FIG. 3.

The HRC enhancer directs cardiac, skeletal, and arterial smooth muscle expression in transgenic mouse embryos. A 2,726-bp fragment of the human HRC gene was fused to a lacZ reporter plasmid and used to generate transgenic mice. This fragment of the HRC gene was sufficient to direct expression to all three muscle lineages in the mouse embryo in the pattern of endogenous HRC. (A to D) Representative X-Gal-stained transgenic embryos are shown at 8.5 dpc (A), 9.5 dpc (B), 11.5 dpc (C), and 13.5 dpc (D). The embryos in panels C and D have been cleared in a 1:1 mixture of benzyl alcohol and benzyl benzoate to help visualization of internal structures. Expression was evident at 8.5 dpc in the heart (hrt) and in the myotomal compartment of the somites (S) and by 11.5 dpc in arterial smooth muscle. No expression was observed in venous smooth muscle or in other smooth muscle cell types. (E to G) Transverse sections from X-Gal-stained transgenic embryos at 11.5 dpc. Expression was evident in somites, heart, and arterial vascular smooth muscle, including the dorsal aorta (DA) and branchial arch arteries (BAA). By contrast, expression was not observed in venous smooth muscle, including the cardinal veins (CV). No expression was observed in the smooth muscle of the trachea (Tr) or esophagus (Es) (G). Expression in the heart was restricted to the ventricles (E and F). Bar, 100 μm. (H) The heart and associated vasculature removed from an HRC-lacZ transgenic embryo at 13.5 dpc and stained with X-Gal. Expression in the heart at 13.5 dpc was restricted to the ventricles. Expression was also evident in arterial vascular smooth muscle, including the aorta (Ao) and subclavian (SubCL) and carotid arteries (Ctd). LA, left atrium; LV, left ventricle; NT, neural tube; RA, right atrium; RV, right ventricle. Seven independent transgenic lines all displayed nearly identical patterns of expression.