Spermatogenesis is the development of spermatogonia into spermatozoa and this process takes place in the seminiferous tubule of the adult testis. Three central processes encompasses spermatogenesis: mitotic replication of spermatogonia, meiotic proliferation of spermatocytes, in which genetic material is recombined and segregated, and post-meiotic differentiation into spermatozoa, also termed spermiogenesis (Russell et al., 1990a), (Soares et al., 2009) (Fig 1.1).
The functional morphology of the seminiferous tubule depicts the different phases of the spermatogenesis (Courot et al., 1970). Myoid cells and fibroblasts encompass the external part of the basement membrane. The blood supply of the testis is confined to the interstitial tissue by the basement membrane, and therefore does not enter the seminiferous tubule itself (Pelletier and Byers, 1992). Groups of Leydig cells are situated between the tubules as a part of the interstitial tissue. They are the central component of the endocrine portion of the testis and synthetises testosterone, which is the androgen hormone that drives the spermatogenesis (Hooker, 1970),(Hall, 2006).
Somatic Sertoli cells are positioned on the internal side of the basal membrane of the seminiferous tubule. They are cylindrical in shape with an elusive outline of the cell membrane and contain an irregularly shaped nuclei located close to the basement membrane, which contains a small amount of chromatin and a distinct nucleolus (Foote et al., 1972), (Russell et al., 1990a). The Sertoli cells provide nutritional and physical support for the germinal cells and therefore extent all through the layers of the seminiferous epithelium, in order to be in close connection to all the different phases of germinal cells in the spermatogenesis (Griswold, 1998). Tight cell junctions, adherent junctions and gap junctions between the Sertoli cells form the blood-testis barrier and separate the seminiferous tubules into a basal and a luminal compartment (Pelletier and Byers, 1992, p. -). This sustains a specialized environment in the luminal compartment and dissociates the germ cells of the meiotic and spermiogenic phases from the immune system (Hochereau-de Reviers et al., 1990), (Pelletier and Byers, 1992). The junctions between the Sertoli cells form around puberty and are androgen-dependent, like the adherence between Sertoli cells and germinal cells (Holdcraft and Braun, 2004a).
Spermatogenesis can be classified based on changes in the shape of the spermatid nucleus, the location of spermatids and spermatozoa in regard to the basement membrane, presence of meiotic figures and the release of spermatozoa from the lumen of the tubule (Foote et al., 1972). These stages can also be identified based on the development of the acrosome system and the morphology of developing spermatids (Russell et al., 1990b), (Soares et al., 2009) as well as tubular morphology, in which the shape and location of the spermatid nucleus are the main aspects of consideration (Courot et al., 1970).
The epithelium of the seminiferous tubule appears as concentric layers of Sertoli cells, spermatogonia, spermatocytes, and spermatids (Parkinson, 2009) (Fig. 1.2)
Numerous generations of spermatogonia, the immature germinal stem cell, are situated along the basement membrane. These undergo several rounds of mitosis to form a reservoir of stem cells, of which some will later undergo meiosis, differentiating spermatogonia, and form spermatozoa (Russell et al., 1990a).
The A-spermatogonia is the least differentiated of the germ cells, and after several rounds of mitosis it gives rise to intermediate spermatogonia, that eventually form B-spermatogonia. The B-spermatogonia move closer to the lumen of the seminiferous tubule and therefore has significantly reduced connection to the basement membrane (Kerr et al., 2006).
After the last round of mitosis the B-spermatogonia give rise to preleptotene primary spermatocytes, which initiates the prophase of the first meiotic cell division, by developing into leptotene primary spermatocytes. They enter the zygotene phase with pairing of homologous chromosomes, and then later the pachytene phase with genetic recombination of the paired chromosomes. Finally the primary spermatocytes undergo diakinesis, the separation phase of meiosis, and give rise to short-lived secondary spermatocytes, which go through another round of meiotic cell division to produce round spermatids (Russell et al., 1990a), (Parkinson, 2009).
Next step in the development of spermatozoa is the maturation of spermatids, termed spermiogenesis. This process consists of the synthesis of the acrosome and the formation of the tail from the centrioles. One centriole forms the piece connecting the neck and the head of the sperm, while the other one forms the axial filament of the tail (Kerr et al., 2006).
Late in the spermiogenesis a helix of mitochondria densify around the proximal part of the flagellum, forming the mid-piece of the spermatozoa.
Just before spermiation, the release of spermatozoon into the lumen of the seminiferous tubule, the Sertoli cell phagocytizes the remaining of the cytoplasm of the spermatid (Fouquet, 1974).
1.2 Endocrine Regulation of Spermatogenesis
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