Читать книгу Bovine Reproduction - Группа авторов - Страница 58

Spermatogenesis in the bull establishes progressively from 16 weeks to 32 weeks of age. Gonocytes are the main germ cells in the ST until one week of age and are sequentially replaced by undifferentiated spermatogonia by 20 weeks. Meiosis starts at about 16 weeks and is completed by 28–32 weeks. Complete spermatogenesis can be observed in cross‐sections of the ST by 32 weeks of age [23].

Histologically, spermatogenesis is evaluated qualitatively by the appearance of the ST and quantitatively by differential cell count in the ST. Daily sperm production (DSP, millions) per gram of decapsulated testes is an appropriate measure of the efficiency of spermatogenesis and is used for species comparison [28]. DSP per gram of decapsulated testes is 4–6 for the human man, 12 for the bull, 16–19 for the stallion, 21 for the ram, 23 for the rhesus monkey and for the boar, 20–24 for the rat, 24 for the hamster, and 25 for the rabbit [28–30]. DSP is affected by longer duration of spermatogenesis, longer cycle of the seminiferous epithelium, the density of germ cells in the ST, and germ cell degeneration during spermatogenesis [30]. The reason for lower efficiency of spermatogenesis in the bull is not understood [16]. In the bull, germ cell degeneration during transition from A₁ to A₄ and at intermediate spermatogonia stage accounts for 30% losses in DSP. Moreover, degeneration at B₁ and during transition from B₁ to B₂ causes another 30% loss. However, there is no loss during meiosis and spermiogenesis in the bull. In the absence of this degeneration, the bull would have a DSP of around 30 × 10⁶/g of testes [16, 25].

When a bull can produce an ejaculate with 50 million spermatozoa with at least 10% motility, he is pubertal [31]. Age at puberty varies from 38 to 48 weeks of age, with an average age of 42 weeks [32]. After maturity, a bull can give two useable ejaculates daily; commercially, bull ejaculates are collected three times a week for a balance between management and number of artificial insemination doses. Both the quality and quantity of spermatogenesis decrease in old age; however, the aging‐related extinction of spermatogenesis in the bull has not been reported. Aging‐related decrease in sperm production has not been studied in the bull as breeding bulls are not maintained for senescence. One of the reasons for age‐related decline in spermatogenesis is decreased Sertoli cell number in old age [16].