The zebrafish young mutation acts non-cell-autonomously to uncouple differentiation from specification for all retinal cells

Embryos from mutagenized zebrafish were screened for disruptions in retinal lamination to identify factors involved in vertebrate retinal cell specifi cation and differentiation. Two alleles of a recessive mutation, young, wer e isolated in which final differentiation and normal lamination of retinal cells were blocked. Early aspects of retinogenesis including the specificat ion of cells along the inner optic cup as retinal tissue, polarity of the r etinal neuroepithelium, and confinement of cell divisions to the apical pig mented epithelial boarder were normal in young mutants. BrdU incorporation experiments showed that the initiation and pattern of cell cycle withdrawal across the retina was comparable to wild-type siblings; however, this proc ess took longer in the mutant. Analysis of early markers for cell type diff erentiation revealed that each of the major classes of retinal neurons, as well as nonneural Muller glial cells, are specified in young embryos. Howev er, the retinal cells fail to elaborate morphological specializations, and analysis of late cell-type-specific markers suggests that the retinal cells were inhibited from fully differentiating. Other regions of the nervous sy stem showed no obvious defects in young mutants,Mosaic analysis demonstrate d that the young mutation auts non-cell-autonomously within the retina, as final morphological and molecular differentiation was rescued when genetica lly mutant cells were transplanted into wild-type hosts. Conversely, differ entiation was prevented in wild-type cells when placed in young mutant reti nas. Mosaic experiments also suggest that young functions at or near the ce ll surface and is not freely diffusible. We conclude that the young mutatio n disrupts the post-specification development of all retinal neurons and gl ia cells.