Mutation of the glass onion locus causes drastic neuronal patterning defects in the zebrafish retina and brain. The precise stratified appearance of the wild-type retina is absent in the mutants. The glass onion phenotype is first visible shortly after the formation of optic primordia and is characterized by the rounding of cells and disruption of the ventricular surface in the eye and brain neuroepithelia. With exception of the dorsal- and ventral-most regions of the brain, neuroepithelial cells lose their integrity and begin to distribute ectopically. At later stages, the laminar patterning of retinal neurons is severely disrupted. Despite the lack of lamination, individual retinal cell classes differentiate in the glass onion retina. Mosaic analysis reveals that the glass onion mutation acts cell nonautonomously within the retina and brain, as neuroepithelial cell morphology and polarity in these tissues are normal when mutant cells develop in wild-type hosts. We conclude that the glass onion mutation affects cell-cell signaling event(s) involved in the maintenance of the neuroepithelial cell layer shortly after its formation. The disruption of neuroepithelial integrity may be the cause of the neuronal patterning defects following neurogenesis. In addition, the expression of the glass onion phenotype in a subset of neuroepithelial cells as well as its onset following the initial formation of the neuroepithelial sheets indicate the presence of genetically distinct temporal and spatial subdivisions in the development of this histologically uniform tissue.
Copyright 2001 Academic Press.