title: Placentation in the anteaters Myrmecophaga tridactyla and Tamandua tetradactyla (Eutheria, Xenarthra).
abstract: BACKGROUND
  Since Xenarthra are serious candidates for being basal to Eutheria, their characteristics, e.g. the placental system, influence perceptions of evolution. However, in the subgroup containing the anteaters, data are very limited. The present study aims to elucidate the nature of the feto-maternal interface in the anteater placenta and to interpret these data within an evolutionary context.
  
  
  METHODS
  Placentas of two species were investigated with histology, immunohistochemistry and transmission electron microscopy.
  
  
  RESULTS
  Remnants of the maternal vessel endothelium were absent, resulting in a fully haemochorial barrier throughout the placenta. Two structurally different parts, the villous and trabecular areas were complex and intermingled. In particular, the trabeculae which consisted of cellular, proliferative trophoblast, associated with connective tissue, were attached to the decidua. The villi contained fetal capillaries and hypertrophied mesenchymal cells that occurred near the surface near the end of gestation. The surface of the villi consisted of flat, syncytial trophoblast, interspersed with proliferative trophoblast cells.
  
  
  CONCLUSIONS
  Based on fundamental differences between anteaters and armadillos, we inferred that placental evolution was more complex than previously thought. The haemochorial pattern of anteaters was likely an ancient condition of xenarthrans. Consequently, villous placentation may be attributed, at least in part, by convergent evolution, but was also characterized by some features that were widespread among xenarthrans.
authors.affiliation: Department of Surgery, Faculty of Veterinary Medicine and Animal Science, University of Sao Paulo, Av. Prof. Dr. Orlando Marques de Paiva, 87, Cidade Universitária, São Paulo, SP, CEP 05508-270, Brazil. drandrmss@aol.com
authors.forename: Andrea M
authors.initials: AM
authors.lastname: Mess
authors.forename: Phelipe O
authors.initials: PO
authors.lastname: Favaron
authors.forename: Christiane
authors.initials: C
authors.lastname: Pfarrer
authors.forename: Christine
authors.initials: C
authors.lastname: Osmann
authors.forename: Allan P F
authors.initials: AP
authors.lastname: Melo
authors.forename: Rosangela F
authors.initials: RF
authors.lastname: Rodrigues
authors.forename: Carlos E
authors.initials: CE
authors.lastname: Ambrósio
authors.forename: Estela
authors.initials: E
authors.lastname: Bevilacqua
authors.forename: Maria A
authors.initials: MA
authors.lastname: Miglino
mesh_terms: D000818:Animals; D005260:Female; D010920:Placenta / Q000166:cytology*; D010929:Placentation / Q000502:physiology*; D011247:Pregnancy; D013045:Species Specificity; D004490:Xenarthra / Q000254:growth & development*