Trophoblastic lesions of the placental site

The trophoblast of the chorionic villi as well as the hydatidiform mole and choriocarcinoma have been recognized and studied for many years. However, the trophoblast comprising the implantation site, chorionic plate, chorion laeve, cell islands and septa have only in recent years received attention in the literature. These "extravillous" trophoblastic cells were originally referred to as "X" cells due to doubt regarding their derivation from either maternal or fetal tissue (70). Subsequent studies determined that they were trophoblastic in origin (42), but the term X-cell is still in use today by some researchers (30, 4). Due to continued uncertainty regarding their nature and origin, many other terms have been used including syncytial wandering cells (21), placental site trophoblast, placental site giant cells (42), extravillous trophoblast (25), extravillous cytotrophoblast, nonvillous trophoblast, and intermediate trophoblast (49). Light microscopic and immunohistochemical studies have led to elucidation of specific morphologic and biochemical features of the extravillous trophoblast (48-50) which is commonly designated as the "intermediate trophoblast". Lesions of the "intermediate" or extravillous trophoblast of the placental site include the exaggerated placental site, the placental site trophoblast tumor and the recently described placental site nodule. This article will review the clinical and pathologic features of these lesions, their differential diagnosis, treatment, and prognostic factors after discussion of the origin, nature and definition of the "intermediate" trophoblast.