Anatomy and Physiology

The Testes

The testes, which produce sperm and also the male sex hormones, lie outside the abdominal cavity of the male within the scrotum. The testes begin their development inside the abdominal cavity but descend into the scrotal sacs during the last two months of fetal development. If, by chance, the testes do not descend and the male is not treated or operated on to place the testes in the scrotum, sterility—the inability to produce offspring—usually follows. This is because the internal temperature of the body is too high to produce viable sperm. A subcutaneous muscle along with an adjoining muscle raise the scrotum during sexual excitement and when a higher temperature is need to warm the testes.

Anatomy of a Testis

A sagittal section of a testis shows that it is enclosed by a tough, fibrous capsule. The connective tissue of the capsule extends into the testis, forming septa that divide the testis into compartments called lobules. Each lobule contains one to three tightly coiled seminiferous tubules (Fig. 17.3a). Altogether, these tubules have a combined length of approximately 250 m. A microscopic cross section of a seminiferous tubule reveals that it is packed with cells undergoing spermatogenesis (Fig. 17.3b), the production of sperm. Delicate connective tissue surrounds the seminiferous tubules. Cells that secrete the male sex hormones, the androgens, are located here between the seminiferous tubules.

Therefore, these endocrine cells are called interstitial cells. The most important of the androgens is testosterone, whose functions are discussed later in this section.

Testicular cancer, or cancer of the testes, is one type of cancer that can be detected by self-examination, as explained in the Medical Focus on page 357.

Spermatogenesis

Spermatogenesis, the production of sperm, includes the process of meiosis as the sperm form. Before puberty, the testes, including the seminiferous tubules, are small and nonfunctioning.

At the time of puberty, the interstitial cells become larger and start producing androgens. Then, the seminiferous tubules also enlarge, and they start producing sperm.

The seminiferous tubules contain two types of cells: germ cells, which are involved in spermatogenesis, and sustentacular (Sertoli) cells. Sustentacular cells are large; they extend from the capsule to the lumen of the seminiferous tubule. The sustentacular cells support, nourish, and regulate the development of cells undergoing spermatogenesis.

The germ cells near the capsule are called spermatogonia. The spermatogonia divide, producing more cells by mitosis. Some of these cells remain as spermatogonia, and some are

primary spermatocytes (Fig. 17.4). The spermatocytes start the process of meiosis, which requires two divisions. Following meiosis I, cells called secondary spermatocytes have the reduced, or n, number of chromosomes (i.e., 23 chromosomes). Following meiosis II, there are four spermatids. Spermatids then differentiate into sperm.

Mature sperm, or spermatozoa, have three distinct parts: a head, a middle piece, and a tail (see Fig. 17.3c). Mitochondria in the middle piece provide energy for the movement of the tail, which has the structure of a flagellum. The head contains a nucleus covered by a cap called the acrosome, which stores enzymes needed to penetrate the egg. Notice in Figure 17.3b, that the sperm are situated so that their tails project into the lumen of the seminiferous tubules.

When formed, the sperm are transported to the epididymis because the seminiferous tubules unite to form a complex network of channels that join, forming ducts. When the ducts join, an epididymis is formed.

The ejaculated semen of a normal human male contains several hundred million sperm, but only one sperm normally enters an egg. Sperm usually do not live more than 48 hours in the female genital tract.