Structure of the synaptic membranes in the inner plexiform layer of the retina: a freeze-fracture study in monkeys and rabbits.

The internal structure of the synaptic membranes in the inner plexiform layer (IPL) of the retina of monkeys and rabbits was studied with the freeze-fracturing technique. In ribbon synapses, the presynaptic active zone is characterized by an aggregate of P-face particles, images of synaptic vesicle exocytosis, and forming coated vesicles which occupy distinct, contiguous membrane domains from apex to base of the synaptic ridge. The postsynaptic membrane contains a prominent aggregate of homogeneous particles which remain associated with the E-face. In the presynaptic membrane of conventional synapses, images of synaptic vesicle exocytosis are intermingled with large P-face particles, whereas forming coated vesicles surround the active zone. Three types of internal organization characterize the postsynaptic membrane of conventional synapses. Usually, the postsynaptic membrane exhibits the same internal structure as the surrounding nonjunctional plasmalemma. A second, less common type of conventional synapse contains a loose aggregate of heterogeneous particles which remain associated with the P-face. Finally, synapses were exceptionally found which are macular in shape and contain an aggregate of E-free particles within the postsynaptic membrane. The freeze-fracture evidence suggests that the axonal endings of bipolar cells--or at least some of them--make excitatory synapses, whereas the vast majority of amacrine cell dendrites make inhibitory synapses. Additional specializations of the cell surface in the IPL include gap junctions, puncta adhaerentia, subsurface cisterns, and cell corner aggregates.