AXONAL OUTGROWTH WITHIN THE ABNORMAL SCAFFOLD OF BRAIN TRACTS IN A ZEBRAFISH MUTANT

The role of specific axonal tracts for the guidance of growth cones wa s investigated by examining axonal outgrowth within the abnormal brain tracts of zebrafish cyclops mutants. Normally, the earliest different iating neurons in the zebrafish brain establish a simple scaffold of a xonal tracts. Later-developing axons follow cell-specific pathways wit hin this axonal scaffold. In cyclops embryos, this scaffold is perturb ed due to the deletion of some ventromedial neurons that establish par ts of the axonal scaffold and the development of an abnormal crease in the brain. In these mutant embryos, the growth cones projected by the neurons of the nucleus of the posterior commissure (nuc PC) are depri ved of the two tracts of axons that they sequentially follow to first extend ventrally, then posteriorly. These growth cones respond to the abnormal scaffold in several interesting ways. First, nuc PC growth co nes initially always extend ventrally as in wild-type embryos. This su ggests that for the first portion of their pathway the axons they norm ally follow are not required for proper navigation. Second, approximat ely half of the nuc PC growth cones follow aberrant longitudinal pathw ays after the first portion of their pathway. This suggests that for t he longitudinal portion of the pathway, specific growth cone/axon inte ractions are important for guiding growth cones. Third, although appro ximately half of the nuc PC growth cones follow aberrant longitudinal pathways, the rest follow normal pathways despite the absence of the a xons that they normally follow. This suggests that cues independent of these axons may be capable of guiding nuc PC growth cones as well. Th ese results suggest that different guidance cues or combinations of cu es guide specific growth cones along different portions of their pathw ay. (C) 1994 John Wiley & Sons, Inc.