Embryo attachment and implantation in the ewe have been examined with a view to (a) elucidating the interrelationships of the trophoblast and the uterine epithelium, and (b) defining accurately the time relationships of the various phenomena. Embryonic attachment occurred during the 15th day of pregnancy and cytological changes within the uterine caruncular epithelium that appeared on the following day and were completed within a week, resulted in the formation of plaques of multinucleate symplasm and the death of some maternal epithelial cells.
High levels of acid phosphatase activity, considered to demonstrate the release of lysosomal hydrolases, were associated with the modification of the maternal epithelium and with the later trophoblastic invasion of the uterine stroma. Alkaline phosphatase activity present in the apical cell membranes of the opposing trophoblast and uterine epithelium is considered to be associated with carbohydrate metabolism and the production of trophoblastic fibrinoid.
No evidence was obtained to support proposals that trophoblastic binucleate giant cells have a major role in modifying the uterine epithelium through their phagocytic activity or as a source of the cryptal epithelium. The cryptal epithelium during the first 6 weeks of pregnancy appears to be derived from the symplasmic plaques of maternal epithelium.