An ultrastructural study of implanting mouse blastocysts: coated vesicles and epithelium formation.

Electron microscopial techniques were used to study mouse embryos at implantation stages (5 1/2 days post coitum). Evidence was obtained indicating that trophoblast cells remove uterine luminal epithelial cells by phagocytosis. Small coated vesicles very probably play an important role in the process of digestion of uterine cells by the trophoblast. Two types of endoderm cell were present: the embryonic endodermal cells contained organelles appropriate to a nutritive pathway; the extra-embryonic endodermal cells were relatively poorly developed compared with those of the embryonic endoderm, and remained as a discontinuous layer. The ectoderm contained few organelles. Epithelium formation is described. Junctions only develop between cells in the same germ layer except for occasional ones, including desmosomes, between trophoblast and uterine epithelial cells. The author wishes to thank Professors J.M.F. Landsmeer and W.T. Daems for their stimulating discussions during this research project and for their helpful comments during the preparation of the manuscript. The Laboratory for Electron Microscopy kindly provided full facilities. The assistance of Mr J. Tinkelenberg in the preparation of the drawing (Fig. 1) is acknowledged. Thanks are also due to Mrs 2. seeger for reading the English manuscript and to Miss Ria Stokman for typing it.