Surface Alterations of the Mouse Zona Pellucida and Ovum following in vivo Fertilization: Correlation with the Cell Cycle

Abstract

The zona pellucida and cell surface of in vivo fertilized mouse ova exhibit time dependent changes which can be detected with the scanning electron microscope. The periods of ovulation, fertilization and first cleavage in superovulated C3D2/F₁ hybrids were determined and times corresponding to G₁, S, G₂ and M were calculated. The zona of a mature unfertilized ovum has a rough texture with deep furrows; at fertilization and thereafter the zona develops a smoother, ropy and seemingly porous surface. The cell surface of the unfertilized ovum is characterized by uniform microvilli, small blebs and rounded, mound-like elevations. After fertilization and development to G₁, the ovum loses its blebs but retains the mound-like elevations and microvilli which are now less uniform. As the ovum progresses toward S, it loses the mound-like elevations but retains microvilli in the same density as found in G₁. The ovum in G₂ exhibits smaller but more numerous microvilli which vary considerably in length. Some appear to bifurcate. The fertilized ovum developing through M and G₁ of the 2 cell stage exhibits a less dense population of relatively uniform microvilli, periodic blebs and, again, rounded elevations. The data are reminiscent of surface changes associated with the cell cycle in tissue culture cells and indicate a cyclic progression of the in vivo ertilized mouse ovum through the first cleavage division to the 2 cell stage.