MULLERIAN-INHIBITING SUBSTANCE GENE-EXPRESSION IN THE CYCLING RAT OVARY CORRELATES WITH RECRUITED OR GRAAFIAN FOLLICLE SELECTION

Throughout the estrous cycle, Mullerian inhibiting substance (MIS) mRNA signals, as detected by in situ hybridization, were found to be intense in granulosa cells of growing preantral and small antral follicles, especially in the layer closest to the oocyte. Neither primordial follicles, typical atretic follicles, nor CL expressed detectable signals. MIS mRNA signals seen in the cumulus cells of preovulatory follicles at 1000 and 1600 h of proestrus sharply declined at 2400 h, just before ovulation. MIS mRNA signals were intense and uniform in all newly recruited growing antral and Graafian follicles (> 350 mu m) at 1000 h of estrus but became heterogeneous in their expression of MIS on metestrus and diestrus and notably absent in atretic follicles; these findings suggest that MIS expression is correlated with recruited follicle selection and suppression with selection for atresia. The fact that all the MIS-positive growing follicles contain oocytes arrested in meiosis, coupled with previous experimental evidence that MIS inhibits germinal vesicle breakdown, suggests that MIS also might act as a regulator of oocyte maturation.