Gap junctions in the outer plexiform layer of the chick retina: thin section and freeze-fracture studies

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Abstract
Summary Previous studies have established that gap junctions between presumptive retinal neurons of the chick retina disappear during the course of embryogenesis. The present study examines the 2–3-week-old chick retina to determine if gap junctions are present in the outer plexiform layer of the more mature animal as would be in accordance with evidence from morphological and physiological studies on a variety of other vertebrates. Thin section and freeze-fracture techniques are used in a complementary manner to demonstrate that gap junctions are present between horizontal cell processes in the distal regions of the outer plexiform layer. These junctions appear to be between axon terminals and between spines that project from axon terminals to rods and double cones. Gap junctions are also observed between photoreceptors. They are seen on the synaptic terminals of all classes of cones and are located between the cone synaptic terminals and cone basal processes. Gap junctions are also seen between unidentified photoreceptor basal processes within the neuropil of both distal and proximal parts of the outer plexiform layer. Gap junctions are also present between cone synaptic terminals and deeply invaginated, vesicle-containing processes the origin of which remains to be determined.