Development of the bovine placentome from days 20 to 29 of gestation.

Nulliparous heifers (N = 21) were killed at each day between Days 20 and 29 after insemination. Placentomes were collected from the region near the embryo and prepared for examination by light electron microscopy. At Day 20 placentomes could be recognized as discrete oval structures on the surface of the uterine lumen. At Day 20-21, the trophoblast was in close apposition to the caruncular epithelium with microvilli from the maternal cells indenting the apical border of the embryonic cells. Mutual interdigitation of microvilli was present by Day 24 and this progressed to intimate attachment by Day 27. In the early states, the maternal cell types ranged from tall columnar to very thin, elongated cells with some excessively large multinucleate cells, but became uniformly cuboidal by Day 29. Trophoblast giant cells, without microvilli, were observed in contact with the maternal epithelium at all stages examined.