Axonal pathfinding and fasciculation behaviour in the embryonic ventral nerve cord. (a) In the ventral nerve cord of stage 13/14 embryos, growth cones of identified neurons (aCC, curved arrow; pCC, arrow; RP2, arrow head) navigate in stereotypic positions (stippled line, midline; asterisk, somata of aCC and pCC). (b) Schematic representations of early growing neurons vMP2, dMP2, MP1, aCC and pCC (colour coded): at stage 13, their axons are partly guided by glia cells (grey circles) and Netrin A and B (light green) bound to lateral fields of Frazzled expression (wave pattern); MP1s and dMP2s grow jointly posteriorward, whereas vMP2s and pCCs fasciculate and grow together anteriorward until all four neurons contact one another midway between adjacent neuromeres and establish a single longitudinal fascicle (stage 13) that splits (stage 14), re-fasciculates (stage 15) and splits again (stage 16), partly mediated by glia cells (pCCs, dMP2s, vMP2s form a common fascicle close to the midline, MP1s a distinct axon tract further lateral; grey stippled line represents the lateral Fas2 fascicle of unknown identity; compare Figure 4, ix). (c,d) The neuroblast lineage NB1-2  illustrates the stereotypic pathfinding choices of individual neurons (curved arrow, ipsilateral longitudinal path; AC/PC, anterior/posterior commissure; arrow, medial contralateral longitudinal; double chevron, lateral contralateral longitudinal; arrow head, soma of identified TB neuron; CX, cortex; NP, neuropile). (e) Regulation of midline crossing and mediolateral longitudinal path choice: ipsilateral neurons don't express Commissureless (Comm), and their combinatorial Robo receptor code determines the mediolateral positioning of their axons; contralateral neurons express Comm (black T), thus preventing transport of Robo receptors to the growth cone (curved red arrow); subsequent downregulation of Comm activity permits the Robo-mediated fascicle choice. (f) Choice of anterior versus posterior commissure during midline crossing is partly determined by posterior expression of Wnt5 (Figure 4, xx), which repels growth cones of Derailed expressing neurons. (c,d) Kindly provided by Janina Seibert, Christoph Rickert and Gerd Technau; (b) redrawn from Hidalgo and Booth .