Freeze-fracture analysis of the neural connections in the outer plexiform layer of the retina of primates (Macaca mulatta and Macaca arctoides) demonstrates a remarkable diversity in the internal structure of the synaptic membranes. In the invaginating synapses of cone pedicles, the plasma membrane of the photoreceptor ending contains an aggregate of A-face particles, a hexagonal array of synaptic vesicle sites, and rows of coated vesicle sites, which are deployed in sequence from apex to base of the synaptic ridge. The horizontal cell dendrites lack vesicle sites and have two aggregates of intramembrane A-face particles, one at the interface with the apex of the synaptic ridge, the other opposite the tip of the invaginating midget bipolar dendrite. Furthermore, the horizontal cell dendrites are interconnected by a novel type of specialized junction, characterized by: (a) enlarged intercellular cleft, bisected by a dense plate and traversed by uniformly spaced crossbars; (b) symmetrical arrays of B-face particles arranged in parallel rows within the junctional membranes; and (c) a layer of dense material on the cytoplasmic surface of the membranes. The plasmalemma of the invaginating midget bipolar dendrite is unspecialized. At the contact region between the basal surface of cone pedicles and the dendrites of the flat midget and diffuse cone bipolar cells, the pedicle membrane has moderately clustered A-face particles, but no vesicle sites, whereas the adjoining membrane of the bipolar dendrites contains an aggregate of B-face particles. The invaginating synapse of rod spherules differs from that of cone pedicles, because the membrane of the axonal endings of the horizontal cells only has an A-face particle aggregate opposite the apex of the synaptic ridge. Specialized junctions between horizontal cell processes, characterized by symmetrical arrays of intramembrane B-face particles, are also present in the neuropil underlying the photoreceptor endings. Small gap junctions connect the processes of the horizontal cells; other gap junctions probably connect the bipolar cell dendrites which make contact with each cone pedicle. Most of the junctional specializations typical of the primate outer plexiform layer are also found in the rabbit retina. The fact that specialized contacts between different types of neurons interacting in the outer plexiform layer have specific arrangements of intramembrane particles strongly suggests that the internal structure of the synaptic membranes is intimately correlated with synaptic function.
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