Figure 2

Immunohistochemical analysis of syncytin 2 (HERV-FRD Env) in human placenta. Top panel. First trimester floating villi (8 weeks of pregnancy). A. Syncytin 2 was detected in some cytotrophoblastic cells (CT). No immunostaining was observed in the syncytiotrophoblast (ST) and in the mesenchymal core (MC). Scale bar = 10 μm. B. This large magnification allows to clearly establish the cytoplasmic localization of syncytin 2 immunostaining in a pair of cytotrophoblastic cells. At this gestational age the cytotrophoblast consists of a continuous single layer of cuboidal cells beneath the syncytiotrophoblast. Scale bar = 10 μm. Arrowhead: non labeled CT, arrow: positively stained CT. Middle panel. Second trimester placenta (16 weeks of pregnancy). C. Immunostaining with anti-syncytin2 antibody shows positive reactivity in some cytotrophoblastic cells. No syncytin 2 reactivity was detected in the extravillous trophoblast (ECT), in the syncytiotrophoblast and in the mesenchymal core. Scale bar = 10 μm. D. In this floating villi, syncytin 2 immunostaining was observed in the cytoplasm of some cytotrophoblastic cells (arrow), in their thin cytoplasmic processes (star) and at the level of the trophoblastic basal lamina (double head arrow). Scale bar = 10 μm. Bottom panel. Term placenta floating villi. E. Syncytin 2 was detected in the cytoplasm surrounding the nuclei of flat cytotrophoblastic cells and in their thin elongated cytoplasmic processes. Staining was absent from some villi. Scale bar = 10 μm. F. This large magnification allows to clearly establish the syncytin 2 immunostaining continuity within cytotrophoblasts between the cytoplasm surrounding the nuclei and that of the thin cytoplasmic processes. Scale bar = 10 μm.