Morphological components associated with frog cone outer segment disc margins

The two margin morphologies of frog retinal cone outer segment (COS) discs were examined by thin sectioning, freeze-fracture, and deep-etch, rotary shadowing techniques. The disc margin adjacent to the connecting cilium, which morphologically resembles the terminal loop of rod outer segment (ROS) discs, exhibited a distinctive staining density tightly apposed to the membrane surface facing the lumen. This density was crescent-shaped in longitudinal sections, and a continuous band in cross-sections. Associated with the disc margin opposite the cilium, two additional extracellular structures were observed: a globular staining density located at the outer edge of the membrane loop forming the margin, and filaments axially interconnecting adjacent margins. The globular densities and filaments were spaced at regular intervals along the margin. Where these margins were adjacent to calycal processes, the globular densities appeared to span the extracellular gap and interconnect the membranes of the COS discs and the calycal process. Distinctive intramembrane particles were observed along both margins by freeze-fracture. The distribution of the globular and filamentous elements suggests that they may have a role in maintaining the radial dimensions and axial spacing of the associated disc margin by forming an extracellular framework.