Surface topography of mouse eggs before and after insemination

The surface configuration of zona‐free mouse ovarian eggs before and after germinal vesicle breakdown (GVBD) and of tubal eggs before and after insemination was studied by scanning electron microscopy. The surface of germinal vesicle‐intact eggs was characterized by uniformly distributed, dense (2–3μm 2 and slender (0.1 × 0.8μm) microvilli. Following GVBD, the mouse oolemma underwent dramatic changes which persisted essentially unmodified in uninseminated tubal eggs. These changes consisted of an apparent conversion of the oolemma from a microvillar to a smooth or ruffled configuration with approximately two ruffle‐like micro‐projections (0.6 × 0.4 μm) perμm 2 of surface. In most eggs, both ruffled and smooth configurations ere present, creating dramatic polarities in surface topography similar to those documented previously by TEM [Nicosia et al, 1977]. Following insemination and presumed sperm–egg fusion, microvilli reappeared either from smooth areas or from the upper edge of ruffled microprojections. Zona removal appeared to induce limited reduction in the size of microvilli in some eggs. The present results emphasize the labile and dynamic nature of the egg surface and suggest a close relationship among nuclear, cytoplasmic, and surface events during the final stages of egg maturation and during fertilization.