Fig. 1. Comparison of human and mouse placentation.

(Left) Before placentation, the blastocysts of humans and mice are similar. (Middle and right) However, upon implantation, placental development progresses differently. (Top middle) After blastocyst implantation, the human syncytiotrophoblast layer burrows into the maternal decidua. By the third week of gestation, the definitive human placenta is formed and is composed of villous trees. However, at this stage of human pregnancy, the fetal-derived placenta does not directly contact maternal blood. (Top right) Extravillous trophoblasts anchor the villi to the decidua and are involved in the remodeling of the spiral arteries to flood the intervillous space with maternal blood toward the end of the first trimester of pregnancy. The surface of the villi are covered by the syncytiotrophoblast layer, which directly contacts the maternal blood and facilitates the transport of nutrients, gases, and waste across the placental barrier. Underlying the syncytiotrophoblast layer are mononucleated cytotrophoblasts that can either fuse to replenish the syncytial layer or differentiate into extravillous trophoblasts. By contrast, mouse placental development and organization is different from that in humans. (Bottom middle) Upon implantation, the trophectoderm differentiates, and trophoblast giant cells encapsulate the developing mouse embryo. Halfway through gestation, the definitive mouse placenta is fully formed and functional, where (bottom right) the folded villi form a labyrinth structure that becomes perfused with maternal blood. The trophoblast giant cells channel maternal blood from the decidua through the spongiotrophoblast layer (a structure not present in the human placenta) toward the labyrinth zone. In the labyrinth zone, the maternal blood makes contact with the cytotrophoblasts that overlay two separate layers of syncytiotrophoblasts. Credit: A. Kitterman/Science Immunology