Two distinct cell populations in the floor plate of the zebrafish are induced by different pathways

The floor plate is a morphologically distinct structure of epithelial cells situated along the midline of the ventral spinal cord in vertebrates. It i s a source of guidance molecules directing the growth of axons along and ac ross the midline of the neural tube, In the zebrafish, the floor plate is a bout three cells wide and composed of cuboidal cells. Two cell populations can be distinguished by the expression patterns of several marker genes, in cluding sonic hedgehog (shh) and the fork head-domain gene fkd4: a single r ow of medial floor plate (MFP) cells, expressing both shh and fkd4, is flan ked by rows of lateral hoot plate (LFP) cells that express fkd4 but not shh . Systematic mutant searches in zebrafish embryos have identified a number of genes, mutations in which visibly reduce the floor plate. In these mutan ts either the MFP or the LFP cells are absent, as revealed by the analysis of the shh and fkd4 expression patterns. MFP cells are absent, but LFP cell s are present, in mutants of cyclops, one-eyed pinhead, and schmalspur, who se development of midline structures is affected. LFP cells are absent, but MFP cells are present, in mutants of four genes, sonic you, you, you-too, and chameleon, collectively called the you-type genes. This group of mutant s also shows defects in patterning of the paraxial mesoderm, causing U- ins tead of V-shaped somites. One of the you-type genes, sonic you, was recentl y shown to encode the zebrafish Shh protein, suggesting that the you-type g enes encode components of the Shh signaling pathway. It has been shown prev iously that in the zebrafish shh is required for the induction of LFP cells , but not for the development of MFP cells. This conclusion is supported by the finding that injection of shh RNA causes an increase in the number of LFP, but not MFP cells. Embryos mutant for iguana, detour, and umleitung sh are the lack of LFP cells with you-type mutants while somite patterning is not severely affected. In mutants that fail to develop a notochord, MFP cel ls may be present, but are always surrounded by LFP cells. These data indic ate that shh, expressed in the notochord and/or the MFP cells, induces the formation of LFP cells. In embryos doubly mutant for cyclops (cyc) and soni c you (syu) both LFP and MFP cells are deleted. The number of primary motor neurons is strongly reduced in cyc;syu double mutants, while almost normal in single mutants, suggesting that the two different pathways have overlap ping functions in the induction of primary motor neurons. (C) 2000 Academic Press.