The distribution of binding sites for human chorionic gonadotropin (hCG) in the preovulatory follicle was studied by autoradiography. An ovulatory dose (10 IU/rat) of [125I]hCG (1.4 muCi/IU) was administered intravenously, and large Graafian follicles were isolated 3 h later by microdissection. Injection of excess unlabeled hCG (500 IU/rat) prevented uptake of radioactivity by the follicle, indicating that binding of iodinated hormone was confined to specific and saturable receptor sites. The density of bound hormone molecules was highest in the theca interna and in three to four layers of mural granulosa cells adjacent to the basement membrane; labeling was chiefly associated with the cell borders. No significant binding could be detected either on the oocyte or on the cumulus cells surrounding the oocyte. We therefore suggest that the induction of ovum maturation does not require attachment of the hormone to the oocyte itself or to follicle cells in its immediate vicinity.

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