What we have learned about placental circulation.

Progress in our knowledge of placental structure and function commenced with the Hunterian establishment of the independence of the maternal and fetal circulation and of the existence of spiral arteries traversing the uterine wall. Among the 19th-and early-20th-century landmarks were the development and progressive modification of the Grosser classification of placental types, recognition of cyclic uterine changes and demonstration of the architecture and histology of the uteroplacental vasculature. Recently attention has focused on the trophoblast, making use of the current technology of electron microscopy, radiology, ultrasound, immunohistochemistry and fluorescence. New information on trophoblastic types, origin, and erosive and secretory activity has come to light.