A freeze-fracture study of synaptogenesis in the distal retina of larval Xenopus.

Synapse formation between photoreceptor, bipolar and horizontal cells of the larval Xenopus retina was studied by the freeze-fracture technique. Photoreceptors and horizontal cells were joined by ribbon synapses; photoreceptor and bipolar cells by basal junctions. Gap junctions were found between photoreceptors and between horizontal cells. Horizontal cell dendrites invaginated receptor bases before the plasma membrane of either cell showed zones of intramembrane (IMP) particle accumulation. Subsequently the receptor cell began to form a synaptic ridge where P-face IMPs aggregated at a protrusion of the surface membrane. The length of the ridge and the density of its IMPs increased between larval stages 40 and 56. Cross-fractured views of receptor cytoplasm at different larval stages showed that synaptic ribbons and synaptic vesicles developed in conjunction with the ridge. Plasmalemmal deformations suggesting sites of vesicle fusion or uptake were noted adjacent to the apex of the ridge. Horizontal cell dendritic membrane first accumulated P-face IMPs at several small regions; subsequently the IMPs became aligned over a broad membrane area. Both rod- and cone-related horizontal cell dendrites also manifested a loose patch of E-face IMPs which subsequently was transformed into a linear array. Basal junctions were characterized by a P-face IMP aggregate in the photoreceptor membrane and an E-face IMP aggregate in the bipolar cell membrane. Basal junctions appeared suddenly in a mature configuration at larval stage 42.