An electron microscopic study of implantation in the cow.

The paper describes the changes occurring in the uterine and chorionic epithelia of the cow between days 18 and 28 of gestation, before and during implantation. The irregular uterine epithelium of the non-pregnant cow was simplified to a tall columnar epithelium before attachment began. Areas of attachment were first observed at day 20 in the region of the embryo. Such sites were characterized by the presence of pale uterine cells containing up to 8 nuclei ("giant cells") which accounted for nearly 50% of the epithelial area by day 24. Fetal binucleate cells were found in contact with, and partly across, the microvillar junction at all stages examined, and the presence of granules characteristic of binucleate cells within the giant cells suggested that the fetal cells contributed to their formation. Many uterine epithelial cells underwent degenerative changes between 22 and 28 days, and residues equivalent to their pyknotic remnants were found at the microvillar junction and within the mononucleate chorion cells. By day 28 some of the giant cells appeared to be degenerating, and areas of low cuboidal epithelium were present. This epithelium resembled the type found in the mature placenta. There was evidence that binucleate cell migration continued at this time. Nevertheless, the definitive membrane in the mature bovine placenta is epitheliochorial.