Studies on the development of hypothalamic regulation of the hypophysial gonadotropic activity in rats

Development of the hypothalamic regulation of gonadotrophs has been studied in rats. Pituitaries of rat fetuses were cut serially at 4 micrometer in the coronal plane, and every 5th section was mounted and stained immunohistochemically with anti-HCG serum. Sum total of the numbers of the immunoreactive cells in each section was used as an index of the number of gonadotrophs of the pituitary. Anti-rLH, anti-rLH beta, anti-rFSH, anti-rTSH and anti-rTSH beta sera were also applied additionally for staining. The anti-HCG serum-positive cells were first evident on day 17.5 of gestation, and the cells reacting for anti-rLH and anti-rLH beta sera started also to appear. On day 19.5, very few cells appeared to react for anti-rFSH serum. Intraperitoneal injection of LHRH to the fetuses induced a remarkable disappearance of the immunoreactive materials, and as a result a numerical reduction occurred in the immunoreactive cells. These phenomena were noted even on day 18.5 of gestation. Encephalectomy of day 16.5 fetuses and daily i.p injection of anti-LHRH serum to 17.5 to 19.5 day old fetuses were carried out with the purpose of blocking the effect of endogenous LHRH to fetal pituitary. Dense staining of the cytoplasm and remarkable reduction in size and in number of gonadotrophs were obtained after day 20.5 of gestation in both animal groups. These findings suggest that the hypothalamus may start to govern virtually the hypophysial gonadotrophs on day 20.5 of gestation in rat.