ANTERIOR NEURECTODERM IS PROGRESSIVELY INDUCED DURING GASTRULATION - THE ROLE OF THE XENOPUS HOMEOBOX GENE ORTHODENTICLE

In order to study the regional specification of neural tissue we isola ted Xotx2, a Xenopus homolog of the Drosophila orthodenticle gene. Xot x2 is initially expressed in Spemann's organizer and its expression is absent in the ectoderm of early gastrulae. As gastrulation proceeds, Xotx2 expression is induced in the overlying ectoderm and this domain of expression moves anteriorly in register with underlying anterior me soderm throughout the remainder of gastrulation. The expression patter n of Xotx2 suggests that a wave of Xotx2 expression (marking anterior neurectoderm) travels through the ectoderm of the gastrula with the mo vement of underlying anterior (prechordal plate) mesoderm. This expres sion of Xotx2 is reminiscent of the Eyal-Giladi model for neural induc tion. According to this model, anterior neural-inducing signals emanat ing from underlying anterior mesoderm transiently induce anterior neur al tissues after vertical contact with the overlying ectoderm. Further patterning is achieved when the ectoderm receives caudalizing signals as it comes in contact with more posterior mesoderm during subsequent gastrulation movements. Functional characterization of the Xotx2 prot ein has revealed its involvement in differentiation of the anterior-mo st tissue, the cement gland. Ectopic expression of Xotx2 in embryos in duces extra cement glands in the skin as well as inducing a cement gla nd marker (XAG1) in isolated animal cap ectoderm. Microinjection of RN A encoding the organizer-specific homeodomain protein goosecoid into t he ventral marginal zone results in induction of the Xotx2 gene. This result, taken in combination with the indistinguishable expression pat terns of Xotx2 and goosecoid in the anterior mesoderm suggests that Xo tx2 is a target of goosecoid regulation.