Rings of cross-striated fibrils within the cat cone pedicle: a computer-assisted serial EM analysis.

Using serial electron micrographs and a computer reconstruction system the authors have examined the three-dimensional cytoarchitecture of the cat cone pedicle. These reconstructions reveal that within each pedicle is a closed 4-6 micron diameter ring of cross-striated fibrils similar to the nonring structures described in photoreceptor inner and outer segments by others in guinea pig, rat, chick, monkey, and humans. Our cone pedicle rings encompass the invaginating synaptic contacts of the cone bipolars and horizontal cells and have periodic 70 nm striations surrounding bundles of fibrils about 100 nm in diameter. The authors suggest that these striated rings may be active contractile elements and could be responsible for shape changes in cone pedicles during dark adaptation.