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The placenta is an organ that is composed of fetal and maternal components. The fetal components of the placenta are formed by cells of the trophoblast and cells that are derived from the epiblast. The epiblast-derived components are the amnion, the umbilical cord, and the mesodermal components of the chorion. The villous parenchyma makes up most of the placenta and consists of 40–60 trophoblast or chorionic villi, which is the functional unit of placenta where diffusion and active transport of nutrients and waste products takes place.
Rodent and human placentas both belong to the hemochorial-type placentas, where the maternal blood directly faces the trophoblast barrier, which separates the fetal from the maternal compartment. The complex architecture of the placenta is formed following embryo implantation in the endometrium. Implantation is a process comprised of three stages: first, free-floating apposition of the blastocyst, then, adhesion between the endometrium and trophoblast, and finally, the fine-tuned process of trophoblastic invasion and differentiation. This dialogue between fetus and maternal tissues is mediated through a broad array of molecules released at the implantation site both by the trophoblast and the endometrium.