Growth and development of the placenta in the capybara (Hydrochaeris hydrochaeris)

Material was collected from six animals at pregnancy stages ranging from the late limb bud stage to mid gestation. Methods included latex casts, standard histology, immunohistochemistry for cytokeratin, vimentin, alpha-smooth muscle actin, and proliferating cell nuclear antigen as well as transmission electron microscopy.At the limb bud stage, the placenta was a pad of trophoblast covered by a layer of mesoderm from which fetal vessels were beginning to penetrate at folds in the surface. By 70 days, the placenta comprised areas of labyrinth (lobes) separated by interlobular areas. Placental growth resulted predominantly from proliferation of cellular trophoblast situated in nests at the fetal side of the placenta and along internally directed projections on fetal mesenchyme. Additional proliferation was demonstrated for cellular trophoblast within the labyrinth.Already at the limb bud stage, there was a prominent subplacenta comprising cellular and syncytial trophoblast with mesenchyme and associated blood vessels. At 90 days, differentiation was complete and similar to that seen in other hystricognath rodents. Overlap of fetal vessels and maternal blood lacunae was confirmed by latex injection of the vessels. At all stages extraplacental trophoblast was associated with the maternal arterial supply and consisted of cellular trophoblast and syncytial streamers derived from the subplacenta.All important characteristics of placental development and organization in the capybara resembled those found in smaller hystricognath rodents including the guinea pig. These features apparently do not dependent on body size. Clearly, placentation in hystricognaths adheres to an extraordinarily stable pattern suggesting they can be used interchangeably as models of human placenta.Rodents are useful models for human reproduction due to the ready availability of laboratory animals [1] and their closeness to the primate lineage [2-4]. Although four suborders are recognized, most specie