EXOGENOUS RETINOIC ACID CAUSES SPECIFIC ALTERATIONS IN THE DEVELOPMENT OF THE MIDBRAIN AND HINDBRAIN OF THE ZEBRAFISH EMBRYO INCLUDING POSITIONAL RESPECIFICATION OF THE MAUTHNER NEURON

Exogenously applied retinoic acid given at the early stages of gastrul ation causes abnormal development of the caudal midbrain and anterior hindbrain in vertebrate embryos. We describe the limits of the brain r egions that are affected using neuroanatomical criteria in the zebrafi sh embryo. Analysis of the reticulospinal complex shows that the Mauth ner cell, which normally differentiates in rhombomere 4, is duplicated either in this rhombomere or in rhombomere 2. Using probes for zebraf ish krx20 and pax2, it is demonstrated that retinoic acid affects the expression domains of these regulatory genes in a manner that is consi stent with the neuroanatomical data. Expression of the goosecoid gene, which expressed in the prospective anterior mesoderm from the onset o f gastrulation, is unaffected by the doses of retinoic acid used in th is study, reflecting the normal development of the anterior end of the embryo.