Fig. 2

Pioneer neurons forming the main neuronal scaffold. The schematic diagram indicates the larval regions shown in the figures. a-d Posterior pioneer neuron (PPN). a Dense tubulin marks the soma of the posterior pioneer neuron (PPN) already at 9 hpf. b Lateral view at 10 hpf showing the orientation of the PPN (inset). c A single dorsal follower neuron of the PPN is visible from 16hpf. d A lateral pair of follower neurons of the PPN at 36hpf. e-g Development of the anterior neuronal scaffold. An apical ganglion cell (AP1) (orange asterisk) on the left body side sends a descending axon towards the right body side. A pair of sensory cells (white asterisk) give rise to the prototroch nerve ring. The dotted circles show the adjacent prototroch cells. Neurites from later appearing neuronal pairs (red asterisk) and peripheral sensory neurons (arrowheads) converge to form the central neuropil. h, i Detailed few the pioneers of the prototroch nerve ring. k, l Detailed view of the apical ganglion cell AP1. m-s Immunohistochmical staining of serotonin (5-HT) and FMFRamide during early nervous system development. m, n 5-HT and FMRFamide are detected in many neurons and neurites at 48 h. o First FMRFamide is detected at 14 hpf in a ciliated sensory cell dorsal and left of the apical tuft. p Dorsal apical pair of FMRFamidergic cells. r, s One FMRFamidergic and one serotogener apical pair of ciliated sensory cells ventral of the apical tuft. Asterisks (o-s): nucleus of cells with FMRFamide or 5-HT. Scale bars: 10 μm, (except F: 20 μm, N-Q: 5 μm). at: apical tuft, cec: circumesophageal connective, cg: central ganglia, prt: prototroch, sec: supraesophageal connective, tel.: telotroch