Studies on the retina of the gecko Coleonyx variegatus: I. The visual cell classification

Abstract

Light microscopy studies have established the retina of Coleonyx as containing only rod-type visual cells of three classes, Single rods, and two classes of Double rods. Three additional classes of rods were found in this electron microscopic study which included a Twin rod and two classes of Triplet rods. A new visual cell class nomenclature was presented and defined, and was based on the general morphology of the receptor cell and the relationship of the receptor members to each other. The terminology presented was: Single, Doublets (D 1 and D 2 ), Twin, Triplet 1 and Triplet 2 . A Quintuplet rod was described, but only one was ever found, and this class may not be of physiological significance. The receptor outer segment disks had a five-layered membrane structure when the tissue was fixed in osmium tetroxide and the sections were stained with saturated uranyl acetate at 60°C. The ellipsoid body was comprised of tightly packed mitochondria which fell into two size groups: small mitochondria (3.0–5.5 m μ ) and large mitochondria (11–18 m μ , with a few up to 35 m μ ). The distribution of the small mitochondria formed a hemisphere about the base and sides of the ellipsoids with the large mitochondria in the ellipsoid center. Two types of radially symmetrical paraboloids were described, a granular type which had a central granular mass surrounded by a system of membrane lamellae, and a membranous type which lacked the central granular mass and consisted only of radially symmetrically arranged paired membrane lamellae between which granules were located. In cross sections of the vitread paraboloid and myoid, radial plasma membrane extensions from the visual cells, the lateral fins, were seen to interdigitate freely with Müller cell cytoplasmic extentions. A function of the inner segment-paraboloid-myoid structures in relation to a possible metabolic pathway was presented and discussed.