Fine structure of the zona pellucida of unfertilized egg cells and embryos

The fine structure of the zona pellucida of mouse unfertilized egg cells and of mouse embryos was investigated. In addition to standard fixation for electron microscopy, fixation with an addition of ruthenium red was used and found to be especially valuable for the purpose of studying the zona pellucida. The zona pellucida of unfertilized egg cells and of embryos consisted mainly of fibrillar and granular material. There were two layers: a thicker internal and a denser external layer in the zona pellucida of the unfertilized egg cells. The zona pellucida of embryos had, in addition to these two layers, a coarse-grained intimal layer inside the internal layer and a fine-grained peripheral cortical reaction whereas the peripheral layer can be accumulated and modified during the passage of the embryo through the female genital tract. The influence of developmental events and environmental factors on the fine structure of zona pellucida was discussed.