Fig. 2. Expression of the myocardin cardiac and smooth muscle enhancer. Embryos and adult tissues from the stable transgenic line harboring construct 5 (MyE1) were stained for ß-galactosidase activity, sectioned and counterstained with Nuclear Fast Red. Note intense staining in the cardiogenic plate at E7.5 (A) and in the bilaterally symmetric cardiac mesoderm at E7.75 (B). At E8.5, expression is detected throughout the primitive myocardium, including the endocardial tissue lining the primitive heart (C). At E12.5 expression is seen in the ventricles and atria (D,E). Note that the endocardial cushion is devoid of lacZ expression. Using dark-field microscopy (E,F) lacZ expression could clearly be observed in the dorsal aorta at E12.5 (E,F). In adult vascular tissues (G-J), lacZ expression was detected in smooth muscle cells of the lung arteries (G) and aorta (H). A cross-section through the esophagus revealed strong expression in the vessel wall of small arteries but not veins (I). A high magnification of adult heart shows strong expression in cardiomyocytes (J). a, artery; ao, aorta; b, bronchus; cm, cardiac mesoderm; cp, cardiogenic plate; ec, endocardial cushion; ra, right atria; rv, right ventricle; sm, splanchnic mesoderm.