THE MORPHOLOGY OF BLASTOCYST IMPLANTATION

Implantation is a process whereby the blastocyst becomes embedded in the endometrium. Firstly, the blastocyst must contact the epithelium, then the trophoblast cells will start invading the endometrium. In species with delayed implantation, the blastocyst requires activation before its attachment. Thus there are three stages involved in delayed implantation: blastocyst activation, trophoblast attachment and trophoblast invasion. The following account indicates what characterizes these three stages at a cellular and subcellular level and what suggestions structural analysis can give as to the mechanisms controlling these processes. DELAYED IMPLANTATION The blastocysts of some mammalian species are normally kept in a state of delay before implantation (Enders, 1963; Lanman, 1970). In some other species, for instance the mouse, a delayed implantation also can be obtained experimentally (Yoshinaga & Adams, 1966; Humphrey, 1967; Smith & Biggers, 1968; McLaren, 1971). This offers a biological