Species differences in the regulation of cumulus expansion by an oocyte-secreted factor(s)

The expansion of the mouse cumulus oophorus in vitro in response to FSH is dependent upon the presence of an enabling factor secreted by the oocyte. The purpose of this study was to determine whether the expansion of cumulus cells from rats and pigs were similarly dependent upon an oocyte-secreted enabling factor. Mouse and rat oocyte–cumulus cell complexes were isolated from pregnant mares' serum gonadotrophin (PMSG)-stimulated animals; pig oocyte–cumulus cell complexes with the attached piece of mural granulosa were obtained from either prepubertal gilts or cyclic sows. FSH (25–1000 ng ml−1) or dibutyryl cyclic adenosine monophosphate (dbcAMP; 0.05–2 mmol l−1) induced a dose-dependent expansion of the oocyte–cumulus complexes from rats and pigs. The hypothesis that the oocyte plays a role in the regulation of cumulus expansion was tested by microsurgically removing oocytes from oocyte–cumulus complexes and the oocytectomized complexes were tested for their ability to undergo expansion in response to FSH. FSH did not induce cumulus expansion in oocytectomized mouse complexes; however, expansion occurred in rat and pig oocytectomized complexes. The pieces of mural granulosa, detached from the pig complexes, also expanded in response to FSH stimulation. Rat and pig oocytectomized complexes were then held in culture for up to 48 h before stimulation by FSH. The degree of expansion in rat oocytectomized complexes decreased as the delay before FSH stimulation increased such that, with an 8 h delay, oocytectomized complexes did not expand. Pig oocytectomized complexes expanded fully even with a 32 h delay before FSH stimulation, and in response to dbcAMP or epidermal growth factor (EGF). Finally, when rat or mouse oocytectomized complexes were cocultured with germinal vesicle-stage mouse, rat or pig oocytes, FSH stimulated expansion by all oocytectomized complexes. These results indicate that a factor(s) secreted by mouse and rat oocytes is necessary for the cumulus cells to undergo expansion in response to FSH, EGF or dbcAMP. Pig oocytes can secrete a cumulus expansion-enabling factor, but expansion of pig complexes does not depend upon this factor.