Fig. 3. Formation and differentiation of the dorsal vessel in the absence of cardiac Tin. Dorsal views of stage 15-16 embryos, with the left column showing wild-type control embryos and the right column homozygous tin-null mutant (tin³⁴⁶) embryos carrying tin-ABD. (A) Control embryo stained for mid RNA and for Mef2 protein. (B) tin-ABD, tin^- embryo with normal mid and Mef2 expression in cardioblasts. (C) Wild-type expression of Hand mRNA in Mef2⁺ cardioblasts (cb), Mef2^- pericardial cells (pc), and the lymph gland (lg). (D) tin-ABD, tin^- embryo expressing Hand mRNA in the developing dorsal vessel. Hand in cardioblasts is present, albeit at reduced levels when compared with pericardial cells and wild-type cardioblasts. (E) Control embryo doubly stained for Mef2 and Tropomyosin (posterior portion of the dorsal vessel). (F) Tropomyosin is present in myocardial cells of tin-ABD, tin^- embryo. (G) Staining for the transmembrane protein Toll and Mef2 shows a characteristic alternation between more cubical `working' myocardial cells and elongated pairs of ostial cells (brackets) in the `heart' region of the dorsal vessel. (H) tin-ABD, tin³⁴⁶ mutant embryo stained as in G showing disorganized tube formation and lack of the correct alternation of cell shapes in the `heart' (bracket). Mef2 channels in G,H to identify cardioblasts were omitted in the higher magnification insets.