Freeze-fracture study of photoreceptor outer segments and pigment epithelium in dystrophic and normal retinas.

The intramembrane organization of outer segment (OS) membranes and pigment epithelial (PE) microvilli has been studied in rats (10-17 postnatal days) with inherited retinal degeneration (RCS) and in normal retinas from genetically controlled rats (RCS-rdy+). The OS plasma membranes of both dystrophic and normal retinas are characterized by large particles surrounding circular, particle-free zones on the P-faces (cytoplasmic leaflets) and a sparse distribution of particles on the E-faces (external leaflets). No regional differences in particle distribution are observed in either basal or distal plasma membrane regions. Outer segment disc membranes are characterized by large, densely packed P-face particles and ridged E-faces with very few particles. Small, particle-free patches of membrane are present in the basal disc P-faces of both normal and dystrophic retinas, which Andrews and Cohen ("79) have described as characteristic of newly added disc membrane. In dystrophic retinas, larger, particle-free domains are observed in the distal disc membranes (P-faces) and accumulating membranous debris. In older retinas, which have accumulated more debris, the particle-free domains occupy vast areas of the membrane faces and it is not possible to identify these membranes as belonging to either discs of plasma membranes. No comparable areas of particle-free membrane are observed in the distal discs and OS plasma membranes of normal retinas. Pigment epithelial microvillus membranes are characterized by an intermixture of large and medium-sized particles surrounding irregular particle-free areas, but no differences between normal and dystrophic PE membranes are observed. The changes in particle distribution observed in the dystrophic retinas suggests that the intramembrane molecular composition of older disc membranes has become altered or rearranged as the OS membranes accumulate as debris.