Dorsal-ventral patterning and differentiation of noggin-induced neural tissue in the absence of mesoderm.

In Xenopus development, dorsal mesoderm is thought to play a key role in both induction and patterning of the nervous system. Previously, we identified a secreted factor, noggin, which is expressed in dorsal mesoderm and which can mimic that tissue's neural-inducing activity, without inducing mesoderm. Here the neural tissue induced in ectodermal explants by noggin is further characterized using four neural-specific genes: two putative RNA-binding proteins, nrp-1 and etr-1; the synaptobrevin sybII; and the lipocalin cpl-1. First we determine the expression domain of each gene during embryogenesis. Then we analyze expression of these genes in noggin-treated explants. All markers, including the differentiated marker sybII, are expressed in noggin-induced neural tissue. Furthermore, cpl-1, a marker of dorsal brain, and etr-1, a marker absent in much of the dorsal forebrain, are expressed in non-overlapping territories within these explants. We conclude that the despite the absence of mesoderm, noggin-induced neural tissue shows considerable differentiation and organization, which may represent dorsal-ventral patterning of the forebrain.