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P.I. Song, J.I. Matsui, J.E. Dowling; Horizontal Cell Types And Connectivity Found In The Adult Zebrafish Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):153.
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The cone photoreceptors in the zebrafish retina are arranged in a regular mosaic pattern, but the way in which horizontal cells connect to the photoreceptors is unclear. We have examined the morphology and connectivity of the different types of horizontal cells found in the adult zebrafish retina.
Tiny crystals of DiI were placed onto the tips of pulled glass electrodes and the crystals were inserted into the inner nuclear layer of whole mounted retinas. The somas, axons, and dendritic terminals of horizontal cells were fully labeled with DiI, and horizontal cells were imaged with a confocal microscope. The images were then analyzed according to dendritic arborization, cell depth from the inner edge of the soma to the dendritic terminals, dendritic terminal morphology, and connectivity with photoreceptors.
Two types of horizontal cells (H1 and H2) were identified using the DiI–labeling method. The H1 cell type represents a large group of horizontal cells that have dendritic arbors that measure approximately 20–30µm across, that are 9–10µm in depth, and have dendritic terminals that are arranged in clusters or "rosettes" that connect to a variety of nearby photoreceptors without any apparent specificity. The H2 cells have dendritic arbors that measure approximately 40–50µm across, are 13µm in depth, and have doublets of dendritic terminals arrayed in a rhomboid lattice pattern similar to those found for the H3 horizontal cell type of the goldfish retina (Stell and Lightfoot, J Comp Neurol, 159:473–502). The rhomboid pattern of the H2 cells corresponds to the dimensions of the photoreceptor mosaic indicating that the H2 cells connect to specific photoreceptors.
The dendritic terminal patterns of the horizontal cells suggest that the H2 cells are selective in terms of the photoreceptor type they connect with, whereas the H1 cells do not exhibit such selectivity. Thus, H1 cells may be luminosity type cells and H2 cells chromatic.
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