A role for midline closure in the reestablishment of dorsoventral pattern following dorsal hindbrain ablation

The cellular and molecular study of dorsal neural tube ablation reported here demonstrates a critical role for midline closure in hindbrain repatterning. This was revealed by detailed analysis of the transcriptional response of two genes, Pax-3 and slug, during repair of the neural tube following ablation. The reexpression of Pax-3 appears to rely on a single surface ectoderm/neuroepithelial contact, while this is insufficient for reexpression of slug. In fact, slug up-regulation only occurred upon midline closure and, strikingly, corresponded to down-regulation of Pax-3. We examined whether a candidate dorsalizing molecule, Bmp-4, was responsible for this reciprocal regulation of Pax-3 and slug at midline closure. However, Bmp-4 was not reexpressed following ablation, indicating not only that it is not responsible for the observed repatterning but that it lies in regulatory pathways distinct from Pax-3 and slug We additionally examined the expression of Pax-6, which, together with assessment of the pattern of cranial ganglia, roof plate morphology, and positioning of branchiomotor exit points, demonstrates that neural crest regeneration is accompanied by reestablishment of a normal dorsoventral pattern within the neural tube. Thus, both local and longer range patterning appears to be restored following ablation, which is reliant dorsally on midline closure of the neural tube. (C) 1997 Academic Press.