Oxytocin: a follicular luteinisation factor in the marmoset monkey

The distributions of oxytocin (OT) and the oxytocin receptor (OTR) have been characterised in preovulatory follicles of the marmoset monkey and are described in relation to the process of luteinisation using a combination of in vitro and in vivo techniques, including immunohistochemistry and granulosa cell culture. Ovaries were collected in the periovulatory phase before and 22h after exogenous gonadotropin (hCG) treatment, but prior to ovulation. Before hCG treatment, OT immunoreactivity (OT-ir) was found mostly in granulosa cells of antral follicles, especially in the layers nearest the antrum. In contrast, oxytocin receptor immunoreactivity (OTR-ir) was observed principally in basal granulosa cells of antral follicles. Progesterone receptor (PR) and 3 beta hydroxysteroid dehydrogenase (3 beta HSD), markers for luteinisation, were negative for all ovarian tissues collected before hCG treatment. After hCG treatment, almost all granulosa cell layers in antral follicles stained positively for both OT-ir and OTR-ir, most prominently in preovulatory follicles and especially in the cumulus oophorus. This increase in staining was associated with an induction of PR and 3 beta HSD activity in the granulosa cells of preovulatory follicles. OT production by granulosa cells in culture was stimulated by the addition of hCG, but only in cultures derived from preovulatory follicles; whereas FSH was without any effect. Addition of extrinsic OT to the cultures elicited an increase in progesterone production only for the granulosa cells derived from preovulatory follicles. Together, the results of the in vivo and in vitro studies point to an involvement of OT in the process of leuteinisation in the marmoset monkey.