AXIAL MESODERM AND PATTERNING OF THE ZEBRAFISH EMBRYO

It has long been recognized that the notochord, a derivative of the ga strula organizer, is important for patterning the vertebrate embryo. E xperimental manipulations of amphibian and chick embryos demonstrate t he notochord's influence on neural tube and somite differentiation. Re cent studies implicate the notochord-derived signal Sonic hedgehog. In zebrafish, genetic strategies have enabled the recovery of mutations that disrupt notochord development. Embryos mutant for floating head o r no tail lack differentiated notochord and, consistent with encoding transcription factors, both genes behave cell autonomously in genetic mosaics: wild-type cells develop into notochord in mutant embryos, whe reas mutant cells fail to contribute to wild-type notochord. In floati ng head mutants, axial mesoderm fated to form notochord respecifies to paraxial mesoderm, differentiating into muscle instead. As a result, fused somites develop across the trunk midline. In no tail mutants, me senchymal cells related by lineage to notochord are found in the midli ne, and although trunk somites are bilaterally paired, they lack Engra iled-expressing muscle pioneer cells. In the presence of wild-type not ochord, however, no tail mutant cells can differentiate muscle pioneer s, suggesting that induction of muscle pioneers is a normal function o f notochord. Notochord is also involved in induction off floor plate a t the ventral midline of the vertebrate neural tube; yet in floating h ead and no tail mutants, floor plate cells develop in the absence of d ifferentiating notochord. Expression of sonic hedgehog by newly-formin g axial mesoderm at. gastrulation may account for floor plate inductio n prior to notochord differentiation. Thus, analyses of zebrafish noto chordless mutants suggest a role for both early and late signaling by cells of the notochord lineage during embryogenesis.