Disk shedding in the absence of a pigment epithelium in the lizard parietal eye

The photoreceptors and pigment epithelia of vertebrate retinas rhythmically synthesize and degrade photosensitive membrane, but the origin of the signal to shed the outer segment tips remains unknown. The parietal eye of lizards contains cone-like photoreceptors but no pigment epithelium. Parietal eye photoreceptors synthesize new disk membrane in a manner similar to rods and cones and also shed their tips rhythmically. The shed material is then engulfed by lumenal macrophages. The signal to shed must originate in, or be transduced by, the photoreceptor.