Cell redundancy in the zona-intact preimplantation mouse blastocyst: a light and electron microscope study of dead cells and their fate

In the zona pellucida-intact 95 h post coitum mouse blastocyst, electron-microscopic studies reveal the presence, as a part of normal development, of 1–2 dead cells lying free on the surface of the inner cell mass (ICM) or trophoblast cells, and of 4–5 dead cells phagocytosed by 1CM cells. Such dead cells are electron-dense and show characteristic chromatopycnosis of the nucleus, and swelling of the endoplasmic reticulum. In addition, 1–2 digestive vacuoles with granular contents and myelin figures are found, principally in ICM cells, and corresponding with basophilic bodies observed with the light microscope. The results are interpreted as indicating that a relatively large number (i.e. a minimum of 6–8, or approximately 10%) of blastocyst cells die and are phagocytosed and digested, usually by the ICM cells, but probably also by trophoblast cells. This process does not, however, affect the future differentiation of the ingesting cell. Simultaneously a small number of epithelial cells adjacent to the blastocyst die, either singly or in small groups. These findings confirm the view that previous reports of penetration of the uterine epithelium by ‘primary invasive cells’ originating in the ICM were due to confusion between two separate groups of dead cells, namely the embryonic dead cells of the ICM and the single dead cells in the adjacent uterine epithelium, which appear to be phagocytosed by trophoblast cells following loss of the zona pellucida.