D

Mitotic

1st meiotic division

2nd meiotic division

Figure 1.23 The products of meiosis during spermatogenesis in humans.

prophase (22 days) followed by rapid completion of meiosis I and formation of secondary spermatocytes. During the second meiotic division, these cells immediately begin to form haploid spermatids (Figs. 1.21-1.23). Throughout this series of events, from the time type A cells leave the stem cell population to formation of spermatids, cytokinesis is incomplete, so that successive cell generations are joined by cytoplasmic bridges. Thus, the progeny of a single type A spermatogonium form a clone of germ cells that maintain contact throughout differentiation (Fig. 1.22). Furthermore, spermatogonia and spermatids remain embedded in deep recesses of Sertoli cells throughout their development (Fig. 1.24). In this manner, Sertoli cells support and protect the germ cells, participate in their nutrition, and assist in the release of mature spermatozoa.

Spermatogenesis is regulated by luteinizing hormone (LH) production by the pituitary. LH binds to receptors on Leydig cells and stimulates testosterone production, which in turn binds to Sertoli cells to promote spermatogenesis. Follicle stimulating hormone (FSH) is also essential because its binding to Sertoli cells stimulates testicular fluid production and synthesis of intracellular androgen receptor proteins.

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