Neural expression of the Xenopus homeobox gene Xhox3: evidence for a patterning neural signal that spreads through the ectoderm.

The Xenopus laevis homeobox gene Xhox3 is expressed in the axial mesoderm of gastrula and neurula stage embryos. By the late neurula-early tailbud stage, mesodermal expression is no longer detectable and expression appears in the growing tailbud and in neural tissue. In situ hybridization analysis of the expression of Xhox3 in neural tissue shows that it is restricted within the neural tube and the cranial neural crest during the tailbud-early tadpole stages. In late tadpole stages, Xhox3 is only expressed in the mid/hindbrain area and can therefore be considered a marker of anterior neural development. To investigate the mechanism responsible for the anterior-posterior (A-P) regionalization of the neural tissue, the expression of Xhox3 has been analysed in total exogastrula. In situ hybridization analyses of exogastrulated embryos show that Xhox3 is expressed in the apical ectoderm of total exogastrulae, a region that develops in the absence of anterior axial mesoderm. The results provide further support for the existence of a neuralizing signal, which originates from the organizer region and spreads through the ectoderm. Moreover, the data suggest that this neural signal also has a role in A-P patterning the neural ectoderm.