Bioscientifica Proceedings

Summary. The defining feature of ruminant placentation is the fusion of binucleate trophoblast cells with uterine epithelium. It was present in the last common ancestor of ruminants and the fusion process is facilitated by the products of endogenous retroviral genes called syncytins. It provides a mechanism to transfer placental hormones to maternal tissues. One of these hormones is placental lactogen, which likewise was present in the ancestral ruminant. An innovation in the pecoran lineage was the placentome, which enabled the exchange area to be increased compared with the diffuse placenta of chevrotains. Duplication of a hemoglobin gene and evolution of a fetal hemoglobin to improve oxygen transfer probably occurred later. Other gene duplications enabled elaboration of the endocrine repertoire of the placenta and occurred at various stages in the evolution of ruminants. The binucleate trophoblast cells express MHC Class I antigens and can be expected to elicit a maternal immune response. A balance needs to be established whereby the semiallogeneic trophoblast is tolerated whilst maintaining vigilance against infection. Uterine macrophages develop along a pathway where they become immunosuppressive, whereas lymphocytes in the uterine epithelium between placentomes remain primed to respond to pathogens. It cannot be determined how such responses evolved, however, due to the paucity of information on the immune system of the uterus in ruminants and other artiodactyls.

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