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Fig. 2 Arterial pole progenitors at the 7-somite stage. (A) Dorsal view of a 7-somite zebrafish embryo, cranial to the top. Photoactivated fluorescein, labeled immunohistochemically (red, arrow), can be identified relative to the hand2 expression field (blue/black). (B) A magnification of the image in A with a grid superimposed on the region targeted for uncaging to show experimental division of the heart field in the right anterior lateral plate mesoderm (ALPM). Gray circles represent ~4-cell clusters that were labeled in the mesodermal heart field. In this specimen, cells were labeled in grid zone F (red). The dashed line highlights the cranial tip of the notochord. (C) Dorsal view of a 7-somite embryo with a map showing the cardiogenic field (red) and arterial pole progenitors within that field (blue), based on Table 1. (D) Overlay of the grid from B and map from C. (E) Diagram of the ventral view of a zebrafish embryo at 72 hpf, with cranial indicated by the eyes (pink) and caudal indicated by the yolk (yellow). The branchial muscles (light blue) are cranial to the bulbus arteriosus (green), ventricle (orange) and atrium (red). (F) Zebrafish in similar orientation as in E. Progeny of cells labeled in grid zone F (black) at the 7-somite stage are shown relative to the bulbus arteriosus (anti-Eln2, green), and striated muscle (MF20, red). (G,H,J,K) Sagittal sections of 72 hpf zebrafish, with cranial to the left, showing progeny of the photoactivated population at grid zone F in the arterial pole (arrows). (I) Diagram showing the various parts of the heart seen in sagittal sections. Caudal is indicated by the yolk (yellow) and cranial by the bulbus arteriosus (green). A, atrium; V, ventricle; Y, Yolk. Scale bars: 10 μm.