Cell death in the mammalian blastocyst.

Cell death is a widespread feature in the blastocysts of many mammals. Isolated cells in both the inner cell mass and the trophectoderm undergo cell death. These dying cells appear morphologically to be undergoing apoptosis. In mouse blastocysts, a wave of cell death is seen in vivo, suggesting that it plays an important role in normal development. However, cell death is increased under suboptimal culture conditions. There is evidence that levels of cell death are regulated by 'survival' factors produced both by the embryo itself and by the maternal reproductive tract. The role of cell death in development is unknown, but could involve the elimination of abnormal cells, or a sublineage of cells with an inappropriate developmental potential. Work in other systems has demonstrated that cell death is regulated by the activity of apoptosis genes. Whether these genes are implicated in blastocyst cell death, and the reasons for apoptosis in the early embryo, remain to be determined.