Light and Electron Microscopic Study on Epithelial Cell Death During Implantation in Rat Uterus

Nam, Kwang Il; Oh, Chang Seok; Ahn, Kyu Youn; Bae, Choon Sang; Kim, Baik Yoon; Yoon, Jae Rhyong; Park, Sung Sik

Abstract

The loss of the uterine epithelium surrounding blastocyst is important in bringing the trophoblast into close association with the endometrial stroma during embryonal implantation in several laboratory rodents. It is usually assumed that the epithelial cells die during this process, but the cause of epithelial cell death remains speculative. This study was performed to investigate the mechanism of epithelial cell death during implantation in the endometrium of rat uterus throughout days 5-9 of pregnancy by light and electron microscopy. Cathepsin-D reactivity, detected by imunohistochemistry, was localized to all viable epithelia, some degenerating epithelial and decidual cells between days 5-9 of pregnancy. DNA fragmentation, detected by TUNEL staining, was observed in degenerating epithelial and decidual cells throughout days 6-9 of pregnancy. Cellular fragments within lumen and phagosome within trophoblasts were reacted positively to TUNEL staining. Electron microscopy revealed that the epithelium lining the antimesometrial chamber was sloughed off into the lumen on day 6 of pregnancy. Its cell organelles appeared healthy but its nucleus was condensed and irregular shaped. The detached epithelial cells lining the antimesometrial chamber on day 7 of pregnancy contained condensed and fragmented nuclei, but exhibited different morphological pattern according to the intactness or loss of cytoplamic organelles and membrane. And these cells were surrounded and phagocytosed by trophoblasts. On day 7 of pregnancy, syncytial knots were formed, and later these knots and most of detached epithelial cells were severely degenerated. Some decidual cells nearby the lumen were also degenerated at this period. These results indicate that the epithelial cell death of rat uterine endometrium during implantation is mainly due to the process of apoptosis and these dying cells are removed by trophoblasts.