The development of the placental lobular pattern in the human. Review and reinterpretation of the material.

The human placental lobule is a dense mass of mostly terminal villi, with a nearly villus-free center. The villi arise from whatever stem is nearby, and branches of these stems anchor near uteroplacental arteries at the base of the loose interlobular areas. There is no one-to-one relation of lobules either to the principal villous stems, or to maternal arteries. In the first trimester the villi are nearly uniform in size and anchor over the entire palcental floor, but better vascularized villi, the later stems, can already be found to anchor near maternal arteries. Thus, a network of uteroplacental arteries with nearby anchoring stems forms on the placental floor during the second month. It becomes accentuated during the following weeks as trophoblastic destruction spares the endometrial stroma around the arteries, and thus leads to the formation of elevations of the decidua, containing arterial ostia and anchoring points of villous stems. The remaining, smaller villi lose their connection with the decidua. By the end of the second trimester, the definitive lobular pattern is complete. How the dense lobules form above the interstices of this network is not known.