The surface structure of photoreceptor outer segment disks.

The double-membrane disks in the outer segment of photoreceptors in the rabbit retina were isolated structurally intact after crosslinking with glutaraldehyde. The following criteria were used to determine that intact whole disks were isolated: (1) the disk diameter, 1.4 microns; (2) the presence of one incision extending toward the center of the disk; (3) the edge structure; and (4) the presence of particulate material with the dimensions of the particles corresponding to those present in the particulate fracture face of freeze-fractured outer segment disks. The disks were analyzed after negative staining or after freeze-drying and surface replication. The surface of the disks appeared smooth. In damaged disks negative staining revealed the presence of particulate material located below the disk surface. The observations are interpreted to reveal that the lipid bilayers of the disks are located at the disk surfaces shielding the rhodopsin molecules from contact with the cytoplasm present between the disks.