Ultrastructure of the synaptic ribbons in photoreceptor cells of Rana catesbeiana revealed by freeze-etching and freeze-substitution.

The three-dimensional structure of synaptic ribbons in photoreceptor cells of the frog retina was studied with freeze-etching and freeze-substitution methods, combined with a rapid-freezing technique. Although the synaptic ribbon consisted of two electron-dense plaques bisected by an electron-lucent layer in conventional thin sections, such lamellar nature was not so evident in freeze-etched replicas. The cytoplasmic surfaces of the synaptic ribbon presented an extremely regular arrangements of small particles 4-6 nm in diameter. Fine filaments 8-10 nm in diameter and 30-50 nm in length connected synaptic vesicles and the ribbon surface. These connections were mediated by large particles on both ends of the filaments. Approximately 3-5 filaments attached to one synaptic vesicle. Synaptic ribbons were anchored to a characteristic meshwork underlying the presynaptic membrane via another group of similar fine filaments. The meshwork seemed to be an etched replicated image of the presynaptic archiform density observed in thin sections.