Ultrastructural aspects of the antimesometral implantation in the rabbit.

A scanning and transmission electron microscope study of blastocysts immediately prior to ovo implantation and of antimesometrial implantation sites was conducted. External membranes of the eggs showed at 6 days post coitum the imprint of the endometrial surface, pointing to the early establishment of contact between egg and uterus. At 7 days the antimesometrial region showed flattening with continued evidence of gland openings that appear to be the elective sites of attraction of the trophoblastic knobs. Attachment of the trophoblastic knobs followed on days 8 and 9.