The structure and relationships of horizontal cells and photoreceptor‐bipolar synaptic complexes in goldfish retina

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Abstract
After impregnation of goldfish retina by the rapid Golgi method, two classes each of photoreceptor, bipolar, and horizontal cells were observed by light microscopy. Interconnections between these elements in the outer plexiform (first synaptic) layer were investigated by electron microscopy of ultrathin sections, in which the processes of impregnated cells are easily distinguished. Dendrites of large bipolar cells (Cajal's “bipolaires destineés aux bâtonnets”) appeared to contact the synaptic endings of both rods and cones, while those of small bipolars (Cajal's “bipolaires destinées aux cónes”) appeared to contact only cones. Processes from horizontal cells of the vitread level (Cajal's “cellules horizontaux intermédiaires”) appeared to contact only rods, while those from horizontal cells of the sclerad level (Cajal's “cellules horizontaux externes”) appeared to contact only cones. The structures formerly called “synaptic vacuoles” are the terminals of horizontal cell processes in the goldfish, and by analogy they should be so identified in all vertebrates. Teleostean horizontal cells are not typical of neurons or glia cells, but are morphologically intermediate between them. Their most interesting properties are their unique relationship to photoreceptor synaptic endings and their segregation into rod and cone subsystems along with the corresponding photoreceptor and bipolar cells. Although their specific function is not clear, some role in information processing therefore appears likely.