Fig. 4. Localization and tissue identity of ß-gal⁺ cells in the dorsal aorta in -cardiac actin^nlaacZ/+ embryos. (A) A section from the long clone, E10.5-788, stained with X-gal shows a ß-gal⁺ cell (arrowhead) in the dorsal wall of the dorsal aorta in a periendothelial position. The dorsal/ventral orientation of the dorsal aorta (DA) is indicated. (B) Immunohistochemistry on the same section using an -smooth muscle antibody (red) shows that the ß-gal⁺ nucleus is in a cell expressing -smooth muscle actin (white arrowhead). DAPI staining shows cell nuclei. (C) A section from the short clone E10.5-5, stained with X-gal, shows labelled cells in the ventral wall of the dorsal aorta in a periendothelial position (black arrowheads). (D) The same section treated with an -smooth muscle actin antibody (red) shows that these cells are positive (white arrowheads). DAPI staining shows the nuclei. When nuclei are strongly ß-gal⁺ they appear dark blue after DAPI staining. (E) Another section of the same clone as in C shows that ß-gal⁺ cells (black arrowheads) are also present immediately adjacent to the lumen of the dorsal aorta in an endothelial cell position. (F) Immunohistochemistry on the same section as in E with an -smooth muscle actin antibody shows that these cells (white arrowheads) are not smooth muscle positive, consistent with an endothelial identity. (G) A section from the single somite labelled clone E10.5-168, where a periendothelial cell (brown arrowhead), and vascular smooth muscle cells (white arrowheads) are stained by X-gal. (H) The same section as in G shows that all X-gal stained cells (brown and white arrowheads) are co-stained with an -smooth muscle actin antibody (red). Scale bars: 20 µm.