Retinoic acid (RA) plays an essential role in establishing anterior-posterior patterning in the hindbrain, where a spatio-temporal gradient of RA is believed to generate domains of RA-responsive gene expression. Given this, it is puzzling that a uniform concentration of exogenous RA can rescue RA-depleted embryos. Cecilia Moens and co-workers now resolve this puzzle by showing that Cyp26 enzymes (which degrade RA) generate the RA-response gene expression patterns required for zebrafish hindbrain development (see p. 177). The researchers first documented dynamic spatio-temporal expression domains for the three zebrafish cyp26 genes - cyp26a1, cyp26b1 and cyp26c1 - in the developing hindbrain. They then showed that, in embryos depleted of all three enzymes, the entire hindbrain expresses RA-responsive genes that are normally restricted to the posterior hindbrain. Finally, they report that cyp26 genes are responsible for the ability of exogenous RA to rescue embryos depleted of endogenous RA. Given these results, the researchers propose a `gradient-free' model for hindbrain patterning in which Cyp26 activity establishes sequential RA-responsiveness boundaries.
Degrading uniformity in hindbrain patterning
Degrading uniformity in hindbrain patterning. Development 1 January 2007; 134 (1): e102. doi:
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