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Figure 1 Stages of heart development in zebrafish from 10 to 96 h post-fertilization (hpf). (A) Atrial and ventricular cardiac progenitors are located in the anterior lateral plate mesoderm by ~15 hpf. (B) The cardiac disc is visible by 22 hpf as cardiac progenitors surround endocardial cells at the midline. (C) At 24 hpf, the linear heart tube forms and jogs to the left in preparation for heart looping. (D) At 36 hpf, the heart tube undergoes rightward looping. The AV canal (orange) begins to develop between the cardiac chambers. (E) At 48 hpf, the cardiac chambers begin to balloon and expand outwards. The bulbus arteriosus (dark blue) and AV canal (orange) continue to develop and mature. (F) From 72 to 96 hpf, the cardiac chambers expand and align beside each other. (G) A cross-section of a 96 hpf heart showing the trabeculae, the finger-like muscular projections on the inner wall of the ventricle, and endocardial leaflets (orange) of the AV canal. (A–D) dorsal views; (E–G) ventral views.