Specialized intramembrane organizations of the cone presynaptic membrane in the pigeon retina. Freeze-fracture study.

The presynaptic membranes of the cone cell endings of the pigeon retina were investigated using the freeze-fracture technique. En face views of the cytoplasmic leaflet (P-face) of the split presynaptic membrane revealed several specialized membrane organizations, 1. membrane particle aggregates composed of 10-20 particles which were larger than the usual ones seen in the cell membrane, 2. fenestration-like circular structures of 30-50 nm in diameter which were not surrounded by membrane particles. 3. similiar circular structures as described above but which were accompanied by a few membrane particles on the circular margin and were considered to be an intermediate form of the first and second membrane structures. These three structures appeared simultaneously in one fracture plane of the presynaptic membrane; were situated at the same intervals from one another and were approximately equal in size to synaptic vesicles (30-50 nm). These findings strongly suggested that these three structures were serial events in presynaptic membrane organization. When fortuitous cross fractures exposed both the P-face of the presynaptic membrane and the adjacent cytoplasm of the cone ending, fusion of the synaptic vesicles to the presynaptic membrane was observed, and was considered to be the opening of the synaptic vesicle to the synaptic cleft. These openings were also situated at the same distance as the structures described above. These findings demonstrate the process of exocytosis of the synaptic vesicles by which the chemical transmitter is probably released to the synaptic cleft.