Autoradiographic localization of specific binding of meiosis-activating sterol to cumulus-oocyte complexes from marmoset, cow, and mouse

The sterol, 4,4-dimethyl-5 alpha -cholesta-8,14,24-trien-3 beta -ol (FF-MAS ), isolated from human follicular fluid, can induce resumption of meiosis i n denuded and cumulus-enclosed mouse oocytes inhibited by hypoxanthine, IBM X, or dibutyric cyclic adenosine monophosphate. In this study the distribut ion of FF-MAS binding sites in denuded oocytes and in cumulus-oocyte comple xes (COCs) was studied using light microscopic (LM) and transmission electr on microscopic (TEM) autoradiography in marmoset, cow, and mouse oocytes. D enuded (n = 39) and cumulus-enclosed (n = 28) marmoset, cow, and mouse oocy tes were cultured in the presence of [H-3]FF-MAS with and without excess of unlabeled FF-MAS, respectively. In denuded oocytes LM autoradiography demo nstrated specific binding to the oolemma and zona pellucida and, to some ex tent, the cytoplasm. In the nucleus, no specific binding of [3H]FF-MAS was demonstrated. In some COCs the labeling was dispersed throughout the zona p ellucida, the oolemma, and the cytoplasm as well as the cumulus cells; wher eas in others, only the outer part of the cumulus cells were labeled. TEM a utoradiograms of denuded cow oocytes (n = 6) demonstrated that specific [H- 3]FF-MAS binding was closely related to the oolemma and that a low level of [H-3]FF-MAS binding to cumulus cell remnants was present. In conclusion, s pecific binding of FF-MAS is predominant at the oolemma of denuded oocytes, suggesting the existence of a plasma membrane-associated molecule with aff inity for FF-MAS (i.e., a putative FF-MAS receptor).