Uterine cell death during implantation and early placentation.

During blastocyst implantation and placentation in common laboratory rodents, trophoblast cells come into increasingly more intimate associations with the endometrium and, eventually, are in contact with maternal blood. Uterine cell death is one mechanism for removing uterine tissues, primarily epithelial cells, and decidual cells that intervene between trophoblast cells and maternal blood. Mechanisms of cell death and the signals that initiate and regulate it are not well understood. According to current theories, cell death is either gene-directed or the result of traumatic injury, and classification of cell death is based on ultrastructural and biochemical criteria that hypothetically reflect underlying molecular mechanisms. Although the term apoptosis is extensively used to describe all aspects of gene-directed cell death and the term necrosis to describe traumatic death, ultrastructural studies indicate that there are morphological variations of the established criteria, and these could reflect variations of underlying mechanisms. Recent light and electron microscopic work has shown that timing and ultrastructure of uterine cell death at the gestation site varies with region suggesting that initiation and control of cell death is complicated and that more than one mechanism of cell death may be operative. Current information indicates that uterine cell death is most likely part of an intrinsic response of the endometrium to the conceptus, and other than acting as a stimulus to elicit the uterine response, the conceptus probably plays only a minor role in regulating the death of endometrial cells in these species.