It is accepted that the implantation consists of establishment of contacts between the trophoblast cells of blastocystic ring and the uterine epithelium. Prior to the contact with uterine epithelium the blastocyst undergoes a series of physiological and developmental changes. The formation and differentiation of blastocystic ring and its ultrastructural and cytochemical aspects have been extensively studied. The stages of blastocystic ring formation, preimplantation, epithelial penetration, and related biochemical events vary significantly among different species of mammals.
Biomedical Reviews 1997; 8: 127-132.
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