Ultrastructure of remnant photoreceptors in advanced hereditary retinal degeneration.

The outer layers of the retinas of pigmented rats affected with hereditary retinal degeneration (rdy) were studied at an advanced stage in the degenerative process, ie, in 200 day old animals. At this age, most of the photoreceptors that survived the degenerative process were cones. The chromatin pattern of their nuclei clearly differentiated them from rods, displaced pigment cells and/or presumptive macrophages that also were found in the outer nuclear layer. None of the cones encountered had outer segments although structures resembling outer segment discs were found with a single cilium. Cones that had inner segments were found in regions of the retina that contained large accumulations of cellular debris. Cones that had lost both inner or outer segments, on the other hand, were found in regions that contained less debris. In such cells, the perikaryon of the cone was rich in mitochondria and other organelles; and the cilium arose directly from the cell body. The morphology of the cones and the fact that they were found in regions of the retina that contained different amounts of cellular debris suggested that cones with inner segments were in an early stage of degeneration while those that had lost inner segments were in a later stage of degeneration. All the cones encountered contained a variety of organelles including: free ribosomes, rough endoplasmic reticulum, and the Golgi apparatus. The cones that survived retinal degeneration therefore appeared to retain the cellular organelles needed for the production of photosensitive pigments. As a consequence, they may be capable of photoreceptor functions.