Expression of TGFβ2, ALK2, ALK5, and Smad6 in PE. Representative sections are depicted that reflect the pattern of expression from stages 14–20. A,B, At stage 18, TGFβ2 mRNA is seen in cells within the body of the PE (asterisks) and in cells forming the primitive epicardium (arrowheads). C, D, ALK2 mRNA in the PE (asterisk) and endocardium (arrow) at stage 18. E, F, ALK5 and G, H, Smad6 are both expressed in the PE (asterisks), shown here at stages 18 and 19, respectively. I, RT-PCR was performed as described in “Experimental Procedures” to amplify specific regions of the mRNAs encoding TGFβ1, TGFβ2, ALK2, ALK5, Smad6 and GAPDH from PEs. GAPDH was not appreciably amplified when the reaction was performed without reverse transcriptase enzyme (-RT) or template (-RNA). All photomicrographs at 200x. avc, atrioventricular cushion; pe, proepicardium; sv, sinus venosus