Localization of 17βHSD2 and KDR proteins in mid-gestation placentas. Tissue sections of human placentas obtained at 19 weeks (Panels A and B), 18 weeks (Panel C), 10 weeks (Panel D) or 20 weeks (Panels E-H) of pregnancy were immunostained with anti-17βHSD2 (Panels A, C-F), with pre-adsorbed antibody, as negative control (Panel B), or with anti-KDR antibody used as endothelial cell marker (Panels G and H). 17βHSD2 protein expression is observed in immature intermediate villi (IIV) (Panels A, C, E, F), in capillary endothelial (CE) cells and in cells forming the stromal channel (SC) (Panel C). Positive cells (most probably endothelial cells (EC)) were found in mesenchymal villi (MV). No signal was observed in trophoblastic sprout (TS), in syncytial layer (SL), and in decidua (DC) (Panel A). In stem villi (SV) (Panel A), EC of large blood vessels and those of paravascular capillary (PC) network are positive. Endothelial cells forming PC network in SV (Panel G) and those composing the wall of the stromal channel were immunostained by the anti-KDR (Panel H). Intervillous space (IS), where maternal blood circulates, is indicated.