Membrane specializations in the outer plexiform layer of the turtle retina

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Abstract
The internal organization of the plasma membrane at specialized contacts in the outer plexiform layer of the turtle, Pseudemys scripta elegans, was analyzed with the aid of the freeze‐fracturing technique. In the invaginating synapse of cone pedicles the plasma membrane of the photoreceptor ending contains an aggregate of P‐face particles, images of synaptic vesicle exocytosis, and rows of forming coated vesicles which are arranged in sequence from apex to base of the synaptic ridge. Thus, freeze‐fracturing provides positive evidence that the synaptic ridge represents the active zone at the surface of the photoreceptor endings. Horizontal cell processes of dyads and triads have an aggregate of P‐face particles opposite the apex of the ridge, but lack images of vesicle exocytosis. Deep‐etching and rotaryshadowing demonstrate that an array of minute protrusions decorates the true outer surface of the horizontal cell membrane at the site of the intramembrane particle aggregate. The membrane of the invaginating bipolar dendrite is unspecialized. At basal junctions, the cone pedicle membrane has a sparse complement of P‐face particles, but images of vesicle exocytosis are absent. The adjoining bipolar membrane is characterized by a prominent aggregate of E‐face particles, often arranged in an orthogonal lattice. The freeze‐fracture profile therefore suggests the existence of (1) a sign‐conserving cone‐to‐horizontal cell synapse; (2) a sign‐inverting synapse between cones and invaginating bipolar dendrites; and (3) a sign‐conserving synapse between cones and bipolar dendrites at basal junctions. No freezefracture evidence was found for a horizontal‐to‐cone or horizontal‐to‐bipolar cell synapse within the synaptic invaginations.