Fig. 2. SMC differentiation is induced in lateral plate mesoderm-derived cells on the ventral side of the dorsal aorta at E9.5. (A-D) lacZ staining of stage- and location-matched sections from E9.5 Hoxb6-cre/R26 reporter and SM22-lacZ transgenic mice at three levels between the forelimb and hindlimb regions. SM22-lacZ was expressed around the full circumference of the aorta in the anterior sections (A), but was confined to the ventral side in more posterior sections (B,C). The Hoxb6-cre-activated R26 reporter gene was expressed in splanchnic and somatic mesoderm, but not in intermediate mesoderm (D). The reporter gene was expressed in all cells on the ventral side of the aorta, including the endothelial cells (arrowheads in A-C). The reporter was also expressed in a single layer of cells on the lateral and dorsal side of the aorta (arrows in A-C). (E-G) TEM of ultrathin sections from lacZ-stained E9.5 Hoxb6-cre/R26 reporter mice. On the dorsal side of the aorta (E,G), the lacZ staining, seen as an electron-dense cytoplasmic precipitate (arrows), was confined to endothelial cells. By contrast, the staining was found in endothelium and SMCs on the ventral side (F). The dorsal-ventral axis (indicated as d-v) runs from top to bottom if not otherwise indicated. nt, neural tube; nc, notochord; da, dorsal aorta; im, intermediate mesoderm; spl, splanchnic mesoderm; som, somatic mesoderm; g, gut; e, endothelium; s, SMC. Scale bars: 25 µm in A-D; 5 µm in E-G.