Mouse-rabbit germinal vesicle transfer reveals that factors regulating oocyte meiotic progression are not species-specific in mammals

A series of experiments were designed to evaluate the meiotic competence of mouse oocyte germinal vesicle (GV) in rabbit ooplasm. In experiment 1, an isolated mouse GV was transferred into rabbit GV-stage cytoplast by electro fusion. It was shown that 71.8% and 63.3% of the reconstructed oocytes comp leted the first meiosis as indicated by the first polar body (PB1) emission when cultured in M199 and M199 + PMSG, respectively. Chromosomal analysis showed that 75% of matured oocytes contained the normal 20 mouse chromosome s. When mouse spermatozoa were microinjected into the cytoplasm of oocytes matured in M199 + PMSG and M199, as many as 59.4% and 48% finished the seco nd meiosis as revealed by the second polar body (PB2) emission and a few fe rtilized eggs developed to the eight-cell stage. In experiment 2, a mouse G V was transferred into rabbit MII-stage cytoplast. Only 13.0-14.3% of the r econstructed oocytes underwent germinal vesicle breakdown (GVBD) and none p roceeded past the MI stage. When two mouse GVs were transferred into an enu cleated rabbit oocyte, only 8.7% went through GVBD. In experiment 3, a whol e zona-free mouse GV oocyte was fused with a rabbit MII cytoplast. The GVBD rates were increased to 51.2% and 49.4% when cultured in M199 + PMSG and M 199, respectively, but none reached the MII stage. In experiment 4, a mouse GV was transferred into a partial cytoplasm-removed rabbit MII oocyte in w hich the second meiotic apparatus was still present. GVBD occurred in nearl y all the reconstructed oocytes when one or two GVs were transferred and tw o or three metaphase plates were observed in ooplasm after culturing in M19 9 + PMSG for 8 hr. These data suggest that cytoplasmic factors regulating t he progression of the first and the second meioses are not species-specific in mammalian oocytes and that these factors are located in the meiotic app aratus and/or its surrounding cytoplasm at MIT stage. J. Exp. Zool. 289:322 -329, 2001. (C) 2001 Wiley-Liss, Inc.