Читать книгу Bovine Reproduction - Группа авторов - Страница 45

The first studies on the process of spermatogenesis and the spermatogonia of the domestic bull were reported in the latter part of the nineteenth century [147], but it was not until 1931 that the chromosome number of spermatogonia and spermatocytes was determined and that the diploid number was 60 [148]. Subsequent studies in the era after the Second World War described the germ cells and their cytoplasmic structures in more detail [147]. It is now well established that spermatogenesis is a complex cellular process whereby spermatozoa, the male haploid germ cell or gamete, are formed from the diploid spermatogonia stem cells through a series of cellular transformations. These complex transformations occur in the seminiferous tubules of the mammalian testes and may proceed over an extended period of time, which is species dependent. Spermatogenesis has been described morphologically in distinct and recognizable cellular “stages” or “phases” that progress through highly organized and precisely timed cycles [35]. During fetal development primordial germ cells migrate to the embryonic testes where they undergo mitotic division to form gonocytes. Just prior to puberty, gonocytes differentiate into the primary pool of A0 spermatogonia, the stem cells from which all subsequent classes of spermatogonia arise. Spermatogenesis proceeds through three distinct stages within the seminiferous tubules of the testis. The first stage is spermatocytogenesis, a proliferative phase where spermatogonia undergo a series of mitotic divisions to form primary spermatocytes. The second stage is meiosis where the primary spermatocytes undergo reduction division of the chromosomal number, from primary spermatocytes (4n) to secondary spermatocytes which are diploid (2n) to the final division that produces round haploid (1n) spermatids. The final stage is differentiation of the haploid spermatids in a process referred to as spermiogenesis, where the rounded spermatids undergo a series of metamorphic changes to form the elongated and flagellated spermatozoa [35, 149].