It has long been proposed that concentration gradients of morphogens provide cues to specify cell fate in embryonic fields. Recent work in a variety of vertebrate systems give bona fide evidence that retinoic acid, the biologically active form of vitamin A, is a candidate for such a morphogen. In the developing chick wing, for example, locally applied retinoic acid triggers striking changes in the pattern along the anteroposterior axis. Instead of giving rise to a wing with the normal 234 digit pattern, wing buds treated with retinoic acid develop a 432234 mirror-image symmetrical digit pattern.
For this review, we focus on three aspects of limb morphogenesis. (1) We summarize the experimental evidence supporting the notion that retinoic acid is a candidate morphogen. (2) Limb buds contain high levels of cellular retinoic-acid-binding protein (CRABP). Using order of magnitude calculations, we evaluate how the concentration of CRABP might affect the occupancy state of the retinoic acid receptor. (3) We discuss the spatio-temporal expression pattern of homeobox-containing genes in the developing limb and speculate about the possibility that retinoic acid influences the pattern of expression of homeobox genes.