Hindbrain patterning revisited: timing and effects of retinoic acid signalling

Retinoids play a critical role in patterning, segmentation, and neurogenesis of the posterior hindbrain and it has been proposed that they act as a posteriorising signal during hindbrain development. Until now, direct evidence that endogenous retinoid signalling acts through a gradient to specify cell fates along the anteroposterior axis has been missing. Two recent studies tested the requirement for retinoid signalling in the developing hindbrain through systematic application of a pan‐retinoic acid receptor antagonist.(1,2) They demonstrate a stage‐dependent requirement for increasing retinoid signalling activity along the hindbrain that proceeds from anterior to posterior. Together these findings challenge the concept of a stable gradient of retinoic acid across the hindbrain and warrant a re‐interpretation of the phenotypes obtained by genetic and nutritional disruption of retinoid signalling in the amniote embryo. BioEssays 23:981–986, 2001. © 2001 John Wiley & Sons, Inc.