A key problem in the study of vertebrate development is to determine the molecular basis of positional value along a developmental axis. In amphibian regeneration, retinoic acid is able to respecify positional value in a graded fashion that is dependent on its concentration. In view of the fact that retinoic acid is a naturally occurring metabolite of vitamin A, this raises the possibility that it is deployed in vivo as an endogenous morphogen. Furthermore, the recent evidence that its effects are mediated by nuclear receptors of the steroid/thyroid hormone superfamily suggests the possibility of understanding the mechanism of its graded effects on morphogenesis. Such insights would be of crucial importance for our understanding of vertebrate patterning along an axis.

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