Placental alkaline phosphatase expression at the apical and basal plasma membrane in term villous trophoblasts

Human placental alkaline phosphatase (PLAP) was localized at the apical and basal plasma membrane of syncytiotrophoblasts and at the surface of cytotr ophoblasts in term chorionic villi using immunoelectron microscopy. Similar ly, apical and basolateral PLAP expression was found in polarized trophobla st-derived BeWo cells. Trophoblasts isolated from term placentas exhibited mainly vesicular PLAP immunofluorescence staining immediately after isolati on. After in vitro differentiation into syncytia, PLAP plasma membrane expr ession was upregulated and exceeded that observed in mononuclear trophoblas ts. These data call for caution in using PLAP as a morphological marker to differentiate syncytiotrophoblasts from cytotrophoblasts or as a marker enz yme for placental brush-border membranes.