Retinoic acid and the late phase of neural induction.

Regional neural gene expression in Xenopus is the result of a number of processes that continue well beyond the end of gastrulation. By considering two of the basic features of neural induction, the duration of contact between mesoderm and ectoderm and the timing of neural competence, it has been possible to distinguish two phases in neural tissue formation. The late phase includes the period following gastrulation. A factor in determining regional neural gene expression is the difference in inducing ability of the mesoderm that develops during gastrulation along the anterior-posterior axis. The resulting ability to express regional neural genes is subsequently refined during the late phase by a signal that progresses from the posterior part of the embryo. Using a dorsal explant system, it is shown that this progressive signal can be mimicked by the addition of retinoic acid (RA). However, the observation that regions along the anterior-posterior axis respond in different ways to the addition of RA suggests that additional factors are also important in defining regional neural gene expression. One possibility is that the expression of retinoic acid receptors along the axis may demarcate regions that respond to RA in particular ways.