Three-dimensional membrane crystals in amphibian cone outer segments: 2. Crystal type associated with the saddle point regions of cone disks.

In light-adapted, perfusion fixed retinas of the Congo eel salamander, Amphiuma, we have observed distinctive 3-D crystalline domains within the axial array of cone outer segment disks. These crystalline domains, each involving 2-12 disks, have been observed in the distal half of cone outer segment, and are associated with saddle point regions and immediately adjacent segments of the cone disk perimeter. In longitudinal sections, the crystals typically display an axially oriented array of cytoplasmic filaments with lateral spacings in the range of 12-13 nm. The width of the intradiskal compartment is expanded to 8-10 nm within the crystal, and approximates the width of the cytoplasmic compartment. In some sections, the cytoplasmic filaments are axially aligned with intradiskal filaments of similar length and width. In transverse sections, the projected lattice appears to be approximately rectangular, with unit cell dimensions of approximately 12 nm x 12.5 nm. In shape, orientation, dimension, location, projection symmetry and associated membrane spacing relationships, the cone outer segment crystal filaments share a strong resemblance with lattice filaments located along the perimeters and preincisures of rod disks. These similarities suggest that the cone crystal filaments may be related to the rim protein of rod disks. Lastly, the preferential association of these crystals with saddle point regions indirectly supports the hypothesis that reductions in COS disk area with apical displacement are accomplished by resorption of disk membrane components through the saddle points.