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// / » 7-. .r • • > > \i\i figure 12-9 Cellular organization of cells of the seminiferous tubules of a human testis. Leydig cells occur in clusters surrounded by several seminiferous tubules as well as capillaries. Reproduced with permission from Christensen, A. K. (1975). Leydig cells. In "Handbook of Physiology" (D. W. Hamilton and R. O. Greep, eds.), Section 7, Vol. V, pp. 57-94. Williams & Wilkins, Baltimore, MD.

important functions: (1 ) production of unique proteins, including androgen-binding protein (ABP; see later); (2) nourishment of developing spermatozoa; (3) phagocytosis of damaged spermatozoa; (4) production of a bicarbonate- and potassium-rich fluid used for transport of the mature sperm; and (5) production of estra diol from testosterone.

FSH actions on the seminiferous tubules are initiated by binding to specific receptors on the external plasma membrane of the Sertoli cells. The FSH receptor is a member of the G-protein-coupled seven-transmembrane protein receptor superfamily (see

Figure 1-23). Occupancy of the FSH receptor leads to an increase in cAMP, which in turn stimulates the phosphorylation of a number of unknown cellular proteins, as well as to an increase in intracellular Ca2+. Collectively these responses mediate FSH stimulation of mRNA and protein synthesis by the Sertoli cells.

Some of the proteins whose production is stimulated by FSH include the androgen-binding protein (ABP; see later discussion), transferrin, P450, GnRH-like peptides, ceruloplasmin, Mullerian duct-inhibiting substance (MIS; see Chapter 14), and inhibin (see Table 12-4).

A major action of FSH on the Sertoli cells is to stimulate the production and secretion of inhibin. Inhibin, along with testosterone and estradiol, is a negative feedback regulator of the pituitary secretion of FSH (see Figure 12-8). The chief evidence supporting the existence of inhibin is that, following orchiectomy (removal of the testes), FSH secretion rises. Further elevation in FSH cannot be blocked by only the administration of testosterone or estradiol. A further discussion of the actions of inhibin is given in Chapter 13.

FSH also stimulates the production of androgen-binding protein (ABP) by the Sertoli cell. The production of ABP in the Sertoli cell is stimulated by testosterone as well. In this regard, ABP is unique in that its biosynthesis is stimulated by both a peptide hormone and a steroid hormone. ABP is a protein of molecular mass 90 kDa, which binds T and DHT with high affinity (Kd = 10 9 M). The ABP is secreted by the Sertoli cells into the tubular lumen (see Figure 12-3) so as to ensure that the androgen concentration of the intraluminal fluid is maintained at a high level. The detailed role of ABP is not known, although it may function to deliver testosterone to the seminiferous tubule and epididymis. ABP normally does not circulate in the blood.

In addition, FSH stimulates mitosis in immature Sertoli cells. The complement of Sertoli cells in the adult human testes is known to be largely dependent upon FSH-mediated amplification in the late phases of fetal development.

D. Spermatogenesis

The process of gametogenesis in the male is termed spermatogenesis. In contrast to the comparable process in the female (oogenesis), which occurs exclusively in the embryonic phase, the process of spermatogenesis occurs in the male from puberty throughout the bulk of adult life. Some of the other fundamental differences between the processes of male spermatogenesis and female oogenesis are summarized in Table 12-5.

The overall process of the production of mature spermatozoa by the process of gametogenesis is depen dent on both a specialized cellular anatomical relationship between the developing germ cells and the surrounding cells and the presence of the gonadotropins, FSH and LH. Figure 12-9 illustrates the complexity of the local cellular organization in a section of the testis. At least five cell types are involved in the overall process of spermatogenesis: (1) Sertoli cells, (2) Leydig cells, (3) developing germ cells, (4) myoepithelial cells, and (5) epithelial cells of the duct system.

The Sertoli cell is unusual in that it possesses receptors for both the steroid hormone, testosterone, as well as the peptide hormone, FSH. While both hormones are critical for the process of spermatogenesis, FSH is only mandatory for the maturation and testosterone-sensitizing process of the Sertoli cell that occurs during puberty. After puberty, if FSH is removed by hypophy-sectomy, spermatogenesis can be sustained in the rat by the immediate administration of large doses of testosterone. In man, however, there is a continuing requirement for FSH along with testosterone or LH to attain spermatogenesis.

The entire process of spermatogenesis takes place while the developing germ cell is completely embedded in the seminiferous tubule wall. The three main phases of spermatogenesis, which requires 70 days in man, are as follows: (i) proliferation and differentiation of the germ cell, known as a spermatogonium; (ii) mei-otic maturation to yield primary spermatocytes, then secondary spermatocytes, and finally spermatids; and (iii) transformation of spermatids into mature spermatozoa. These stages are illustrated in Figure 12-10A,B.

When embryonic gonocytes become committed to the future production of sperm cells, they are known as spermatogonia; they remain in this state until puberty. After puberty, selected spermatogonia are converted into a primary spermatocyte, which in turn can yield, after meiosis, two secondary spermatocytes. Secondary spermatocytes then divide again to generate two hap-loid spermatids. The cellular structures of a spermatocyte and a spermatid are shown in Figure 12-11.

As emphasized in Figure 12-9, there is exquisite local cellular architecture describing the relationship of the cells of the seminiferous tubules. Here the seminiferous epithelium is composed of a very tight packing of the spermatogonia in intimate contact and supported by adjacent tall columnar Sertoli cells. The Sertoli cells, which rest upon the basement membrane (outer side of tubule), extend apically toward the lumen of the tubule. Thus, as the spermatogonia divide and mature to spermatids, they migrate from a peripheral position adjacent to the Sertoli cell(s) outer membrane to a position inward on the Sertoli cells closer to the lumen of the seminiferous tubules. In the very final stages of maturation, the mature spermatozoon

Hormones, Second Edition TABLE 12-5 Comparison of Male Spermatogenesis and Female Oogenesis

Topic

Period of process

Number of functional germ cells produced in a lifetime

Time required for production of a mature germ cell

Type of cell division

Structural organization of the mature germ cell

Male spermatogenesis

After puberty, throughout adulthood Many trillions over a lifetime or about 30 x 106/ day

Approximately 60-65 days for sperm production followed by 10-14 days for epididymal transport "Even" division of cytoplasm

Has a specialized anatomical structure (see Figure 12-4)

Female oogenesis

Embryonic life

7,000,000 (20th week of fetus), 1-2 million at birth, ~40,000 ova available at puberty

12-50 years for a mature ovum; ~4000 ova used from puberty to menopause.

"Reductive" division yields production of one potential ovum and three polar bodies

Relatively uncomplicated anatomical structure (see Figure 13-4C)

Diagram Rat Spermatozoa Morphology

FIGURE 12-10 Summary of the spermatogenic cycle occurring over 70 days in the seminiferous tubules of man. (A) Sequence of steps in the transformation of a spermatogonium into a spermatozoon. (B) Representation of the six stages of the spermatogenic cycle, with their complement of germ cells arranged to show how their sequence of development in collaboration with the Sertoli cells corresponds to the geometry of spiral pathways leading conically toward the lumen of the seminiferous tubule. Definition of the symbols for both panels A and B: Ad, Ap, and B, spermatogonia! cells, dark type A, pale type A, and type B; primary spermatocytes, PL, preleptotene, L, leptotene, Z, zygotene, P, pachytene; M, meiotic division; II, secondary spermatocyte; Sa, Sb, Sc, and Sd, spermatids; RB, excess spermatid cytoplasm or residual body. A was modified with permission from DeKretser, D. M., Risbridger, G. P. and Kerr, J. B. (1995). Basic endocrinology of the testis. In "Endocrinology" (L. J. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaus, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd ed., Vol. 3, pp. 2307-2335. W. B. Saunders Co. Philadelphia, PA. B was modified from Kerr, J. B. (1992). Functional cytology of the human testis. In "Bailliere's Clinical Endocrinology and Metabolism" (D. M. de Kretser, ed.), Vol. 6, pp. 235-250. Bailliere Tindall, Philadelphia.

FIGURE 12-10 Summary of the spermatogenic cycle occurring over 70 days in the seminiferous tubules of man. (A) Sequence of steps in the transformation of a spermatogonium into a spermatozoon. (B) Representation of the six stages of the spermatogenic cycle, with their complement of germ cells arranged to show how their sequence of development in collaboration with the Sertoli cells corresponds to the geometry of spiral pathways leading conically toward the lumen of the seminiferous tubule. Definition of the symbols for both panels A and B: Ad, Ap, and B, spermatogonia! cells, dark type A, pale type A, and type B; primary spermatocytes, PL, preleptotene, L, leptotene, Z, zygotene, P, pachytene; M, meiotic division; II, secondary spermatocyte; Sa, Sb, Sc, and Sd, spermatids; RB, excess spermatid cytoplasm or residual body. A was modified with permission from DeKretser, D. M., Risbridger, G. P. and Kerr, J. B. (1995). Basic endocrinology of the testis. In "Endocrinology" (L. J. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaus, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd ed., Vol. 3, pp. 2307-2335. W. B. Saunders Co. Philadelphia, PA. B was modified from Kerr, J. B. (1992). Functional cytology of the human testis. In "Bailliere's Clinical Endocrinology and Metabolism" (D. M. de Kretser, ed.), Vol. 6, pp. 235-250. Bailliere Tindall, Philadelphia.

Testis Structure DiagramDiagram Spermatozoa

figure 12-11 (A) Schematic diagram of a spermatocyte. The cytoplasmic structure of a spermatocyte is unspecialized. The process of division of spermatozoa (giving rise to spermatocytes) and spermatocytes is incomplete, so that the daughter calls remain connected to intercellular bridges (IB). Abbreviations: M, mitochondria; SxV, sex vesicle; Ce, centriole. Meiotic division of a spermatocyte yields two secondary spermatocytes, which then divide again to generate two haploid spermatids. (B) Schematic diagram of a spermatid. Spermatogenesis is the process of differentiation of the spermatid into the mature sperm. As the spermatid develops, proacrosomal granules (AcG) spread to cover the tip of the mature spermatozoon (see Figure 12-4). The cell centrioles migrate from the Golgi zone distally to give rise to the flagellum. Abbreviations: Mt, mitochondria; AcV, acrosomal vesicle; StC, stritiated columns; CB, chromatoid body; RC, ring centriole; PcB, paracentriolar body. Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure," pp. 243-245. W. B. Saunders Company, Philadelphia, PA.

separates from the Sertoli cell outer membrane and is released into the lumen of the seminiferous tubule. There is significant evidence to support the two-way paracrine effects of Sertoli cells on developing spermatids and vice versa.

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