Expression of zebrafish nmyc1 highlights restricted neural and endodermal domains at embryonic stages. Expression of nmyc1 revealed by in situ hybridization with a digoxygenin-labelled nmyc1 antisense riboprobe on whole-mount embryos (anterior left; lateral views in A,C,F,H; dorsal views in B, C inset, F inset; stages indicated bottom left of each panel). (D) and (E) are cross-sections at the 20-somite stage at the midbrain level to compare dlx2 and nmyc1 expression in neural crest cells (one half of the embryos only is shown). (G) and (I) are sections of the embryos shown in (F) and (H), respectively, at the levels indicated. Expression at early stages is ubiquitous but progressively resolves into restricted positive domains in endodermal derivatives (g), migrating neural crest cells (arrows in D, E), branchial arches (ba) and limited brain territories. The latter include the epiphysis (ep), the eye (e) and the midbrain-hindbrain (MH, bracket in C,F,H). MH expression is restricted to the alar plate (al) and absent from the basal plate (bas) (see G,I), thus labels the presumptive optic tectum (tec) and dorsal anterior hindbrain.

Zebrafish nmyc1 is prominently transcribed in tectal and retinal growth zones at early larval stages. Expression of nmyc1 on whole-mount embryos at 72 hpf, revealed by in situ hybridization with a digoxygenin-labelled nmyc1 antisense riboprobe (blue staining, left panels) or in combination with the M phase marker phospho-histone H3 (brown nuclei, right panels except I′) (all embryos anterior left, dorsal views in A,B, lateral views in C,D). (E–H) are frontal sections of the embryos in (C, D) at the levels indicated, (I–J) are parasagittal sections of similar embryos, with magnification of the caudal tectal border in (I′) and (J) (boxed area in I). Central nervous system expression of nmyc1 labels the circumference of the optic tectum (ot, black arrowheads), the retinal marginal zone (r, red arrowheads), the cerebellar plate (cb, green arrowheads) and rhombic lips in rhombomere 2 (rl, blue arrowheads). Note the striking coincidence between these sites and proliferating, PH3-positive cells.