Placental Evolution within the Supraordinal Clades of Eutheria with the Perspective of Alternative Animal Models for Human Placentation

Figure 2

Placentation in Rodentia, derived from [32, 36, 223]. ((a), (b)) Necromys lasiurus (Muridae, Sigmodontinae = New World mice). The placenta consisted of the junctional zone and the labyrinth. The latter contains fetal vessels (arrows) and the trophoblast lining the maternal blood spaces. Immunostaining: vimentin. (c) Transmission electron microscopy (TEM) in Euryoryzomys sp. from the same group. The placental barrier is built by three layers of trophoblast above the endothelium of the fetal capillaries. (d) Galea spixii (Caviomorpha, Caviidae), TEM. The barrier between the maternal blood spaces and the fetal capillaries is thin and syncytial. Cellular trophoblast is present towards the inside. ((e), (f)) Trophoblast invasion in another caviid species, the capybara Hydrochaeris hydrochaeris, TEM, immunostaining for cytokeratin. Extravillous trophoblast giant cells and syncytial streamers have migrated towards the spiral arteries in the decidua and started to remodel their walls. The endothelium of the maternal blood system is discontinuous. d = decidua, EC = endothelium of fetal capillary, end = endothelium of the maternal system, fc = fetal capillary, Jz = junctional zone, LAB = labyrinth, MBS = maternal blood space, syn = syncytial trophoblast, TI–TIII = trophoblast layers I to III, and tgc = trophoblast giant cells.

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