Exogenous retinoic acid induces a stage-specific, transient and progressive extension of Sonic hedgehog expression across the pectoral fin bud of zebrafish.

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We have performed a time course analysis of the expression of Sonichedgehog (shh) and patched1 (ptc1) in response to exogenous retinoic acid (RA) application to get some insight into the mechanism(s) underlying the formation of a mirror-image duplication of shh and ptc1 domains of expression in the pectoral fin buds of zebrafish. We have shown that RA exposure during the early stages of pectoral fin development first results in a rapid decrease or complete loss of shh/ptc1 expression. This is followed by reappearance of transcripts in the normal posterior domain, then by a stage-dependent and progressive expansion of the shh domain from the ZPA towards the anterior margin of the bud. Shh transcripts are induced in mesenchymal cells underlying the ventral ectoderm at the base of the bud. Once shh expression is activated in the most anterior cells, the number of shh-expressing cells increases in this region, possibly through an amplification mechanism involving signals from the apical ectodermal ridge. At this time, shh expression disappears from cells centrally located in the bud, resulting in the formation of the two distinct domains. An anterior extension of shh expression is also obtained in syu mutants with impaired shh function, suggesting that shh induction across the fin bud is independent of shh signaling. This study suggests the existence of complex mechanisms controlling the spatial and temporal expression of shh in the developing fin bud.