Normal placental development. Extravillous cytotrophoblasts proliferate in anchoring columns to successful invade through the decidua (1) and transform the distal spiral arteries (2). These changes mediate high volume flow at low pressure into the intervillous space (3). The placental villi are covered by the villous trophoblast compartment (4), comprising cytotrophoblasts that proliferate to generate the outer syncytiotrophoblast in direct contact with maternal blood.