Comparative histological study of ruminant placentomes

Abstract

Histological structures of placentomes and cotyledons were studied in eight antelope species (Uganda kob, common duiker, common eland, greater kudu, wildebeest, sable antelope, steenbok, impala) and domestic cattle. A few samples were also available from other species such as bongo, sitatunga, nyala, deer, giraffe, okapi, domestic sheep, and a surrogate heifer carrying a gaur embryo after interspecies embryo transfer. The common pattern of placentome development started with the apposition of chorion to caruncles and development of the chorionic "milky patches" in contact with caruncles. The patches had thick epithelium and mesenchymal blood vessels. Chorionic villi proliferated from the patches and entered the preformed cryptal network in caruncles. Penetration of villi proceeded in a zone until they reached the basal layer separating placentomes from glandular endometrium. Further growth of placentomes continued by increasing the villi length, diameter, branching, and surface corrugation. All species studied had epitheliochorial type of placenta. Differences among species were observed in the degree of villi branching and surface corrugation and in the epithelial or syncytial type of lining in maternal crypts. The eight species studied were ordered by increasing complexity of their utero-placental junction. Uganda kob had the simplest structure with minimal villi branching, round to oval villi cross-section, and clearly epithelial crypt lining. Common duiker had minimally branched villi with polygonal cross-section and epithelial linin g of crypts. Common eland and greater kudu had moderately branched villi with polygonal to mildly corrugated cross-sections. Crypt lining was epithelial with slight tendency toward formation of a syncytium. Wildebeest and sable antelope had moderately branched villi with corrugated surface and rather syncytial crypt lining. Steenbok and impala had the most complicated branching of villi and corrugation and their crypt lining was clearly syncytial. Placental structure has implications for interspecies embryo transfer. Bongo and eland (successful donor-recipient pair in embryo transfer) had very similar placental structure. Interspecies transfer of gaur embryos into heifers showed problems with placental development. Giraffe and okapi had very different placental structures that may be an obstacle for intended embryo transfer between them.