Fig. 5. Innervation of neuroepithelial cells in developing gill filaments of 7 and 9 d.p.f. larvae. (A) Serotonin (5-HT)-immunoreactivity (IR) of an isolated gill from a larva at 7 d.p.f. At this stage, filaments (f) were longer and primordium of respiratory lamellae (lam) were observed. A single 5-HT-IR neuroepithelial cell (NEC, green) resided within the developing filament. Scale bar, 10 µm. (B) Image in A shown with zn-12-IR indicates that the NEC was associated with zn-12-IR nerve fibres (arrowheads) that arose from a nerve bundle (nb) of the gill arch (ga). A zn-12-IR neuron (n) is also visible. (C) 5-HT-IR of an isolated gill from a larva at 9 d.p.f. A 5-HT-IR NEC (green) of the filament (f) and a 5-HT-IR NEC of the gill arch (ga) were present. Scale bar, 10 µm. (D) Image in C shown with zn-12-IR reveals that both of these cells were associated with zn-12-IR (red) nerve fibres (arrowheads) arising from a nerve bundle (nb) of the gill arch (ga).