Fig. 3. Confocal image of an isolated zebrafish gill at 3 d.p.f. (A) Labelling with antibodies against serotonin (5-HT) demonstrated that O₂-sensitive neuroepithelial cells (NECs) were absent in gill filament primordia (f), but two NECs (green) of an unidentified type were located in the gill arch (ga). Scale bar, 10 µm. (B) Image in A shown with zn-12-IR (red). 5-HT-immunoreactive (IR) NECs of the gill arch were associated with nerve fibres (arrows) of zn-12-IR nerve bundles (nb). Two zn-12-IR neurons (n) are also visible. Although NECs were not found in developing gill filaments at this stage, zn-12-IR nerve fibres (arrowheads) were present. (C) Higher magnification image of a single developing gill filament primordium (f) from another larva showing the absence of filament NECs and the presence of a zn-12-IR nerve fibre (arrowheads) as it entered the filament from the gill arch (ga). Scale bar, 5 µm. (D) Image in C shown with only zn-12-IR.