Ultrastructural features of the cell surfaces of uterine and trophoblastic epithelia during embryo attachment in the cow.

Ultrastructural features of the bovine uterine and trophoblastic epithelial cell surfaces were studied by scanning electron microscopy (SEM) at the time of attachment. Apical cytoplasmic protrusions were observed on the uterine cell surface from cyclic animals on days 12-16 during the luteal phase and disappeared thereafter. However, in pregnant animals, these cytoplasmic protrusions were observed until day 21 (attachment stage). These structures suggest that the uterine cells possess secretory and/or endocytotic properties. Before the attachment stage the trophoblastic cell surface was uniformly covered by slender microvilli. At the beginning of conceptus attachment the microvilli disappeared and the trophoblastic cell surface became smooth. On areas of the conceptus facing uterine gland openings, papillae developed and filled the glandular lumen. These were interpreted to be a system by which the conceptus is immobilized on the uterine epithelium and/or a histotrophic mechanism for absorbing glandular secretory products.