Morphological types and connectivity of horizontal cells found in the adult zebrafish (Danio rerio) retina

We describe here different types of horizontal cells in the zebrafish retina and how they connect to photoreceptors. To label horizontal cells, crystals of DiI were placed onto the tips of pulled glass pipettes and inserted into the inner nuclear layer of fixed whole-mount retinas. The DiI-labeled horizontal cells were imaged by confocal microscopy and analyzed according to dendritic arborization, cell depth, dendritic terminal morphology, and connectivity with photoreceptors. Three types of horizontal cells were unequivocally identified: two cone-connecting (H1/2 and H3) and one rod-related cell. H1/2 cells have dendritic terminals that are arranged in "rosette" clusters and that connect to cone photoreceptors without any apparent specificity. H3 cells are larger and have dendritic terminal doublets arranged in a rectilinear pattern. This pattern corresponds to the mosaic of the single cones in the zebrafish photoreceptor mosaic and indicates that H3 cells connect specifically to either the blue-sensitive (long-single) or ultraviolet-sensitive (short-single) cones. Thus, H3 cells are likely to be chromaticity-type cells that process specific color information, whereas H1/2 cells are probably luminosity-type cells that process luminance information. Rod horizontal cells were identified by their shape and dendritic pattern, and they connect with numerous rod photoreceptors via small spherical terminals.