Male Reproductive System

The male reproductive system, like that of the female, may be divided into two groups of organs: primary and accessory (see Fig. 19-1).
* The primary organs are the gonads, or sex glands; they produce the germ cells and manufacture hormones. The male gonad is the testis. (In comparison, the female gonad is the ovary, as explained below.)
* The accessory organs include a series of ducts that transport the germ cells as well as various exocrine glands.

The Testes

The male gonads, the testes are located outside the body proper, suspended between the thighs in a sac called the scrotum. The testes are oval organs measuring about 4.0 cm (1.5 inches) in length and about 2.5 cm (1 inch) in each of the other two dimensions. During embryonic life, each testis develops from tissue near the kidney. A month or two before birth, the testis normally descends (moves downward) through the inguinal canal in the abdominal wall into the scrotum. Each testis then remains suspended by a spermatic cord (Fig. 19-2) that extends through the inguinal canal.

This cord contains blood vessels, lymphatic vessels, nerves, and the tube (ductus deferens) that transports spermatozoa away from the testis. The gland must descend completely if it is to function normally; to produce spermatozoa, the testis must be kept at the temperature of the scrotum, which is several degrees lower than that of the abdominal cavity.

Figure 19-1 Male reproductive system. Organs of the urinary system are also shown.

Internal Structure: Most of the specialized tissue of the testis consists of tiny coiled seminiferous tubules. Primitive cells in the walls of these tubules develop into mature spermatozoa, aided by neighboring cells called sustentacular cells. These so-called “nurse” cells nourish and protect the developing spermatozoa. They also secrete a protein that binds testosterone in the seminiferous tubules. Specialized interstitial cells that secrete the male sex hormone testosterone are located between the seminiferous tubules. Figure 19-3 is a microscopic view of the testis in cross-section, showing the seminiferous tubules, interstitial cells, and developing spermatozoa.

Testosterone: After its secretion, testosterone is absorbed directly into the bloodstream. This hormone has three functions:
* Development and maintenance of the reproductive structures;
* Development of spermatozoa;
* Development of secondary sex characteristics, traits that characterize males and females but are not directly concerned with reproduction. In males, these traits include a deeper voice, broader shoulders, narrower hips, a greater percentage of muscle tissue, and more body hair than are found in females.

The Spermatozoa: Spermatozoa are tiny individual cells illustrated in Figure 19-4. They are so small that at least 200 million are contained in the average ejaculation (release of semen). After puberty, sperm cells are manufactured continuously in the seminiferous tubules of the testes.

The spermatozoon has an oval head that is largely a nucleus containing chromosomes. The acrosome, which covers the head like a cap, contains enzymes that help the sperm cell to penetrate the ovum. Whiplike movements of the tail (flagellum) propel the sperm through the female reproductive system to the ovum. The cell’s middle region (midpiece) contains many mitochondria that provide energy for movement.

Figure 19-2 Structure of the testis. The epididymis and spermatic cord are also shown.

Figure 19-3 Microscopic view of the testis.

Figure 19-4 Diagram of a human spermatozoon.