Figure 3. Stages of cardiac development in the zebrafish embryo.

(A) Lateral view. At 5 h post fertilization (hpf), the blastula (white) covers approximately 50% of the large yolk cell (yellow). Cardiac progenitor cells are located bilaterally in the lateral marginal zone. Atrial progenitor cells (pink) are located more ventrally (v) than the ventricle progenitor cells (light blue) and pharyngeal mesoderm progenitors (red circles). During gastrulation, the cardiac progenitor cells move dorsally (d) towards the midline to end up in the anterior lateral plate mesoderm by the 12-somite stage (B; B–F are dorsal views). Pharyngeal mesoderm progenitors remain rostral to this region. (C) The endocardial cells (light green) are the first to migrate towards the midline, with the myocardial cells following slightly later. When the bilateral heart fields fuse at the midline, they form a shallow cardiac cone with the endocardial cells in the center, ventricular myocytes at the circumference and atrial myocytes at the periphery (D). Proliferation transforms the cardiac cone into a cardiac tube. The endocardium forms the inner lining of the myocardial tube. (E) By 28 hpf, the elongating heart tube has reoriented from the dorsal-ventral axis to the rostal-caudal axis, with the venous pole swinging to the left while the arterial pole remains fixed at the midline. New cardiomyocytes are added to the arterial pole from the pharyngeal/cranial mesoderm (red). At 36 hpf, cardiac looping is well underway, and the constriction at the position of the AV canal is obvious (F). The heart tube continuous to loop and forms an S-shaped loop (G, ventral view). Ellipsoid extracardiac pro-epicardial cells (brown) are located near the AV canal (yellow), from where they start to cover the myocardium with an epicardial layer. The pacemaker is present in the inner curvature of the atrium near the venous pole (dark green). This figure is modified from Bakkers (2011) and used with permission.