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There are two local subtypes of extravillous trophoblast (EVT): one is the proliferative phenotype of EVT, which primarily consists of proximal cells and the other is the invasive phenotype of EVT, which is composed mainly of distal cells of cell columns. The mechanism of invasion of EVT to the decidua remains obscure. In order to elucidate the potential role of apoptosis along the invasion of EVT to the decidua, we have assessed the expression of apoptosis-regulating proteins including Fas antigen (Fas), Fas-ligand (Fas-L) and Bcl-2 protein, and apoptosis in the proliferative phenotype of EVT and the invasive phenotype of EVT in term (37 to 38 wk) placenta Fas, Fas-L and Bcl-2 protein expression were examined by avidin/biotin immunoperoxidase method. Apoptosis was assessed by in situ DNA 3'-end labeling method. Appearance of apoptotic nuclei in EVT was also examined by transmission electron microscopy. Mean percentage of apoptosis-positive nuclei in the invasive phenotype of EVT was significantly higher than that in the proliferative phenotype of EVT. Transmission electron microscopy revealed the presence of apoptotic nuclei in the invasive phenotype of EVT. Immunohistochemical analyses revealed that Fas and Fas-L expression in the invasive phenotype of EVT were more abundant than those in the proliferative phenotype of EVT, while Bcl-2 protein expression in the invasive phenotype of EVT was less abundant than that in the proliferative phenotype of EVT. The present findings suggest that Fas/Fas-L and Bcl-2 protein expression participate in the regulation of apoptosis in EVT along the invasion to the decidua, and that the increased occurrence of apoptosis in the invasive phenotype of EVT may be attributable to the increased expressions of Fas and Fas-L and decreased expression of Bcl-2 protein in those cells in term placentas.