Widespread defects in crestin expression in sox10 mutants extend to premigratory stages of neural crest development. Lateral views of head and trunk of 24 hpf wild-type (A) and sox10 mutant (B) zebrafish to show expression of crestin. Whilst in wild-types (A) crestin is strongly expressed in premigratory neural crest cells (asterisk) and in neural crest cells migrating on the medial pathway (large arrow), expression in these sites is dramatically reduced in sox10 mutants (B). In contrast, expression in neural crest cells in the branchial arch streams (arrowhead) is unaffected (A, compare B). A few cells on the medial pathway with relatively normal crestin expression levels in sox10 mutants (small arrow, A) may represent sensory neuron precursors (see Kelsh and Eisen (2000)).