CYTOKERATIN DYNAMICS DURING OOCYTE MATURATION IN THE HAMSTER REQUIRESREACHING OF METAPHASE-I

Cytoskeletal components like microfilaments and microtubules are known to play important roles during the processes of oocyte maturation, fe rtilization and early embryonic development in mammals. However, the r oles of other components such as cytoplasmic intermediate filaments, d uring these critical events remain largely unknown. Oocyte maturation is the final step of oogenesis, immediately before ovulation. Several cytological changes involving the cytoskeleton take place during the t he maturation process, including meiotic spindle formation, redistribu tion of cell organelles, membrane polarization and first polar body em ission. In this study we determined the organization and rearrangement s of cytokeratins during hamster oocyte maturation. Fully grown oocyte s were cultured and then visualised using microscopic immunolabelling techniques to monitor the cytokeratin dynamics at specific meiotic sta ges of the maturation process. In prophase-I-arrested fully grown hams ter oocytes, cytokeratins are confined to 4-10 large cortical aggregat es, corresponding to extensive mesh-works of intermediate filaments. T hese large aggregates disperse into multiple small spots starting at m etaphase I until the end of the maturation period at metaphase II, whe re cytokeratin exhibits a homogeneously distributed spotted pattern. H owever, meiotic progression to metaphase II is not necessary for cytok eratin redistribution to occur, since precociously arrested metaphase I oocytes also exhibit dispersed cytoplasmic foci at the end of the cu lture period. The redistribution of cytokeratins is insensitive to noc odazole and cytochalasin D suggesting it occurs independent of microtu bules and microfilaments. In contrast, both cumulus cells and protein synthesis are required for cytokeratin modifications to take place dur ing oocyte maturation. These results show that cytokeratin intermediat e filaments are present in the fully grown hamster oocyte, and that a striking reorganization of cytokeratins, triggered by attainment of th e metaphase I stage, occurs during maturation.