Reasons for Diversity of Placental Structure

Hemomonochorial placentas. A. Note the cytotrophoblast (Langhans) cells underlying the syncytial trophoblast in this villous hemomonochorial placenta of the human. B. Mature villus in the hemomonochorial placenta of the armadillo. Note the epithelioid mesenchymal cells. Inset: Growing villi in a mid-term placenta. Note that the cytotrophoblast cells (ct) are only at the tip of the villus. C. Midgestation labyrinthine placenta of the marmot, a sciuromorph rodent. Note the intrasyncytial bays (arrow). D. Labyrinth of the guinea pig, a hystricognath rodent. Syncytial trophoblast lines the maternal blood space but there are no intrasyncytial bays. (Transmission electron microscopy is definitive in determining the syncytial or cellular nature of trophoblast lining maternal blood spaces and in confirming the presence or absence of intrasyncytial bays.) fc = fetal capillaries. All micrographs except the inset are at the same magnification. Scale bar = 12 μm. Inset: 31 μm.

Hemomonochorial placentas. A. Labyrinth of a midgestation placenta of the hyena, showing intrasyncytial bays (arrows). The hyena placenta has a number of different hemophagous regions (not shown). B. Labyrinth of the placenta of a molossid bat, Tadarida. This cellular hemomonochorial placenta has intratrophoblast bays (arrows) that are difficult to see with light microscopy. C. Placental labyrinth of the jumping mouse, Zapus. The cytotrophoblast (ct) lining maternal blood spaces does not have intratrophoblast bays. D. Placental labyrinth of the hedgehog tenrec, Echinops. The cytotrophoblast (ct) lining maternal blood spaces lacks intratrophoblast bays. In addition to the labyrinth there is a hemophagous region (not shown) in this placenta. Scale bar = 12 μm.

Electron micrographs of interhemal regions. A. Labyrinth of a sciuromorph rodent showing the intrasyncytial bays and contained laminar material (arrows). B. Placental labyrinth of a molossid bat, Tadarida, showing intratrophoblast bays and lamina (arrows) in a cellular hemomonochorial placenta. mbs = maternal blood space. Scale bar: A = 2 μm, B = 4 μm.