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Cell-surface morphological events relevant to human implantation.

Morphological evidence on early stages of human implantation is limited to very few sporadic observations. The nature of implantation which requires the presence of both maternal and embryonic tissues, combined with the currently existing ethical constraints on human studies, appear to preclude generation of new data. However, research on relevant animal and in-vitro models as well as studies on human endometrium and in-vitro embryos, allow some indirect insights to this phenomenon. This review summarizes information on cell-surface morphological events relevant to implantation initiation, derived from scanning electron microscopy studies on the above systems. A central part of this article deals with the formation of epithelial cell projections known as pinopodes, for there is increasing evidence suggesting that these structures are closely associated with the development of endometrial receptivity for blastocyst implantation in humans.