Chorioallantoic placenta formation in the rat. III. Granulated cells invade the uterine luminal epithelium at the time of epithelial cell death.

Rat gestation sites were microscopically examined on Days 7, 8, and 9 of pregnancy to determine the fate of epithelial cells lining the uterine lumen mesometrial to the implantation chamber. This region of the uterine lumen and the mesometrial region of the implantation chamber comprise the site of chorioallantoic placenta formation. The fate of the uterine luminal epithelium was closely associated with a subepithelial accumulation and epithelial invasion of granulated cells (granulated metrial gland cells). On Day 7, mature granulated cells were not detected in the subepithelial stroma, and epithelial cells appeared healthy. On Day 8, when granulated cells began populating the subepithelial stroma and a few crossed the basal lamina, some epithelial cells degenerated in situ. On Day 9, when large numbers of granulated cells invaded the epithelium, large numbers of epithelial cells were displaced from their basal lamina and were degenerating. Degenerating cells were of two morphological types. One type conformed to the standard description of apoptosis; that is, cells rounded up, detached from their neighbors, and their nuclei fragmented, although other organelles remained intact. The other type of cell degeneration resembled apoptosis in some respects, but the cells were very electron-dense, adjacent cells often remained attached, membranous organelles including mitochondria were swollen, and nuclei did not fragment. Granulated cells are related to natural killer cells, cells that induce target cell apoptosis, and we suggest that granulated cells may be involved in uterine luminal epithelial cell death. The conceptus remained in an antimesometrial region of the implantation chamber during the times studied and did not come into direct contact with these epithelial cells.