Placental villi are the functional elements of the placenta contained in the intervillous chamber and which, in contact with the maternal blood, allow fetomaternal exchanges. 

The trophoblast begins at the outer envelope of the early blastocyst and constitutes the supply route between the maternal endometrium and the developing embryo. Trophoblast adhesion to the uterine wall is the first step necessary for implantation and, subsequently, placentation. The human villous tryophoblasts (HVT) that cover the villi of the placenta provide the surface for oxygen and nutrient exchange with the maternal circulation. They synthesize and release chorionic gonadotropin, placental lactogen and angiogenin and express the CXCR4, CCR5 and prolactin gene families. They acquire CCR1 as they differentiate into an invasive phenotype at the villous anchor sites. The characteristics of HVT, together with the recent establishment of trophoblast stem cells, make it an ideal genetic platform to study cell differentiation and organogenesis.