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Implantation is initiated by an interaction of trophoblast with the uterine epithelium via the apical cell poles of both partners. Aspects of changes that must take place in the glycocalyx and in the plasma membrane, to allow this process to be initiated are discussed in other contributions to this volume. There appears to be good reason, however, to bring the cytoskeleton into the picture, since findings obtained in other systems give strong evidence for interactions between it and the cell membranes (Tachi et al., 1970; Jones and Goldman, 1985; Perides et al., 1986a,b; Traub et al., 1987; Lazarides, 1980, for review see: Cowin et al., 1985; Geiger et al., 1985). There are only a few reports on the occurrence of intermediate filaments (IF) in the uterine epithelium. Franke et al. (1986) have studied IF in the proliferative phase of the human endometrial epithelium. Khong et al. (1986) concentrated on the expression of IF in the placenta, amniochorion, and placental bed in humans and Dabbs et al. (1986) used IF as diagnostic tools for histologic differentiation of uterine adenocarcinomas. Viale et al. (1988) described coexpression of vimentin and keratin in endometrial glands.

Lacunae are maternal blood vessels, bordered by trophoblast cells, which begin to develop during late implantation. They are typical of hemochorial placentation. As shown in this comparative study by high resolution light microscopy and SEM micrographs of vessel casts, lacunae are formed from the preimplantational endometrial vasculature by the loss of vessel walls and partly by changing of the vessel’s architecture.