Maturational Effects of Gonadotropins on the Cumulus-Oocyte Complex of the Rat

The temporal relationship between maturational events in the cumulus oophorus and the oocyte of adult rats was examined. The time of preovulatory surge of Lii was estimated from analysis of the critical period. Cyclic rats were killed on the afternoon and night of proestrus; hCGtreated rats were injected during the early morning hours of proestrus and were killed up to 13 h later. At necropsy, the ovaries were removed and the oocytes with their cumuli were isolated from the large preovulatory follicles. The intercellular mucification and tissue dissociation characteristic of cumuli around ovulated oocytes began to appear 4.24 ± 0.15 h after endogenous LH secretion or 4.31 ± 0.25 h after hCG. Transformation of the cumulus started at its periphery and spread centripetally. By 5.70 ± 0.21 and 7.17 ± 0.34 h, respectively, the cumuli had completed their differentiation. Maturation of the oocytes began before and ended after maturation of the cumuli. Timing of the maturational events preceding spontaneous cyclic ovulation and hCG-induced ovulation was similar. Small differences were attributed to differences in circulating levels of gonadotropin as a function of time. It was concluded that cumulus maturation is induced by the ovulatory surge of LH.