Changes at the hamster oocyte surface from the germinal vesicle stage to ovulation

Immature and ovulated hamster oocytes were studied with the scanning electron microscope. Immature oocytes at the germinal vesicle stage have their surface uniformly covered by microvilli. When meiosis has progressed to the first meiotic metaphase the overlying surface shows the differentiation of a circular area 19 μm in diameter with a low density of microvilli. Later, from this region the first polar body emerges, and the oocyte surface at the point from which it was extruded shows a cluster of cytoplasmic, conical projections. When the zona‐free oocytes are cultured at 37°C for 5 minutes these projections disappear and the oocyte surface at that point becomes smooth. However, when the oocytes remain in the oviduct for several hours after ovulation these projections remain unchanged. The in vitro interactions of capacitated hamster sperm with the immature oocyte was always seen at microvillus surfaces and never associated with the differentiated regions.