Epithelial autolysis during implantation of the mouse blastocyst: an ultrastructural study

An ultrastructural investigation was made of cell death in the uterine epithelium during the implantation of the mouse blastocyst. Cell death occurs in two phases: (i) early individual cell death, and (ii) later general cell death. Individual dead epithelial cells are phagocytosed and digested by trophoblast cells. At 95 h post coitum (p.c.) there are 1-4 dead cells; at 105 h and 113 h, 6-8 dead cells, and at 116 h, 2-3 dead cells. No lysosomal involvement could be identified in the death of these cells. The general breakdown of the uterine epithelium around the implanting blastocyst is first recognizable at the ultrastructural level at 113 h p.c., and continues until the adjacent deteriorated epithelium has been phagocytosed by the trophoblast cells at 119 h p.c. Ultrastructurally, from 113 h p.c. cytoplasmic portions of epithelial cells are trapped within cytosegrosomes, and there is an increase in size of the dense lysosomal bodies found in viable epithelial cells at 105 h p.c. The dense bodies, which are positively stained for acid phosphatase enzyme at 105 h p.c., increase in diameter approximately 3-5 times between 105 h p.c. and 119 h p.c. These results provide evidence of intracellular digestion of small portions of the cytoplasm possibly through the formation of cytosegrosomes which then fuse with residual bodies already present. Thus the evidence favours a process of autolytic breakdown of the uterine epithelium around the implanting blastocyst.