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Fig. 1 Trabeculation initiates between 55 and 60 hours postfertilization (hpf) in a characteristic location. A–F: Confocal reconstructions of the ventricular myocardium in wild-type (wt) embryos expressing Tg(myl7:dsredt4) and Tg(myl7:egfp-hshras). Volume reconstruction (A,D), lumenal surface reconstruction (B,E), and optical sections (C,F) are shown as described in the Experimental Procedures section. In A and D, the atrioventricular (AV) canal is outlined with a dotted white line. White asterisks indicate areas where the acquisition of fluorescent signal was blocked by overlying melanocytes. Yellow asterisks indicate examples of cells displaying epigenetic silencing of Tg(myl7:dsredt4). G,H: Diagrams depict the orientations of the hearts and optical sections analyzed, as described in the Experimental Procedures section. The axes indicate the orientation of the embryo: V, ventral; D, dorsal; P, posterior; A, anterior; L, left; R, right. Additional abbreviations used are A for atrium and V for ventricle. A–C: At 55 hpf, the lumenal surface of the ventricle is generally smooth with no prominent protrusions. See also Supp. Movie S1. D–F: By 60 hpf, lumenal protrusion becomes evident (E, red arrowheads), most notably at a characteristic position on the ventricular outer curvature (E, top white arrowhead, and F, white arrowheads). Additional portions of the lumenal surface of the chamber exhibit more subtle and variable protrusions (E, yellow arrowhead). See also Supp. Movie S2. Scale bars = 50 μm.