Placental site nodule and characterization of distinctive types of intermediate trophoblast

Abstract Both placental site nodule and exaggerated placental site are described as being composed of intermediate trophoblast (IT), yet their morphological features and clinical presentation differ significantly. This study was undertaken to evaluate the morphological and immunohistochemical features of trophoblastic cells in placental site nodules and compare them with the trophoblastic cells in exaggerated placental sites as well as in different anatomic locations in the developing placenta to evaluate these differences. Forty-two placental site nodules, 20 abortus specimens ranging from 3 to 13 weeks, 8 second- and 10 third-trimester placentas, and 12 exaggerated placental sites were studied by conventional light microscopy and immunohistochemistry. This analysis showed that the trophoblastic cells in the placental site nodule closely resemble those in the chorion laeve. We have designated these cells "chorionic-type IT cells." They are composed of two populations of cells, one with eosinophilic and the other with clear (glycogen-rich) cytoplasm. The eosinophilic cells tended to be larger with more pleomorphic nuclei, whereas the clear cells were smaller with more uniform nuclei. Chorionic-type IT cells in the chorion laeve and placental site nodule were diffusely positive for placental alkaline phosphatase but were only focally positive or negative for human placental lactogen (hPL), Mel-CAM (CD146), and oncofetal fibronectin. In contrast, hPL, Mel-CAM, and oncofetal fibronectin were diffusely expressed in IT cells in the placental site, both normal and exaggerated. The chorionic-type IT cells in placental site nodule and chorion laeve showed mild proliferative activity as indicated by an increased Ki-67 labeling index (3% to 10%). In contrast, the Ki-67 labeling index in normal and exaggerated implantation sites was zero. The morphological and immunohistochemical features of chorionic-type IT cells contrast with the IT cells in the implantation site that we have designated "implantation site IT cells." Both types of IT cells develop from a population of trophoblastic cells in the trophoblastic columns that we have tentatively termed "villous IT cells." Four of 42 placental site nodules were larger (>5 mm) than the remainder and showed transitional features between a typical placental site nodule and an epithelioid trophoblastic tumor, a recently described distinctive gestational trophoblastic tumor. There were no recurrences among the placental site nodules regardless of size. All placental site nodules were immunoreactive for inhibin-α and cytokeratin 18, whereas 33 squamous cell carcinomas of the cervix, which can at times be confused with placental site nodules, were negative. In conclusion, there appear to be three subpopulations of IT cells with distinctive morphological and immunohistochemical features. Different subpopulations can be related to different trophoblastic lesions: implantation site IT cells to an exaggerated placental site and its neoplastic counterpart, placental site trophoblastic tumor and chorionic-type IT cells to a placental site nodule and its neoplastic counterpart, epithelioid trophoblastic tumor.