Fig. 1. Hypochord precursors are closely associated with notochord and muscle precursors near the edge of the shield. (A) View of dorsal margin region of 6 hpf (shield stage) embryo. Caged fluorescein was photoactivated in a cluster of three to five cells near the margin (broken black line) and centered about 15 cell diameters from the dorsal midline (broken white line). (B) Side view of trunk region of living embryo at 24 hpf showing labeled hypochord (hc) and notochord (nc) cells. Floor-plate (fp) cells were not labeled. (C) Transverse section of 24 hpf embryo in which the photoactivated fluorescein signal was converted to a blue precipitate. Hypochord, muscle (mu) cells near the periphery of the myotome and periderm (asterisk) were labeled. The periderm precursors were enveloping layer cells that were above the hypochord and muscle precursors at the time of photoactivation. (D) Transverse section of 30 hpf embryo labeled with F59 antibody to reveal slow muscle cells (red). Hypochord (green) and three slow muscle cells (yellow, arrows) were labeled by photoactivated fluorescein. (E) Dorsal view, anterior towards the top, of 9.5 hpf living embryo. Cells labeled by photoactivated fluorescein (green) are next to notochord cells (between arrows). Scale bar: 20 µm for all panels.
