IMMUNOHISTOCHEMICAL LOCALIZATION OF ENDOTHELIAL NITRIC-OXIDE SYNTHASEIN HUMAN VILLOUS AND EXTRAVILLOUS TROPHOBLAST POPULATIONS AND EXPRESSION DURING SYNCYTIOTROPHOBLAST FORMATION IN-VITRO

We have examined the distribution of the endothelial isoform of nitric oxide synthase (eNOS) in villous and extravillous trophoblast populat ions by immunohistochemistry and have further studied expression of eN OS during differentiation of cytotrophoblast into syncytiotrophoblast in culture. In first trimester villous tissue, NADPH diaphorase activi ty and eNOS immunostaining were present in syncytiotrophoblast but not the progenitor cytotrophoblast layer. Extravillous trophoblast in the basal plate of the placenta was identified by anticytokeratin immunos taining and displayed NADPH diaphorase activity, but not eNOS immunost aining. Both amnion epithelial cells and chorion cytotrophoblast had N ADPH diaphorase activity but no eNOS immunostaining, whereas eNOS immu nostaining was seen in the fibroblast layer of amnion. Purified villou s cytotrophoblast cells from term placentae aggregated and fused to fo rm a syncytium with increasing time in culture as assessed by antidesm osomal protein and antinuclear antibody immunostaining. Following 24 h in culture, the majority, of cells were still mononucleate cytotropho blast which did not display eNOS immunostaining, whereas a few syncyti al aggregates had formed which were both eNOS positive and hPL positiv e. By 3 to 5 days in culture, the majority of cells were present as sy ncytiotrophoblast. However, eNOS and hPL immunostaining was more diffu se and not all syncytial aggregates were positive. Of the trophoblast populations, only syncytiotrophoblast appears to express eNOS. Differe ntiation of cytotrophoblast into syncytotrophoblast is associated with eNOS expression.