Fig. 8. Model of the mechanisms patterning the pathways of mouse ncc migration. Each rhombomere has the capacity to generate ncc. From r2 and r4, ncc migrate laterally into ba1 and ba2, respectively. r3 and r5 both generate neural crest that migrates anteriorly and posteriorly to join the laterally migrating even-rhombomere ncc streams. Note that r3 produces fewer ncc (small green circles) than r5 (yellow curves). This creates crest-free zones adjacent to the odd-numbered rhombomeres. Neural crest generation from r3 is regulated by inhibitory signals (?) in the adjacent environment (green bar). This crest-free zone adjacent to r3 is maintained by multiple mechanisms. ErbB4 signalling (black arrows and bar) from the hindbrain to the adjacent mesenchyme keeps even-rhombomere ncc streams segregated dorsally, and Eph/ephrin signalling (grey bars) maintains the segregation ventrally. This prevents the infilling of neural crest from flanking regions. In the case of the neural crest-free zone adjacent to r5 this pattern is generated by the physical inhibition created by induction and formation of the otic vesicle. Adapted from fig. 6F in Golding et al. (Golding et al., 2000).