Monday, 18 March 2013

Placentation in anteaters

Giant anteater (Myrmecophaga tridactyla) at Lisbon Zoo

The armadillos, sloths and anteaters of South America (Xenarthra) are of interest because they occupy a basal position in the tree of placental mammals. Studies on anteater placenta are few. Therefore it is exciting to find a paper that applies modern techniques to specimens from two species of anteater.

Villi of a giant anteater placenta
© 2012 Mess et al.; licensee BioMed Central Ltd.

In common with armadillos, anteaters have a villous placenta with a true intervillous space. Thus there is a superficial resemblance to the villous placentas of Old World monkeys, great apes and humans. No other mammal is known to have a definitive placenta of this type.   

The origin of the intervillous space in these mammals is different from in primates. Even before pregnancy the uterine wall contains an extensive network of venous sinuses. In armadillos the placental villi grow into the blood sinuses and the sinuses supply the intervillous space – a process described in detail by Enders and others.

Almost forgotten is the work of Becher on anteater placentas in which he showed an intervillous space first appeared between the villi and only later established contact with the maternal blood sinuses (Gegenbauers Morphol. Jahrb. 67:381-458, 1931). Thus there are subtle differences between placentation in armadillos and anteaters.

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