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Bone morphogenetic proteins (Bmps) are key regulators of dorsoventral (DV) patterning. Within the ectoderm, Bmp activity has been shown to inhibit neural development, promote epidermal differentiation and influence the specification of dorsal neurons and neural crest. In this study, we examine the patterning of neural tissue in mutant zebrafish embryos with compromised Bmp signalling activity. We find that although Bmp activity does not influence anteroposterior (AP) patterning, it does affect DV patterning at all AP levels of the neural plate. Thus, we show that Bmp activity is required for specification of cell fates around the margin of the entire neural plate, including forebrain regions that do not form neural crest. Surprisingly, we find that Bmp activity is also required for patterning neurons at all DV levels of the CNS. In swirl/bmp2b(-) (swr(-)) embryos, laterally positioned sensory neurons are absent whereas more medial interneuron populations are hugely expanded. However, in somitabun(-) (sbn(-)) embryos, which probably retain higher residual Bmp activity, it is the sensory neurons and not the interneurons that are expanded. Conversely, in severely Bmp depleted embryos, both interneurons and sensory neurons are absent and it is the most medial neurons that are expanded. These results are consistent with there being a gradient of Bmp-dependent positional information extending throughout the entire neural and non-neural ectoderm.

Here we report the expression of the zebrafish zic1 gene, also known as opl, a homologue to other vertebrate Zic genes and the Drosophila odd-paired gene. zic1 expression starts during epiboly stages in lateral parts of the neural plate and eventually comes to lie in dorsal regions of the developing brain following the morphogenetic movements of neural tube formation. To address the question whether BMP2 signalling affects the extent of zic1 expression, we analysed swirl and chordino mutant embryos. Expanded Zic1 expression in swirl and reduced expression in chordino as well as in bmp2 injected embryos suggest that BMP2 and its antagonists define the extent of zic1 expression in the neural plate. By searching for factors responsible for the dorsal restriction of Zic1 expression, we found zic1 expression is eliminated in sonic hedgehog (shh) injected embryos. The most rostral expression however is not affected by Shh suggesting that Shh plays a different role in dorso-ventral patterning of the future telencephalon. During somitogenesis zic1 is expressed in the dorsal most part of the developing somites. Here zic1 marks cells that are distinct from the main adaxial somite portion, the future myomere. zic1 expression in the somites is expanded in swirl but reduced in shh injected embryos, suggesting these factors have opposing activity in dorsoventral patterning of the somites. Later, a growing mass of zic1 expressing cells occurs in a dorsal mesenchyme that eventually invades the dorsal fin fold, suggesting a somitic contribution to the dorsal fin mesenchyme.