| Popis: |
In the ram, the size of the testes is related to the concentrations of FSH, LH and testosterone in the blood. A significant linear relationship is observed between testicular size and the level of FSH until a maximum after which no further increase in testis size is observed. Testicular size is also linearly related to the mean level (25 consecutive hours of sampling) of LH and testosterone in the blood, with apparently no upper limit, and to the frequency of peaks of these hormones in the sampling period. Spermatogenesis in the ram is sensitive to variations in the levels of circulating hormones; there is a positive correlation between the number of renewing spermatogonia or the efficiency of spermatogonial multiplication and the mean LH value in the peripheral blood of the adult. Some of these relationships operate over long periods and involve the Sertoli cells. The level of circulating LH in the non-pubertal lamb is directly correlated with number of Sertoli cells per testis, and the latter is correlated with the number of renewing spermatogonia per testis in the adult ram. Treatment of hypophysectomized rams with PMSG, hCG or testosterone shows that spermatogonial divisions are sensitive to the hormonal milieu with specific stages being controlled by the LH-like activity of hCG (A1 spermatogonia), and the FSH-like activity of PMSG (transition from intermediate spermatogonia to leptotene spermatocytes). Testosterone has only a small effect at the beginning of the spermatogenic cycle (production of leptotene spermatocytes) and quantitatively maintains meiosis and spermiogenesis, but the differentiation of spermatids is dependent on information stored at the beginning of meiosis and requires the support of both testosterone and other factors. |
