The placental site nodule: an immunohistochemical study.

The placental site nodule and plaque (PSN-P) is a recently described, benign proliferation of intermediate trophoblast cells (ITs) in the endometrium or endocervix occurring after an intrauterine gestation. We performed an extensive immunohistochemical study of 11 cases of PSN-P. Cytokeratins (AE1/AE3 and MAK 6) were strongly positive in all cases stained. Epithelial membrane antigen (EMA) was positive in all cases, in 5% to 75% of lesional cells. Expression of human placental lactogen (hPL) was weak and focal, and a minority of cases were positive for human chorionic gonadotropin (hCG). More helpful in identifying the trophoblastic nature of the lesion was pregnancy-specific beta-1 glycoprotein (SP1), which was present in 100% of cases, and placental alkaline phosphatase (PLAP), present at least focally in 90% of cases stained. Vimentin was strongly positive in all cases stained. The presence of vimentin, SP1 and PLAP in PSN-P has not been documented previously. In our opinion cytokeratin, vimentin, and SP-1 are the most important monoclonal antibodies to aid in the differential diagnosis of PSN-P.