The classic complete hydatidiform mole is readily recognized. It is composed of free vesicles (“bunches of grapes”) without recognizable placenta or fetus (Fig. 111-6). All villi are dilated to varying degrees, ranging from a few millimeters to several centimeters. Histologically, the villi are largely avascular, and larger ones show central cisternae. No blood is present in the occasional vessel. Proliferation of cytotrophoblast and syncytiotrophoblast is present on the surface of the villi. This shows a haphazard arrangement without the typical cap of syncytiotrophoblast on cytotrophoblast noted on the young avascular villi of an early pregnancy. Free aggregates of trophoblast are also noted in the intervillus space. An exaggerated placental site reaction with numerous giant cells in the myometrium is often present.4,5,6 and 7

A syndrome of partial hydatidiform mole has been distinguished from complete mole. In these, the villus tissue is organized into a placenta, and a fetus or embryo is often found. The villi display a variable pattern. Some show hydatidiform change, whereas others are small. Capillaries are frequently present and may contain blood. The surface trophoblast, predominantly syncytiotrophoblast, is somewhat hyperplastic. The outlines of the villi tend to be irregular and map-like, leading to the frequent presence of stromal trophoblastic inclusions on section (Fig. 111-7).