13.3: Male System - Structures and Functions

The male reproductive system contains several paired tubular structures, including the testes, the epididymides (sing., epididymis), and the ductus deferentia (sing., ductus deferens) (Figure 13.2, Figure 13.3). There are also two pairs of glands: the seminal vesicles and bulbourethral glands. The other important structures in this system occur singly and include the scrotum, the prostate gland, the urethra, and the penis. Finally, the scrotum and the area near the base of the penis are covered with pubic hair.

Figure 13.2 Structure of the male reproductive system: Sagittal section of male pelvis - two versions. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 13.3 Structure of the male reproductive system: 3D view of male reproductive system. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Testes

The two testes rest within the scrotum, a sack of skin and fibrous material suspended near the front of the body between the thighs (Figure 13.4, Figure 13.5). Each oval-shaped testis is divided into 250 to 300 sections by fibrous sheets, and each section contains up to four long, highly coiled tubes called seminiferous tubules. Each tubule may be up to 100 feet long, and the total length of all tubules in one testis is approximately up to 1/8 mile. Spaces among these tubules contain blood vessels and special cells called interstitial cells (Leydig's cells).

Figure 13.4 Structure of the testes. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 13.5 Structure of the seminiferous tubules. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Vessels and Interstitial Cells

Blood flow through the testes delivers needed materials and removes wastes and sex hormones produced by the testes. The interstitial cells produce and secrete two male sex hormones: testosterone and dihydrotestosterone (DHT). These hormones are essential for proper sperm production, development and maintenance of male reproductive structures, development and maintenance of other male characteristics (e.g., deep voice, beard), and interest in sexual activity (libido). They also influence several other activities (Chapter 14).

Seminiferous Tubules and Sperm Production

The wall of each seminiferous tubule is many cells thick (Figure 13.5). Many cells in the outer region of the tubule wall reproduce rapidly, and most of the newly produced cells move toward the central channel (lumen). As each cell moves toward the lumen, it undergoes a special type of cell division called meiosis, which results in the formation of four cells called spermatids. In men, meiosis is also called spermatogenesis. Each spermatid then matures into a long sperm cell through the process of spermiogenesis. Later, when fully mature, each sperm cell can initiate the life of a new human being by entering an ovum during the process of fertilization. Since sperm production occurs continuously throughout the nearly one-eighth of a mile of seminiferous tubules in each testis, a man produces several hundred million sperm cells a day.

The seminiferous tubule wall also contains large sustentacular cells (Sertoli's cells) (Figure 13.5), which promote sperm production in three ways. First, they produce and retain androgen-binding protein (ABP), which binds testosterone and concentrates it in the tubules, stimulating sperm production. Second, these cells protect sperm-producing cells from attack by the immune system. Third, they nourish spermatids as they mature into sperm cells. The sustentacular cells also produce a sex hormone (inhibin) that helps regulate testosterone levels.

Ducts

Epididymis

A series of conducting tubes connect the seminiferous tubules in each testis to an epididymis, a coiled tube behind the testis. Sperm cells and the small amount of fluid produced by the seminiferous tubules move through the conducting tubes into the epididymis. While being stored in the epididymis for 10 days or more, sperm cells become fully mature and capable of swimming. The epididymis also secretes a small amount of fluid that seems to contain nutrients for the sperm cells.

During sexual arousal and activity, contractions of the epididymis push the sperm cells and fluid into the tubular ductus deferens. Mature sperm cells that are not released from the epididymis within about a month are broken down chemically.

Ductus Deferens and Ejaculatory Duct

Each ductus deferens (also called vas deferens) passes up from the scrotum and into the body cavity (Figure 13.2, Figure 13.3). There, the ductus deferens loops over the urinary bladder from front to back and widens just before entering the rear of the prostate gland. Upon entering the prostate gland, the ductus deferens becomes the ejaculatory duct, which leads into the center of the prostate gland and joins the urethra. Rhythmic peristaltic contractions that occur during sexual activity propel the sperm from the ductus deferens into the urethra.

Urethra

The urethra passes through the prostate gland, exits from the body cavity, and extends through the penis to its external opening. Rhythmic peristaltic contractions of the urethra, which occur during the peak of male sexual response (i.e., during ejaculation), propel the sperm cells and accompanying fluids through the urethra and out of the body.

Glands

The successful delivery of functional sperm cells into the female reproductive system depends not only on the functioning of the reproductive ducts but also on secretions from reproductive glands (seminal vesicles, prostate, bulbourethral glands) (Figure 13.2, Figure 13.3). The mixture of sperm cells and secretions released from a man's body during sexual activity usually contains 300 million to 500 million sperm cells and totals approximately 3 ml. to 5 ml. Almost all of this mixture—semen—consists of secretions from reproductive glands.

Seminal Vesicles

Approximately two-thirds of the semen comes from the seminal vesicles, which lie behind the urinary bladder just above the prostate gland. The wall of each seminal vesicle consists of three layers: an inner epithelial layer of secreting cells, a middle layer of smooth muscle, and an outer layer of fibrous connective tissue. Within the gland the wall is highly folded, producing many interconnected spaces that resemble a sponge.

The thick secretion from the inner layer of the seminal vesicle is stored in the internal spaces of the gland. This secretion contains a variety of substances, including water, fructose, and alkaline materials. During sexual activity contraction of the smooth muscle layer forces the secreted materials out of the spaces, through the duct in the seminal vesicle, and into the ductus deferens where it leads into the ejaculatory duct. The secretion then mixes with the sperm and fluids passing from the ductus deferens into the ejaculatory duct.

The water in this secretion dilutes the sperm cells so that they have more room to move. The fructose provides energy that allows the sperm cells to swim actively while trying to reach the ovum. The alkaline materials protect the sperm cells by neutralizing acid materials in the urethra and the female reproductive system.

Prostate Gland

The prostate gland surrounds the upper end of the urethra. It is slightly flattened and is little more than 2.5 cm (1 inch) in diameter. It actually consists of numerous small glands that contain secreting cells and are surrounded by smooth muscle. In addition, fibrous material is found throughout the prostate gland and a distinct layer of fibrous material surrounds the entire gland.

During sexual activity, contraction by the smooth muscle forces the secretion from each small gland through its duct and into either the ejaculatory ducts or the urethra. The secretion then mixes with the sperm cells and other fluids. It contains water and alkaline materials, which serve the same functions as the corresponding substances in the secretions from seminal vesicles. Secretion from the prostate gland constitutes approximately 15 percent of the semen.

Bulbourethral Glands

The bulbourethral glands (Cowper's glands) are located on opposite sides of the urethra just below the prostate gland. Each round bulbourethral gland is less than a quarter inch in diameter.

At the beginning of sexual arousal these glands secrete no more than a few drops of a clear slippery alkaline liquid into the urethra. The alkaline material helps neutralize acid materials in the urethra before contractions in the ejaculatory ducts push sperm cells into the urethra. This protects sperm cells from any acid urine in the urethra. In addition, some of the secretion often leaks out of the external urethral opening onto the end of the penis. This material (precoital fluid) lubricates the end of the penis and aids in inserting it into the vagina at the beginning of sexual intercourse.

Penis

The penis contains the urethra, three elongated masses of spongy erectile tissue, and several arteries and veins, all of which run parallel to the urethra ((Figure 13.2, Figure 13.3) and Figure 13.4). One portion of the erectile tissue (corpus spongiosum) surrounds the portion of the urethra that passes through the penis. The other two erectile tissue masses (corpus cavernosa) are above the urethra and its erectile tissue. The spaces within the erectile tissue can be filled with blood from the penile arteries. Sheets of fibrous tissue surround all these penile components, and the surface of the penis is covered with skin.

Under resting conditions, the erectile tissues are narrow and soft because they contain little blood and the penis is flaccid (limp and flexible). In this state the penis is only a few inches long, though its length varies both from time to time and between individuals.

Erection

During sexual arousal dilation of penile arteries causes blood to enter and fill the erectile tissue spaces faster than the veins carry it away (Figure 13.6). As a result, the erectile tissues expand and become firmer, causing the penis to widen and lengthen. As the erectile tissue expands, it compresses the veins and slows the exit of blood. This further increases the extent of erectile tissue engorgement. The restricting nature of the surrounding fibrous sheets causes the pressure in the filling erectile tissue to increase, and the penis straightens and stiffens. Collectively, these changes constitute the process of erection. Once erection is complete, the penis can be inserted into a vagina and semen can be deposited into the female reproductive system.

Figure 13.6 Penile structure and erection. (Sources of images and videos below. Used with permission.)

An erect penis is often more than 1 inch in diameter and measures 6 inches to 7 inches in length. The final dimensions of the erect penis vary somewhat between individuals and bear only a small correspondence to its dimensions in the flaccid condition.

Erection is a reflexive action controlled primarily by autonomic nerves. It can be caused by erotic physical stimulation (stimulation of the penis or scrotum) or by erotic mental processes (sexual fantasies). Other types of physical stimulation, such as having a full urinary bladder or mild irritation of the urethra, and certain other mental processes, such as those which occur during sleep, can also result in erection. Conversely, erection can be prevented or reversed by other physical stimuli (e.g., pain) and mental processes (e.g., fear).

The results of erection are reversed when blood flow into the erectile tissue is slowed by penile artery constriction. Then blood leaves the erectile tissue faster than it enters, and the penis returns to its flaccid state.

Pubic Hair

The skin of the scrotum is covered with fairly sparse pubic hair. A thicker mat of pubic hair covers the skin near the base of the penis.