Partial hydatidiform mole histology

In this example of partial hydatidiform mole, the associated fetus was triploid and had syndactyly of the third and fourth digits involving the hands. At lower magnification of the placenta, the most distinctive feature is presence of two distinct populations of chorionic villi based on size and morphology. Villi of the two populations are randomly intermixed. The smaller villi are relatively normal histologically, but the larger villi are irregularly shaped with scalloped contours (eg, see lower left corner). Because of their architectural complexity, the villous surface can fold into the stromal interior, forming trophoblastic inclusions (arrows) that give the illusion of glands in two-dimensional histology sections. Trophoblasts covering the villous surface proliferate roughly in parallel with villous size, thus increasing their cell number (hyperplasia). In some of the largest villi, the stroma breaks down, forming fluid-filled cavities (C), earning its names as "partial" and "hydatidiform mole."