Photoreceptor fine structure in the archerfish

Abstract

The fine structure and arrangement of the photoreceptor cells of the archerfish (Toxotes jaculatrix) have been studied by electron microscopy.Rods, twin cones, and single cones are present. In the light-adapted state, the rods are very tall cells reaching almost to the base of the retinal epithelial cells. The outer segment is composed of membranous discs of uniform diameter displaying a single incisure. The rod inner segment displays a distal small ellipsoid and an extremely thin myoid region. The nuclei of rods are electrondense,and the synaptic spherule displays two or three invaginated sites. The single cone is similar to the individual members of a twin cone and displays a tapering outer segment and accompanying accessory outer segment. The wider cone inner segment contains a large, centrally located ellipsoid and a peripheral region rich in endoplasmic reticulum, polysomes, and microfilaments.Twin cones display subsurface cisternae along their entire contiguous surfaces.The cone nuclei are large and vesicular and located vitread to the external limiting membrane. The synaptic pedicle of cones is larger and more electronlucent and contains more invaginated "ribbon" synaptic sites (ten to 12) than do rods. In addition, small "coated" invaginations and larger synaptic vesiclefilled processes are also seen within cone pedicles. In the light-adapted state the cone photoreceptors are arranged in a repeating square mosaic pattern with one single cone surrounded by four twin cones.