Penetration of the uterine epithelium during implantation in the rabbit

Abstract The structure of interacting trophoblast and uterine epithelium was studied in 16 rabbits from seven days zero hours to seven days 20 hours post coitus. The most common type of penetration of the uterus by the trophoblast consisted of a peg from a trophoblastic knob that extended to the basal lamina of the uterine luminal epithelium and was bordered by uterine epithelial cells with which it shared junctional complexes. In early implantation sites there was direct evidence of fusion of the syncytial trophoblast with apical ends of individual uterine epithelial cells. Since no evidence of epithelial dissociation or mechanical intrusion of trophoblast between epithelial cells was found, it is suggested that fusion is the normal method of epithelial penetration by the trophoblast in the rabbit. The cytoplasm of the fused uterine cells was apparently converted into syneytium of the trophoblastic knob. Subsequent to formation of the trophoblastic peg, penetration of the basal lamina and of maternal blood vessels occurred, and the attached trophoblastic knobs increased in width. At later stages when symplasma formation by the uterine epithelium was extensive, fusion included trophoblast between knobs. Since the penetration peg is formed by fusion of trophoblast with uterine epithelium, the cell membrane first associated with the maternal connective tissue is of maternal origin. However, new membrane formed after fusion would be expected to have histocompatibility factors from both the trophoblast and the fused maternal epithelial cells.