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B.E. Reese, R. Farajian, M.A. Raven, K. Cusato; Cellular Positioning and Dendritic Field Size of Cholinergic Amacrine Cells are Impervious to Early Ablation of Neighboring Cholinergic Amacrine Cells in the Mouse Retina . Invest. Ophthalmol. Vis. Sci. 2004;45(13):5340.
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Purpose: To investigate the role of homotypic neighbor–relationships upon the positioning and dendritic field size of cholinergic amacrine cells in the mouse retina. Methods: Partial ablations of the population of cholinergic amacrine cells were made during early development using L–glutamate as an excitotoxin. Subcutaneous injections (4mg/g) made on P–3 and thereafter were found to produce a near–complete elimination, while injections at P–2 were ineffective. Lower doses on P–3 produced only partial reductions, and were subsequently used to examine the effect of partial ablation upon mosaic organization and dendritic growth of the remaining cells within the ganglion cell layer. Results: Four different Voronoi–based measures of mosaic geometry were examined in L–glutamate treated and normal (saline–treated) retinas. Partial depletions of around 40% produced cholinergic mosaics that, when scaled for density, only approximated the mosaic geometry of the normal retina, being slightly less regular. Separate comparisons simulating a 40% random deletion of the normal retina produced mosaics that were no different from those experimentally depleted retinas. Consequently, no evidence was found for positional regulation in the absence of normal neighbor relationships. Single cells in the ganglion cell layer were intracellularly filled with Lucifer Yellow to examine the morphology and dendritic field extent following partial ablation of the cholinergic amacrine cells. No discernable effect was found on their starburst morphology, and total dendritic field area, number of primary dendrites and branch frequency were not significantly different. Conclusions: Somal positioning of cholinergic amacrine cells is independent of neighbor–relationships, at least after P–3. At this age, dendritic field area still undergoes a seven–fold increase, while retinal expansion undergoes at most a tripling in size. This active growth in dendritic field area must therefore be independent of the density of neighboring cholinergic processes. Cell–intrinsic instructions rather than environmental constraints may determine the maximum size of the dendritic field of cholinergic amacrine cells.
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