Fig. S1.

Expression of Pref-1 in embryonic epicardium. (A) Mosaic immunofluorescence images of transverse sections, including whole heart, reveal strong Pref-1 expression in the mesothelium at E12.5. Images of sections from the same embryo at a higher magnification, with additional α-actinin immunostaining, show strong expression of Pref-1 in the genetically labelled epicardium. [Scale bars: 500 µm (whole hearts); 50 µm (images of ventricle and atria).] (B) In E16.5 heart, Pref-1 is strongly expressed in the epicardium and in some EPDCs of the ventricle. (Insets) In particular, Pref-1 is strongly expressed by atrial epicardial cells (n = 3). (Scale bars, 50 µm.)