Ultrastructure of a seven-cell human embryo

The ultrastructural features of a seven-cell human embryo recovered from the oviduct are described. Two adjoining blastomeres located in the periphery exhibited greater electron density than the others and gave a metachromatic rection with toluidine blue. A distinct junctional complex was observed at the external border between a dark and a light cell, but most areas of cell contact were devoid of visible membrane differentiations. Organelles were found unevenly distributed in the cytoplasm and appeared to be in a more advanced stage of development in the dark blastomeres. Endoplasmic reticulum cisternae tended to accumulate between a subcortical and a perinuclear band, whereas large vesicles were present in the subcortical area. Mitochondriae were distributed throughout the cytoplasm but definitely in greater numbers at one pole of the cells. Extensive blebbing of the nuclear envelope was seen in some nuclei. Numerous groups of vesicles and small corpuscles were present in the matrix of the zona pellucida. In general, the features described were consonant with those found in eggs of other mammals.