The fine structure of the retina in the newt, Triturus viridescens.

Abstract

AbstractThe normal adult retina of the North American Urodele, Triturus viridescens, has been studied with the electron microscope. Three basic types of photoreceptors are observed: rods, possibly distinguishable as either red or green rods, single cones, and double cones. The photoreceptors differ from those reported in other amphibia in that: (1) none of photoreceptors contain an oil droplet; (2) all photoreceptors possess an enlarged paraboloid, and, (3) the double cones consist of two complete photoreceptors, a principal cone member and an accessory cone member. The two members are closely related to one another along the length of the inner segment through a clasping of the principal member within cytoplasmic expansions of the accessory member. The principal member inner segment bears strong resemblance to the single cone while the accessory member inner segment is rod‐like in nature.The sustentacular elements of this avascular retina display a diversity of scleral processes. The presence of specific collar‐like glial ensheathments of the photoreceptors at the external limiting membrane contrasts with the extensive interreceptor contacts between all photoreceptor elements throughout the outer nuclear layer. The synaptic bases are characterized by the presence of three types of specific contact sites: (1) interreceptor contacts among all receptor bases; (2) clusters of fine horizontal cell dendrites penetrate the receptor base and terminate in a superficial spray of typical axodendritic synapses; (3) prominent bipolar dendrites enter within the horizontal cell processes terminating in the typical retinal ribbon synapse.The significance of these observations are discussed with respect to the origin of the amphibian double cone, the metabolic support of the retinal, and the potential integration among the receptor cells.