SIR--Mesoderm formation and its dorsoventral spedf kation are essential processes in vertebrate early development. The mesoderm produces many types of tissues such as notochord, muscle, bone and blood cells, arranged in the correct positions along thedorsoventral axis. The mechanisms underlying these processes are best understood in the amphibian Xenopus, whereas little is known about this process in oilier vertebrates, In amphibia, the mesoderm differentiates in the equatorial region of the blasiula-stage embryo through inductive interactions with vegetal blastomeres; the dorsoventral polarity of the mesoderm also depends on signals from dorsovegetal blastomeres. Are similar mechanisms involved in the development of tcieost fish? Which tissues are responsible for mesudenn induction in the teleosls? We have addressed these questions using zebrafish embryos.
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