Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

1294 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and FertilizationSertoli cellspermatogoniumbasal laminaMITOSISspermatogoniumprimary spermatocyte200 mmlumenMEIOSIS IsecondaryspermatocyteMEIOSIS IIspermatiddifferentiatingspermatidbasal laminasurroundingseminiferoustubuleLeydig cellsSertoli cellspermatozoonin lumen(A)(B)meiosis I to produce two secondary spermatocytes, each containing 22 duplicatedautosomal chromosomes and either a duplicated X or a duplicated Y chromosome.The two secondary spermatocytes derived from each primary spermatocyteproceed through meiosis II to produce four spermatids, each with ahaploid number of single chromosomes. The haploid spermatids then undergodramatic morphological changes as they differentiate into sperm, which escapeinto the lumen of the seminiferous tubule. The sperm subsequently pass intothe epididymis, a coiled tube overlying the testis, where they are stored andundergo further maturation. The stored sperm are still not ready to fertilize anegg, however; as we discuss later, they undergo further maturation in the femalegenital tract—a process called capacitation.Sperm Develop as a SyncytiumAn intriguing feature of spermatogenesis is that once a spermatogonium beginsto mature, its progeny no longer complete cytoplasmic division (cytokinesis)during mitosis and subsequent meiosis. Consequently, large clones of differentiatingdaughter cells that have descended from one maturing spermatogoniumremain connected by cytoplasmic bridges, forming a syncytium (Figure 21–31).The cytoplasmic bridges persist until the very end of sperm differentiation,when individual sperm are released into the lumen of the seminiferous tubule.As a result, mature sperm are produced in synchronous batches in any givenarea of a seminiferous tubule. What is the function of this syncytial arrangement?We saw earlier that oocytes grow and differentiate while containing thediploid set of chromosomes in duplicate. Sperm, by contrast, do not grow, andthey undergo most of their differentiation after their nuclei have completedmeiosis to become haploid. The presence of cytoplasmic bridges betweenthem, however, means that each developing haploid sperm shares a commoncytoplasm with its neighbors. In this way, it can be supplied with all the geneFigure 21–29 Highly simplified drawingsof a cross section of a seminiferoustubule in a mammalian testis. (A) All ofthe stages of spermatogenesis showntake place while the developing germlinecells are in intimate association withSertoli cells. Sertoli cells direct sexualdifferentiation along a male pathway.They are large cells, extending from thebasal lamina to the lumen of theseminiferous tubule; they are required forthe survival of the spermatogonia andare analogous to follicle cells in the ovary(see Figure 21–19). Spermatogenesis alsodepends on testosterone secreted byLeydig cells, located between theseminiferous tubules. (B) Spermatogoniadivide by mitosis at the periphery of theseminiferous tubule. Some of these cellsenter meiosis I to become primaryspermatocytes; they then completemeiosis I to become secondaryspermatocytes. The secondaryspermatocytes then complete meiosis IIto become spermatids, whichdifferentiate into spermatozoa (sperm)and are released into the lumen of thetubule (see Figure 21–30). In man, it takesa spermatogonium about 24 days fromthe onset of meiosis to emergence as aspermatid and another 5 weeks for thespermatid to develop into a sperm.