Development of the intercaruncular areas during early gestation and establishment of the bovine placenta.

Specimens were obtained from the uteri of heifers on Days 17, 18, 20 to 30 and at 3-day intervals up to Day 45 of gestation and this material was examined by light microscopy, transmission and scanning electron microscopy. A reduction in height of the uterine epithelium on Day 18 was the first detectable maternal response to the presence of the blastocyst within the lumen. The trophoblast could be preserved in contact with the uterine epithelium by Day 20 and the attachment became progressively more complex between Days 21 and 30. Attachment occurred simultaneously in the intercaruncular and caruncular regions near the embryo and subsequently spread towards each end of the blastocyst. Agranular leucocytes were present in the uterine epithelium until Day 22 but were almost totally absent in later specimens.