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J.M. Fadool, A. Davis, A.C. deCarvalho, M. Merritt, A.C. Morris; Genetic Analysis of Development of the Photoreceptor Mosaic in Zebrafish . Invest. Ophthalmol. Vis. Sci. 2004;45(13):5307.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The well characterized, laminar organization of the vertebrate retina is complemented by the non–random, mosaic organization of the neuronal populations within each of the layers. This is most obvious in the highly ordered, crystalline–like arrangement of the photoreceptors in the teleost. The necessity of the regular distribution of cells for uniform light gathering and parallel processing is self–evident. However, developmental mechanisms leading to mosaic formation remain largely unknown. In this study, the role of Notch signaling in the formation of the photoreceptor mosaic in zebrafish (Danio rerio) was tested. Methods: A GAL4/UAS system controlled by a heat–shock promoter was used to drive ubiquitous but transient expression of a constitutively active form of zebrafish Notch1a (the notch intracellular domain: NICD). Trans–heterozygous embryos ranging in age from 32 hpf to several days were incubated at 38º C for 30 minutes. Alterations in the mosaic were determined by whole–mount immunolabeling and in situ hybridization. Results: Previously, we described the development and arrangement of the rod photoreceptor cell mosaic. Using transgenic zebrafish demonstrating rod–specific expression of eGFP, the spatial and temporal pattern of rod differentiation was observed. In embryo chimeras of transgenic and non–transgenic zebrafish, the regular arrangement of the eGFP expressing rods of the donor clones suggested a role for local signaling mechanisms in mosaic formation. The origin of the cell–cell signaling was tested genetically, by blocking neurogenesis at various stages of development by overexpression of the NICD. Induction of NICD expression at early stages of retinal neurogenesis dramatically decreased retinal lamination and blocked the differentiation of neurons, including photoreceptors. However, induction of NICD at later stages of development, i.e. during photoreceptor cell genesis, had little affect upon the inner retina but resulted in dramatic changes in the pattern and arrangement of the cone photoreceptors, a reduction in the number of rods and alterations in their distribution. Surprisingly, in embryos allowed to survive for several days after heat shock, the photoreceptor cell mosaic recovered in the newly generated cells at the retinal margin. Conclusions: These data provide the first genetic evidence that the photoreceptor cell mosaic is derived by local cell–cell interactions between differentiating neuroblasts of the outer retina and suggest an autonomous property of the neuroepithelial cells in forming neuronal mosaics.
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