Gonadotropin-Binding Sites in the Rhesus Monkey Ovary: Role of the Vasculature in the Selective Distribution of Human Chorionic Gonadotropin to the Preovulatory Follicle*

Abstract

These experiments were initiated to determine if differences exist in the vasculature of individual follicles in the rhesus monkey ovary during the late follicular phase of the menstrual cycle and to determine whether differences in vascularity result in differential exposure of certain follicles to gonadotropic hormones. The density of blood vessels within the thecal layer of the dominant follicle and other antral follicles was determined in ovaries from four animals removed on day 9 or 10 of the menstrual cycle. Blood vessels were identified using a histochemical stain for hemoglobin. Morphometric analysis indicated that the percentage of the thecal layer occupied by blood vessels in the dominant follicles (48%) was significantly greater (P less than 0.005) than that of other smaller antral follicles either within the same ovary as the dominant follicle (24%) or in the contralateral ovary (26%). To determine if differences in vascularity result in a differential supply of gonadotropins to the dominant follicle, we studied, by autoradiography, the in vivo and in vitro binding of [125I]hCG in four rhesus monkeys on day 9 of the menstrual cycle. Results of in vitro binding studies indicated that the thecal layer of virtually every antral follicle possessed hCG-binding sites. However, when [125I]hcg was injected iv into animals and allowed to distribute via the vasculature, the dominant follicle was heavily labeled while other smaller antral follicles accumulated little, if any, radioiodinated hCG. These observations indicate that increased vascularization of individual follicles results in preferential delivery of gonadotropins, and suggest that blood flow to individual follicles may play an instrumental role in the selective maturation of the preovulatory follicle in the rhesus monkey.