Meiosis‐activating sterol protects oocytes from precocious chromosome segregation

BACKGROUND: Follicular fluid meiosis‐activating sterol (FF‐MAS) overcomes hypoxanthine (HX)‐mediated meiotic arrest in mammalian oocytes. METHODS: In order to determine whether chromosome segregation was normal in oocytes matured in FF‐MAS, the development, chromosomal constitution and chromosome alignment was analysed in spontaneously matured as well as HX‐arrested mouse oocytes cultured in the absence or presence of FF‐MAS. RESULTS: FF‐MAS‐induced meiotic maturation was significantly less effective compared with spontaneous maturation in supporting cytokinesis (∼40 and ∼90% polar body formation respectively). The majority of oocytes stimulated by FF‐MAS to overcome the HX block developed to metaphase II (MII), but 23.4% of meiosis II oocytes were diploid. Chromosomes were well aligned on the spindle, and hyperploidy was low in spontaneously matured oocytes and HX‐arrested oocytes cultured with or without FF‐MAS. Unexpectedly, almost 40% of spontaneously matured MII oocytes contained chromatids/monads. Precocious loss of chromatid cohesion was significantly reduced in spontaneously matured as well as HX‐arrested oocytes cultured in the presence of FF‐MAS but not lanosterol. CONCLUSIONS: FF‐MAS induces full nuclear maturation to MII, and chromosomes segregate with high fidelity. However, in delayed FF‐MAS‐stimulated meiotic maturation, anaphase I may occur in the absence of cytokinesis. FF‐MAS appears to protect mammalian oocytes from precocious chromatid segregation.