Basal junctions at synaptic endings of turtle visual cells.

Abstract

It has been often assumed that the first synapse of the visual pathway is confined to the region where the visual cell ending is indented by processes of other retinal elements (4, 9, 15), identified as horizontal and bipolar cells (11, 16). In the vicinity of the indented area, the visual cells contain small vesicles similar to those found at other synapses (3, 13, 14), and dense lamellae are conspicuously placed between the branches of the indenting processes (9). This recessed junction has received considerable attention in the past, but recently the existence of other junctional areas between visual cells and second order neurons has been suggested in the primate and frog retinas (6, 7, 11). In those instances junctions were found at the basal surface of cone pedicles, and structures interpreted as filaments extending between both cell membranes were noted in the frog retina (6). During the course of previous work on toad and rat retinas, I have also occasionally noticed the presence of opaque cross-bars in the intercellular gaps surrounding visual cell endings. These structures did not seem to be located within the area of the recessed junction, but the complex architecture of the toad and rat retinas made this interpretation