Studies on regulation of testicular function by gonadotropins: Regulation function of spermatogenesis and leyding cell function in the rat by follicle stimulating hormone

Abstract

The role of FSH in maintaining adult rat testicular function, as indicated by the present investigation, can be summarized as follows:

Maintenance of testicular weight requires the presence of FSH, but largely depends on T. Testicular DNA, RNA, and protein synthesis are predominantly under the regulation of T; however, FSH also influences RNA and protein synthesis of the germ cells. Proliferation of spermatogonia and the onset of meiotic prophase seem to be independent of FSH, LH, or T. Completion of meiotic division requires both FSH and LH. Absence of either hormone results in a similar degree of inhibition in the 1C:4C ratio, and this cannot be prevented by exogenous T supplementation. Post meiotic differentiation of round spermatids beyond step 12 of spermiogenesis is dependent on the presence of FSH. The process of chromatin condensation via basic nuclear protein transitions may be under the regulation of FSH. FSH has a role in maintaining testicular LDH X activity. Hyaluronidase shows only marginal dependence on FSH. SGP 2, a Sertoli cell protein, is not under stringent regulation by gonadotropins or T. FSH dependent paracrine regulators may maintain Leydig cell responsiveness to LH/hCG. Part of this regulatory system seems to affect the steps distal to cAMP production and prior to pregnenolone production, such as cholesterol transport to mitochondria and its conversion to pregnenolone.

Although FSH deprivation resulted in a significant reduction in hCG stimulated T production in vitro by purified Leydig cells, the responsiveness to in vivo hCG injection in terms of serum T levels was similar to control rats. It is speculated that FSH may modulate Leydig cell steroidogenesis through both stimulatory and inhibitory factors, which probably contribute to the widely observed heterogeneity in the steroidogenic potential of the perivascular and peritubular Leydig cells associated with different stages of the seminiferous epithelial cycle. The present study has demonstrated the effect of FSH deprivation on specific germ cell types, strengthening the physiological relevance of earlier reported stage specific binding and response of adult rat seminiferous tubules to FSH in vitro. Modulation of steroidogenesis by FSH dependent paracrine factors indicates that FSH may indirectly regulate spermatogenesis by affecting localized T production along the seminiferous tubules. In conclusion, the present investigation has shown that the dependence of normal testicular function on FSH is not restricted only to the immature state in the rat but is demonstrable even in the adult rat.