability.

  • The bulbourethral glands secrete an alkaline fluid into the spongy urethra. The fluid neutralizes acidic urine in the urethra before ejaculation occurs.

• The penis is a cylindrical organ that passes urine and delivers sperm. It consists of a root that attaches the penis to the perineum, a body (shaft) that makes up the bulk of the penis, and the glans penis, the enlarged end of the body. The glans penis is covered by a prepuce (foreskin), which may be surgically removed in a procedure called circumcision. Internally, the penis consists of three cylindrical masses of tissue, each of which is surrounded by a thin layer of fibrous tissue, the tunic albuginea. The three cylindrical masses, which function as erectile bodies, are as follows:

  • Two corpora cavernosa fill most of the volume of the penis. Their bases, called the crura (singular, crus) of the penis, attach to the urogenital diaphragm.

  • A single corpus spongiosum encloses the urethra and expands at the end to form the glans penis. The bulb of the penis, an enlargement at the base of the corpus spongiosum, attaches to the urogenital diaphragm.

Figure 1. View of the male reproductive system.

During erection, parasympathetic neurons stimulate dilation of the arteries that deliver blood to the corpus cavernosa and spongiosum. As a result, blood collects in these blood vessels and causes the penis to begin to become erect. The developing erection also constricts the exiting veins of the penis. This causes even more erection. Ejaculation occurs when sympathetic neurons stimulate the discharge of sperm and supporting fluids from their various sources. During ejaculation, the sphincter muscle at the base of the urinary bladder constricts, preventing the passage of urine.

Spermatogenesis

The cells that line the walls of the seminiferous tubules are collectively called spermatogenic cells. Those cells nearest the basement membrane are called spermatogonia. These cells are stem cells—that is, they are capable of continuous division and remain undifferentiated, never maturing into specialized cells. Extending from the spermatogonia toward the lumen of the tubule are cells at various levels of maturity, with the most mature cells—the sperm—facing the lumen.

Spermatogenesis begins at puberty within the seminiferous tubules of the testes. The spermatogonia, each of which contains 46 chromosomes, divide by mitosis repeatedly and differentiate to produce primary spermatocytes (still diploid cells with 46 chromosomes each). The primary spermatocytes begin meiosis. During the first meiotic division (meiosis I, or the reduction division), each primary spermatocyte divides into two secondary spermatocytes, each with 23 chromosomes (haploid cells). During the second meiotic division (meiosis II, or the equatorial division), each secondary spermatocyte divides again, producing a total of four spermatids. Each spermatid still contains 23 chromosomes, but these chromosomes consist of only one chromatid (rather than the normal two chromatids).

Spermiogenesis describes the development of spermatids into mature sperm (sperm cells, or spermatozoa). At the end of this process, each sperm cell bears the following structures:

• The head of the sperm contains the haploid nucleus with 23 chromosomes. At the tip of the sperm head is the acrosome, a lysosome containing enzymes that are used to penetrate the egg. The acrosome originates from Golgi body vesicles that fuse to form a single lysosome.

• The midpiece is the first part of the tail. Mitochondria spiral around the midpiece and produce energy (ATP) used to generate the whiplike movements of the tail that propel the sperm.

• The tail is a flagellum consisting of the typical 9 + 2 microtubule array.

Hormonal regulation of spermatogenesis

The production of sperm is regulated by hormones, as shown in Figure 2:

• The hypothalamus begins secreting gonadotropin releasing hormone (GnRH) at puberty.

• GnRH stimulates the anterior pituitary to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH).

• LH stimulates the interstitial cells in the testes to produce testosterone and other male sex hormones (androgens). (In males, LH is also called interstitial cell stimulating hormone, or ICSH.)

• Testosterone produces the following effects:

  • Testosterone stimulates the final stages of sperm development in the nearby seminiferous tubules. It accumulates in these tissues because testosterone and FSH act together to stimulate sustentacular cells to release androgen‐binding protein (ABP). ABP holds testosterone in these cells.

  • Testosterone entering the blood circulates throughout the body, where it stimulates activity in the prostate gland, seminal vesicles, and various other target tissues.

  • Testosterone and other androgens stimulate the development of secondary sex characteristics, those characteristics not directly involved in reproduction. These include the distribution of muscle and fat typical in adult males, various body hair (facial and pubic hair, for example), and deepening of the voice.

Figure 2. Processes of hormone regulation in the male and female reproductive systems.

Levels of testosterone are regulated by a negative‐feedback mechanism with the hypothalamus. When the hypothalamus detects excessive amounts of testosterone in the blood, it reduces its secretion of GnRH. In response, the anterior pituitary reduces its production of LH and FSH, which results in a decrease in the production of testosterone by interstitial cells. GnRH secretion is also inhibited by inhibin, a hormone secreted by sustentacular cells in response to excessive levels of sperm production.