Fine structure of the afferent synapse and gap junctions on the sensory hair cell in the saccular macula of goldfish: a freeze-fracture study.

Membrane specializations at the active zone of the afferent synapse in the saccular macula of goldfish are described. Those of the presynaptic membrane consist of three to six elongated aggregates of intramembrane particles separated by particle-free furrows on the concave part of the P face and its complementary figure, that is, an alternate arrangement of elongated aggregates of pits and smooth ridges on the convex part of the E face. The size of the specialized area is about 0.5 micrometer x 0.3 micrometer. Vesicle fusion sites are situated at the margin of the particle-free furrow and ridge of the presynaptic active zone. Round pores about 30-50 nm in diameter are seen on the P face around the active zone. They are probably openings of the anastomosing tubules or coated pits. A focal aggregate of intramembrane particles is observed on the E face of the postsynaptic membrane apposing the presynaptic active zone. The P face of the postsynaptic active zone shows pits and particles. Small gap junctions are found between hair cells and adjacent supporting cells. They are frequently associated with desmosomes. The possible functional significance of these gap junctions is discussed.