Читать книгу Bovine Reproduction - Группа авторов - Страница 57

The cycle of the seminiferous epithelium refers to is the histological fact that at a given section of the ST, different types of germ cells appear cyclically at species‐specific time intervals [16]. The notion of the cycle of the seminiferous epithelium was established toward the end of the nineteenth century [19]. Several generations of germ cells are present along the ST because spermatogenesis is a continuous process and subsequent spermatogenesis does not pause to wait for completion of the first generation to the end‐product spermatozoa and the release of these spermatozoa in the lumen of the ST [20]. A_{undifferentiated} spermatogonia differentiate to A_{differentiating} spermatogonia (A₁ and subsequent spermatogenesis) regularly after a species‐specific fixed time interval. This A_{undifferentiated} to A₁ transition does not wait for the preceding process of “A_{undifferentiated} to A₁ transition and finally to spermatozoon” to complete [21]. Along the section of the ST, the development of germ cells is influenced by neighboring cells, resulting in specific germ cell associations following one another in time at a given section of the ST in a perfectly organized pattern, termed the cycle of the seminiferous epithelium. This results from the fact that (i) at specific points of the ST new spermatogonia enter spermatogenesis at constant time intervals, and (ii) once the cells are engaged in spermatogenesis, their rate of differenation to the next cell type is always a constant and each step of spermatogenesis has a constant and fixed time duration. The specific germ cell associations at a given point of the ST during the cycle of the seminiferous epithelium are termed as stages, or less commonly as phases of the cycle. In the bull, the cycle is divided into eight stages and spermiation is taken as the reference point [22, 23]. In bulls, the duration of the seminiferous epithelium cycle is 13.5 days. The total duration of spermatogenesis is 4.5 times the duration of the cycle, that is 61 days. The durations of spermatocytogenesis, meiosis, and spermiogenesis are 21, 23, and 17 days, respectively [16].

The spermatogenic wave is “a sequence of segments showing the complete series of cell associations corresponding to the stages of the cycle of the seminiferous epithelium. One or more modulations must, however, be excluded from the sequence of the segments considered.” While the cycle of the seminiferous epithelium refers to the temporal arrangement of the germ cells at a given point of the ST, the spermatogenic wave refers to the spatial arrangement of germ cells along the ST [24]. However, the wave is not in space what the cycle is in time – the wave is not a dynamic process but is a static way to describe the spatial distribution of the associations along the tubule [16]. Spermatogenesis progresses along the ST in wave form, and the wave is formed by the fact that specific germ associations, i.e. stages, start again and again at specific distance. The regulated order of the wave emergence along the ST follows the numerical order of the stages of the cycle of the seminiferous epithelium [25, 26]. Different waves start as we move along the ST, and at a specific section of the ST, cells from different waves are seen together in a cross‐section of the ST according to the stage of the cycle of the seminiferous epithelium. The waves move along the ST in an inward spiral fashion, with A_{1 undifferentiated} at the outer border of the wave and spermatozoa at the inner border of the wave. One spermatogenic wave starts with initiation of spermatogenesis, i.e. from conversion of type A_{undifferentiated} to type A₁ differentiating spermatogonia under the repeated actions of retinoic acid, and ends with formation of spermatozoa [27]. Histologically, stages of the cycle are identified by two methods: (i) by meiotic status of spermatocytes and changes in the shape and position of nuclei of spermatids; or (ii) by differential morphology of the acrosome of spermatids [24].