Expression of Inhibin/Activin Subunits, Aromatase and 3β-Hydroxysteroid Dehydrogenase in Various Sized Ovarian Antral Follicles of the Mare

Abstract

Localization of inhibin subunits and steroidogenic enzymes in the various-sized antral follicles of the mare was examined. Antral follicles of four sizes (1, 2.5, 4.5 and 6 cm in diameter) were collected from the ovary and subjected to histological analysis. Inhibin α-subunit mRNA was localized in granulosa cells of follicles of all sizes, and inhibin βA-subunit mRNA was confined to granulosa cells of the 1-4.5-cm follicles. Inhibin α-subunit protein was found in granulosa cells of follicles of all classes, and theca cells of the 6-cm follicle. Inhibin βA-and βB-subunit proteins were found in granulosa cells of the 1-4.5-cm follicles, and theca cells of the 2.5-6-cm follicles despite the absence of inhibin/activin subunit mRNAs in theca cells. This suggests that inhibin and/or activin regulates the function of theca cells in a paracrine fashion. Aromatase protein was confined to granulosa cells of the 2.5-and 4.5-cm follicles, and to theca cells of the 4.5-cm follicle. Strong staining for aromatase protein was noted in the 4.5-cm follicle as compared to the 2.5-cm follicle. Positive immunostaing for 3β-hydroxysteroid dehydrogenase was observed in granulosa cells of the 2.5-6-cm follicles, and in theca cells of the 4.5-and 6-cm follicles. These data suggest that inhibin is predominantly secreted from 1-4.5-cm antral follicles and estradiol is predominantly secreted from antral follicles of about 4.5-cm in diameter. Further, it is suggested that progesterone is the predominant hormone secreted form the preovulatory follicle in the mare.