ABSTRACT
Implantation (nidation) is penetration of the trophoblast of the blastocyst into the compact functional layer of the endometrium. During implantation:
(1) The blastocyst attaches by the polar trophoblast to the epithelium of the endometrium; (2) The blastocyst collapses, losing fluid, and reexpands after implantation; (3) In contact with the endometrium, the trophoblast proliferates. The proliferating cytotrophoblast of the
blastocyst changes into the trophoblastic shell which consists of intermingled cytotrophoblast and syncytiotrophoblast (days 7-11).
