A GRADED RESPONSE TO BMP-4 SPATIALLY COORDINATES PATTERNING OF THE MESODERM AND ECTODERM IN THE ZEBRAFISH

The effects of signal perturbation on expression domains of molecular markers for the mesoderm and ectoderm have been analysed across the do rso-ventral axis in zebrafish embryos. Injection of RNA encoding bone morphogenetic protein-4 (BMP-4) ventralised the embryo, expanding the intermediate mesoderm and non-neural ectoderm at the expense of the do rso-anterior mesoderm and neural plate. A dose-dependent response was observed both morphologically and in expression of gta3, MyoD and paw2 . Conversely, increases in dorso-anterior mesoderm and neurectoderm we re generated by injection of RNA encoding either a dominant-negative B MP receptor (Delta BMPR) or noggin, as demonstrated by goosecoid and p ox2 expression. Ventral BMP-4 expression was also inhibited. Thus, pat terning of both the mesoderm and the ectoderm during gastrulation appe ars to depend, directly or indirectly, on the level of BMP activity. C onsistent with their locations prior to formation of the neural tube, elevated BMP-4: increased the number of dorsal spinal cord neurons whi lst sonic hedgehog and islet1 expression in the ventral spinal cord we re reduced. However, the ectopic neurons were not positioned more vent rally, implicating a prepattern in the dorsal neural tube that is inde pendent of the ventral central nervous system. (C) 1997 Elsevier Scien ce Ireland Ltd.