Freezing of zona-free mouse embryos: characteristics of the plasma membrane and subsequent development of the embryos.

Frozen-thawed mouse embryos with (+ZP) and without (-ZP) zona pellucida have been studied at the Scanning Electron Microscope (SEM) to determine how the process affects the plasma membrane and the subsequent embryo development. The main difference observed in -ZP embryos immediately after thawing is the abnormal morphology and distribution of microvilli. This could explain the spontaneous separation of blastomeres in -ZP embryos, and the decrease in their survival rate. If thawed -ZP embryos are allowed to recover in culture, their plasma membrane characteristics and survival rate are identical to those of control embryos.