Trophoblast differentiation in late cultures from normal (NP; PT1) and abnormal placentae (AP; PT3). Cells were grown in complete DMEM-HG, which contains 45 μM phenol red (PhR+), or phenol red-free DMEM-HG without estrogens (PhR-/E2-) of with 20 nM E2 (PhR-/E2+). Each medium was supplemented with 5% heat-inactivated FBS. *indicates vesicles and vacuoles characteristic of multinucleated syncytiotrophoblast ; s, small; m, mid-sized; l, large; xl, extra large syncytial aggregates (dashed line). Short arrows = dividing/post-mitotic trophoblasts; long arrow, adjacent trophoblast cells with preservation of cell membranes; arrowhead, isolated trophoblast cells. Details in Materials and Methods and in the text.