Figure 1.

Key steps of embryonic heart development in zebrafish. In the late blastula (5 hpf), myocardial progenitor cells are found in bilateral zones near the embryonic margin. Fate maps indicate that ventricular (red) and atrial (green) progenitor cells are already spatially organized at this stage (Keegan et al., 2004), even though they are not yet distinguished by molecular markers. After gastrulation is complete (12 hpf), the myocardial progenitors (ventricular progenitors, red; atrial progenitors, green) reside in bilateral heart fields within the anterior lateral plate mesoderm (ALPM). Ventricular and atrial progenitor cells retain their spatial organization as the gastrula forms (Schoenebeck et al., 2007), and myocardial differentiation begins within the ALPM. Next, the process of cardiac fusion brings cardiomyocytes to the embryonic midline, where they form a cardiac cone. By this stage (19 hpf), ventricular and atrial cardiomyocytes can be distinguished by the expression of characteristic molecular markers (ventricular myosin heavy chain (vmhc), red; atrial myosin heavy chain (amhc), green) (Berdougo et al., 2003). Continued migration of cardiomyocytes elongates the cardiac cone to assemble the heart tube (shown here at 30 hpf). By 48 hpf, further growth and morphogenesis of the heart tube results in the formation of a morphologically distinct ventricle (red) and atrium (green) that are separated by a constriction at the atrioventricular canal. Adapted from Miura and Yelon (2011).