Relation between estrogen secretion and follicular morphology in the rat ovary under the influence of ovulating hormone or exogenous gonadotropins.

Abstract

A correlation between the morphology of mature follicles and estrogen secreting activity of ovaries was investigated on the proestrous rat before and after the exposure of endogenous ovulating hormone or ovulatory dose of exogenous gonadotropins. Histology of mature follicles until 5 p.m. in proestrus, when estrogen secretion was highly stimulated, was characterized by a granulosa layer with smooth inner surface, in which many mitoses were seen, regularly arranged nuclei of basal granulosa cells, and relatively thin theca interna sharply bordered from granulosa layer. During the process of ovaries losing estrogen secreting activity after exposure of ovulating hormone, a slight thecal swelling with hyperemia, accumulation of intercellular fluid in granulosa layer were recognizable in mature follicles. At 11 p.m., when estrogen secretion had fallen to the lowest level, orientation of nuclei in basal granulosa cells became irregular and a sharp border between granulosa layer and theca layer became indistinct. It was also noticed that the glucose-6-phosphate dehydrogenase activity distributed in theca interna decreased slightly at this time. If FSH or LH was administered 6 hr before the normal time of ovulatory surge, the same histological and histochemical changes described above were advanced approximately 6 hr. In addition, the minimal doses of gonadotropins required for causing such morphological changes in mature follicles were exactly the same as those to induce termination of estrogen secretion and ovulation. These findings support our previous explanation for the mechanism by which gonadotropin causes termination of estrogen secretion in proestrus, i.e. ovulation inducing hormone initiates the differentiation of follicular elements towards ovulation, and during the process of differentiation, unfavorable conditions may be brought about for estrogen synthesis.