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Amphibians and Fish
The patterning of the neural ectoderm along the anterior-posterior axis in the zebrafish appears to be the result of the interplay of FGFs, Wnts, and retinoic acid, similar to that seen in Xenopus. In fish embryos, there seem to be two separate processes. First a Wnt signal represses the expression of anterior genes; then Wnts, retinoic acid, and FGFs are required to activate the posterior genes.
This regulation of anterior-posterior identity appears to be coordinated by retinoic acid-4-hydroxylase, an enzyme that degrades RA (Kudoh et al. 2002; Dobbs-McAuliffe et al. 2004). The gene encoding this enzyme, cyp26, is expressed specifically in the region of the embryo destined to become the anterior end. Indeed, this gene’s expression is first seen during the late blastula stage, and by the time of gastrulation it defines the presumptive anterior neural plate. Retinoic acid-4-hydroxylase prevents the accumulation of RA at the embryo’s anterior end, blocking the expression of the posterior genes there. This inhibition is reciprocated, since the posteriorly expressed FGFs and Wnts inhibit the expression of the cyp26 gene, as well as inhibiting the expression of the head-specifying gene Otx2. This mutual inhibition creates a border between the zone of posterior gene expression and the zone of anterior gene expression. As epiboly continues, more and more of the body axis is specified to become posterior.
Retinoic acid acts as a morphogen, regulating cell properties depending on its concentration. Cells receiving very little RA express anterior genes; cells receiving high levels of RA express posterior genes; and those cells receiving intermediate levels of RA express genes characteristic of cells between the anterior and posterior regions. This morphogen is extremely important in the hindbrain, where different levels of RA specify different types of cells along the anterior-posterior axis (White et al. 2007).
Literature Cited
Dobbs-McAuliffe, B., Q. Zhao and E. Linney. 2004. Feedback mechanisms regulate retinoic acid production and degradation in the zebrafish embryo. Mech. Dev. 121: 339–350.
Kudoh, T., S. W. Wilson and I. B. Dawid. 2002. Distinct roles for FGF, Wnt, and retinoic acid in posteriorizing the neural ectoderm. Development 129: 4335–4346.
White, R. J., Q. Nie, A. D. Lander and T. F. Schilling. 2007. Complex regulation of cyp26a1 creates a robust retinoic acid gradient in the zebrafish embryo. PLoS Biol. Nov. 5(11): e304.