The relationship between cumulus cell-oocyte coupling, oocyte meiotic maturation, and cumulus expansion

The timing of the reduction of cumulus cell-oocyte coupling was correlated with oocyte meiotic maturation and the expansion (mucification) of the cumulus oophorus using immature mice treated with gonadotropins. Three hours after the injection of an ovulatory dose of human chorionic gonadotropin (hCG), more than 90% of the oocytes isolated from large Graafian follicles had undergone germinal vesicle breakdown, indicating that oocyte meiotic maturation had been initiated. However, no cumulus expansion or reduction of intercellular coupling was detected at this time. By 6 hr after hCG injection, the index of oocyte-cumulus cell coupling was still not less than that found in oocyte-cumulus cell complexes isolated from control mice not receiving hCG. Cumulus expansion at 6 hr post-hCG was limited to the outer cumulus cells while those adjacent to the oocyte were still tightly packed. Cumulus expansion appeared complete by 9 hr after hCG injection and the cumulus cell-oocyte coupling index was greatly reduced. These results show that oocyte meiotic maturation in the mouse is not initiated by a reduction in cumulus cell-oocyte coupling or by cumulus expansion. However, the results suggest that the reduction of intercellular coupling in vivo may be a result of cumulus expansion.