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Placentation in the anteaters Myrmecophaga tridactyla and Tamandua tetradactyla (Eutheria, Xenarthra)

Andrea M Mess1*, Phelipe O Favaron1, Christiane Pfarrer2, Christine Osmann3, Allan PF Melo1, Rosangela F Rodrigues1, Carlos E Ambrósio4, Estela Bevilacqua5 and Maria A Miglino1

Author Affiliations

1 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

2 Institute of Anatomy, University of Veterinary Medicine Hannover, Bischofsholer Damm 15, 30173, Hannover, Germany

3 Zoo Dortmund, Mergelteichstr. 80, 44225, Dortmund, Germany

4 Department of Basic Science, Faculty of Animal Sciences and Food Engineering, University of Sao Paulo, Av. Duque de Caxias Norte, 225, ZAB, Pirassununga, CEP 13635-900, Brazil

5 Institute of Biomedical Sciences, University of Sao Paulo, Av. Prof. Lineu Prestes, 1524, Cidade Universitária, São Paulo, SP, CEP 05508-900, Brazil

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Reproductive Biology and Endocrinology 2012, 10:102  doi:10.1186/1477-7827-10-102

Published: 30 November 2012



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.


Placentas of two species were investigated with histology, immunohistochemistry and transmission electron microscopy.


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 occured near the surface near the end of gestation. The surface of the villi consisted of flat, syncytial trophoblast, interspersed with proliferative trophoblast cells.


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.

Evolution; Vermilingua; Trophoblast; Interhaemal barrier; Villous placenta