The germ cells that migrate from the yolk sac to the testes during early embryonic development become spermatogenic stem cells, called spermatogonia, within the outer region of the seminiferous tubules. Spermatogonia are diploid cells (with forty-six chromosomes) that ultimately give rise to mature
First meiotic division
Second meiotic division
■ Figure 20.16 Spermatogenesis. Spermatogonia undergo mitotic division in which they replace themselves and produce a daughter cell that will undergo meiotic division. This cell is called a primary spermatocyte. Upon completion of the first meiotic division, the daughter cells are called secondary spermatocytes. Each of these completes a second meiotic division to form spermatids. Notice that the four spermatids produced by the meiosis of a primary spermatocyte are interconnected. Each spermatid forms a mature spermatozoon.
haploid gametes by a process of reductive cell division called meiosis. The steps of meiosis are summarized in chapter 3, figure 3.33.
Meiosis involves two nuclear divisions (see fig. 3.33). In the first part of this process, the DNA duplicates and homologous chromosomes are separated into two daughter cells.
Since each daughter cell contains only one of each homologous pair of chromosomes, the cells formed at the end of this first meiotic division contain twenty-three chromosomes each and are haploid. Each of the twenty-three chromosomes at this stage, however, consists of two strands (called chromatids) of identical DNA. During the second meiotic division, these duplicate chromatids are separated into daughter cells. Meiosis of one diploid spermatogonium cell therefore produces four haploid cells.
Actually, only about 1,000 to 2,000 stem cells migrate from the yolk sac into the embryonic testes. In order to produce many millions of sperm throughout adult life, these sper-matogonia duplicate themselves by mitotic division and only one of the two cells—now called a primary spermatocyte— undergoes meiotic division (fig. 20.16). In this way, spermato-genesis can occur continuously without exhausting the number of spermatogonia.
When a diploid primary spermatocyte completes the first meiotic division (at telophase I), the two haploid cells thus produced are called secondary spermatocytes. At the end of the second meiotic division, each of the two secondary spermato-cytes produces two haploid spermatids. One primary spermato-cyte therefore produces four spermatids.
The sequence of events in spermatogenesis is reflected in the cellular arrangement of the wall of the seminiferous tubule. The spermatogonia and primary spermatocytes are located toward the outer side of the tubule, whereas spermatids and mature spermatozoa are located on the side of the tubule facing the lumen.
At the end of the second meiotic division, the four sper-matids produced by meiosis of one primary spermatocyte are interconnected—their cytoplasm does not completely pinch off at the end of each division. Development of these interconnected spermatids into separate mature spermatozoa (singular, spermatozoon)—a process called spermiogenesis—requires the participation of the Sertoli cells (fig. 20.17).
The nongerminal Sertoli cells form a continuous layer connected by tight junctions around the circumference of each tubule. In this way, they constitute a blood-testis barrier; molecules from the blood must pass through the cytoplasm of the Sertoli cells before entering germinal cells. Similarly, this barrier normally prevents the immune system from becoming sensitized to antigens in the developing sperm, and thus prevents autoimmune destruction of the sperm. The cytoplasm of the Ser-toli cells extends from the periphery to the lumen of the tubule and envelops the developing germ cells, so that it is often difficult to tell where the cytoplasm of the Sertoli cells ends and that of germ cells begins.
The Sertoli cells help to make the seminiferous tubules an immunologically privileged site (protected from immune attack) through another mechanism as well. As described in chapter 15, the Sertoli cells produce FAS ligand, which binds to the FAS receptor on the surface of T lymphocytes. This triggers apoptosis (cell suicide) of the T lymphocytes and thus helps to prevent immune attack of the developing sperm.
Spermatozoa (23 chromosomes)
Interstitial tissue with Leydig cells
Spermatids (23 chromosomes)
Secondary spermatocytes (23 chromosomes)
Primary spermatocytes (46 chromosomes)
Lumen of — seminiferous tubule
■ Figure 20.17 A photomicrograph and diagram of the seminiferous tubules. (a) A cross section of the seminiferous tubules also shows surrounding interstitial tissue. (b) The stages of spermatogenesis are indicated within the germinal epithelium of a seminiferous tubule. The relationship between Sertoli cells and developing spermatozoa can also be seen.
In the process of spermiogenesis (conversion of spermatids to spermatozoa), most of the spermatid cytoplasm is eliminated. This occurs through phagocytosis by Sertoli cells of the "residual bodies" of cytoplasm from the spermatids (fig. 20.18). Phagocytosis of residual bodies may transmit informational molecules from germ cells to Sertoli cells. The Ser-toli cells, in turn, may provide molecules needed by the germ cells. It is known, for example, that the X chromosome of germ cells is inactive during meiosis. Since this chromosome contains genes needed to produce many essential molecules, it is believed that these molecules are provided by the Sertoli cells during this time.
Sertoli cells secrete a protein called androgen-binding protein (ABP) into the lumen of the seminiferous tubules. This protein, as its name implies, binds to testosterone and thereby concentrates it within the tubules. The importance of Sertoli cells in tubular function is further evidenced by the fact that FSH receptors are confined to the Sertoli cells. Any effect of FSH on the tubules, therefore, must be mediated through the action of Sertoli cells. These include the FSH-induced stimulation of spermiogenesis and the autocrine interactions between Sertoli cells and Leydig cells previously described.
The formation of primary spermatocytes and entry into early prophase I begins during embryonic development, but spermatoge-nesis is arrested at this point until puberty, when testosterone secretion rises. Testosterone is required for completion of meiotic division and for the early stages of spermatid maturation. This effect is probably not produced by testosterone directly, but rather by some of the molecules derived from testosterone (the 5a-reduced androgens and estrogens, described earlier) in the tubules. The testes also produce a wide variety of paracrine regulators— transforming growth factor, insulin-like growth factor-1, inhibin, and others—that may help to regulate spermatogenesis.
The later stages of spermatid maturation during puberty appear to require stimulation by FSH (fig. 20.19). This FSH effect is mediated by the Sertoli cells, as previously described. During puberty, therefore, both FSH and androgens are needed for the initiation of spermatogenesis.
FSH and testosterone (or its derivatives) stimulate sperm development indirectly, by acting on the Sertoli cells. It is currently believed that these hormones stimulate the Sertoli cells to secrete polypeptides, which in turn act as paracrine regulators to stimulate spermatogenesis.
■ Figure 20.18 The processing of spermatids into spermatozoa (spermiogenesis). As the spermatids develop into spermatozoa, most of their cytoplasm is pinched off as residual bodies and ingested by the surrounding Sertoli cell cytoplasm.
Meiosis (first division)
Required — at puberty
Required " at puberty
■ Figure 20.19 The endocrine control of spermatogenesis. During puberty, both testosterone and FSH are required to initiate spermatogenesis. In the adult, however, testosterone alone can maintain spermatogenesis.
At the conclusion of spermiogenesis, spermatozoa are released into the lumen of the seminiferous tubules. The spermatozoa consist of an oval-shaped head (which contains the DNA), midpiece, and a tail (fig. 20.20). Although the tail will ultimately be capable of flagellar movement, the sperm at this stage are nonmotile. They become motile and undergo other matura-tional changes outside the testis in the epididymis.
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