Читать книгу Bovine Reproduction - Группа авторов - Страница 46

The initial proliferation occurs at the basal membrane compartment of the seminiferous tubules where the germ cells reside. Spermatogonia emerge from the basal germ cell layer, where spermatogonia progress through the differentiation cycle and undergo a series of mitotic divisions, the number of which may be species dependent, forming A1 to A4, I and B spermatogonia. Spermatogonia move forward from the basal lamina of the seminiferous tubules as a cohort connected to each other by intercellular bridges, thereby allowing interconnection of cytoplasm between cells. These cytoplasmic bridges may facilitate intercellular communication and thus support the synchronized development of the cohort of spermatogonia. The bull and ram undergo four mitotic divisions during spermatocytogenesis, yielding 16 primary spermatocytes from each active spermatogonium [18]. During this process, a pool of stem germ cells (dormant spermatogonia A1 and A2) is maintained that provides for new generations of spermatogonia (stem cell renewal). This allows the continual production of spermatozoa in the adult male, but the precise mechanism of this regeneration process or renewal of germinal stem cells is not completely understood [150]. During this process, there may be a level of cell loss resulting from cellular degeneration. These cells are resorbed by Sertoli cell cellular phagocytosis.