Identification of placental-specific antigens in ruminants

© 1998 Journals of Reproduction and Fertility 1359-6004/98 $12.50 The placentation that occurs in ruminant ungulate species, such as cattle, sheep and goats, is superficial, relatively noninvasive, and can be referred to as synepitheliochorial cotyledonary (Wooding, 1992). ‘Synepitheliochorial’ describes the fetal– maternal syncytium formed by the fusion of trophoblast binucleate cells and uterine epithelial cells, whereas ‘cotyledonary’ describes the gross structure of the placenta and specifically the tufts of villous trophoblast (cotyledons) that insinuate themselves into the crypts of the maternal caruncles. These regions of fused fetal and maternal tissue are referred to as placentomes and are the main sites for nutrient and gas exchange in the placenta. The trophectodermal (chorionic) epithelium of the ruminant placenta contains two distinct cell populations: binucleate cells, which represent 15–20% of the total cell population within the mature placenta, and mononucleate cells, which comprise the rest (Wooding, 1983). Binucleate cells have two unique characteristics. First, they migrate from the trophectoderm and fuse with uterine epithelial cells to form a syncytium. In sheep, the syncytium is rather extensive and persists throughout pregnancy. However, in cattle, the syncytium that is formed during implantation is subsequently replaced by an intact epithelium by day 40. After this, the migration and fusion of binucleate cells is limited to the generation of short-lived trinucleated cells (Fig. 1; Wooding, 1992). This fusion event is the extent of invasive implantation in these species, but it allows the conceptus to establish a more direct endocrinological, and presumably immunological, dialogue with the mother than is observed in species such as the pig, where no erosion of the uterine epithelium occurs. Indeed, the second unique characteristic of the binucleate cells is their ability to deliver their secretory products, which are concentrated in dense granules, directly to the maternal system. Within these dense secretory granules are the placental lactogens (Duello et al., 1986) as well as many of the pregnancy-associated glycoproteins, the subject of this review. Identification of placental-specific antigens in ruminants