Fig. 3. Expression of Foxp1 protein during cardiac development. A Foxp1-specific antibody (Lu et al., 2002) was used to determine Foxp1 protein expression at E9.5 (A,B), E11.5 (C,D), E12.5 (E,F,I,J), and E14.5 (G,H,K,L) in the heart and outflow tract vessels. Foxp1 expression is observed in the ventral aspect of the foregut endoderm and the myocardium (A,B, arrows). Foxp1 expression is observed throughout the heart, including myocardium, endocardium and endocardial cushion tissue at E11.5 (C,D). Foxp1 is expressed in cells underlying the mesenchyme of the forming endocardial cushion (C, arrows). Foxp1 is expressed in a decreasing gradient from the compact zone to the trabecular zone as early as E11.5 through E14.5 (D-F,H; note arrows in E,F and H showing decreased expression in trabecular myocardium). By E14.5, expression in endocardial cushion mesenchyme is extinguished but remains high in the overlying endocardial cells (G, arrow). Foxp1 expression is observed in the endothelium of the aorta and pulmonary artery at E12.5 (I) and E14.5 (K). Foxp1 expression is also observed in the VSMCs of these vessels at E12.5 and E14.5 (J,L). The wall of the ductus arteriosus can be observed in J,L. FE, foregut endoderm; myo, myocardium; endo, endocardium; Ao, aorta; Pa, pulmonary artery. Scale bars: 200 µm in A,G,H; 100 µm in B,C-E,I-K; 50 µm in F, 150 µm in L.