Active and Passive Morphological Interactions of Trophoblast and Endometrium During Early Implantation

Abstract

Some 20 years ago, two symposia papers (1, 2) pointed out that common patterns of interaction of trophoblast and endometrial tissues can be found during implantation in different species, regardless of major differences in blastocyst orientation, position, and eventual placentation category. It was argued that implantation in most species can be viewed as starting with apposition of the blastocyst to permit adhesion of trophoblast to uterus, then epithelial penetration, and finally vascular invasion, with various species stopping at different stages in the progression. Although it had been noted earlier that many species “pause” at the residual uterine luminal basal lamina, it was not until it was shown that this step could be accomplished by decidual cell processes that it received recognition as one of the characteristic stages (3). With a few notable exceptions—for example, the excellent studies of Wooding (4, 5) on trophoblast binucleate cell formation and fusion with the uterine epithelium (UE) in ruminants—both the cellular details of these implantation stages and the extension of the information to species other than those of the common laboratory animals initially described have been slow in coming Nevertheless, as we continue to obtain information on the morphology of implantation, we find repeated patterns, not just at the broad level, such as intrusion of trophoblast between epithelial cells as a method of endometrial invasion, but also within stages.