Horizontal cells of the pigeon retina.

Two types of horizontal cells are seen in Golgi-impregnated retinas of the pigeon. Type I horizontal cells are compact, "brush-shaped," and have an axon ending as an irregular spinous arborization. The majority of the dendrites terminate in the distal part of the outer plexiform layer (OPL) as clusters which contact cones, but some terminate as single expansions in the proximal part of the OPL. The axon terminal spines are found only in the distal part of the OPL and contact both rods and cones. Pigeon Type I horizontal cells are Cajal's "brush-shaped" cells, and their axon terminals resemble Cajal's "stellate" cells. Type II horizontal cells have irregular, wavy, multi-branched dendrites, appear horizontally flattened, and lack axons. The dendrites terminate in the proximal part of the OPL as isolated spines and contact only cones. The Type II horizontal cells of the pigeon have not been previously described in the avian retina.