Identification and morphological classification of horizontal, bipolar, and amacrine cells within the zebrafish retina

Horizontal, bipolar, and amacrine cells in the zebrafish retina were morphologically characterized using DiOlistic techniques. In this method, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-coated microcarriers are shot at high speed onto the surfaces of living retinal slices where the DiI then delineates axons, somata, and dendrites of isolated neurons. Zebrafish retinal somata were 5-10 mum in diameter. Three horizontal cell types (H-A-1, H-A-2, and H-B) were identified; dendritic tree diameters averaged 25-40 mum. H-A somata were round. Cells classified as H-A-2 were larger than H-A-1 cells and possessed an axon. H-A somata were flattened, without an axon, although short fusiform structure(s) projected from the soma. Bipolar cells were separated into 17 morphological types. Dendritic trees ranged from 10 to 70 muM. There were six B-on types with axon boutons only in the ON sublamina of the inner plexiform layer (IPL), and seven B-off types with axon boutons or branches only in the OFF sublamina. Four types of bistratified bipolar cells displayed boutons in both ON and OFF layers. Amacrine cells occurred in seven types. A(off) cells (three types) were monostratified and ramified in the IPL OFF sublamina. Dendritic fields were 60-150 muM. A(on) pyriform cells (three types) branched in the ON sublamina. Dendritic fields were 50-170 muM. A(diffuse) cells articulated processes in all IPL strata. Dendritic fields were 15-90 muM. These findings are important for studies examining signal processing in zebrafish retina and for understanding changes in function resulting from mutations and perturbations of retinal organization.