In situ hybridizations on first trimester human placenta. In situ hybridizations on first trimester human placenta showing EPCR (A), CD39 (B), and PN1 (C) expression on trophoblast cells. Panel D shows negative stain with sense control. Bar represents 0.05 mm. EPCR and CD39 are expressed on cytotrophoblast-synctiotrophoblast bilayers of placental villi. PN1 is expressed at a different site on columns of cytotrophoblast cells.