A quantitative study of intramembrane changes during cell junctional breakdown in the dystrophic rat retinal pigment epithelium

Previous electron microscope freeze-fracture and tracer studies have revealed that intercellular junctions in the retinal pigment epithelium (RPE) of Royal College of Surgeons (RCS) rats with inherited retinal dystrophy [5] break down between three and six postnatal weeks [6, 7]. In this study quantitative computer techniques were used to analyze the freeze-fracture changes in the dystrophic RPE. The following parameters were measured: length of tight junctional strands/μm²; number of tight junctional strand anastomoses/μm²; number of gap junctional aggregates/μm²; area of gap junctional aggregates/μm²; and density of background intramembrane particles/μm². At three postnatal weeks, the dystrophic junctional complex membrane is similar to normal, but at 10 weeks and later there are dramatic decreases in tight junctional strand length/μm² and number of anastomoses/μm², as well as in the number/μm² and area of gap junctions/μm², while the density of background particles/μm² is dramatically increased. Correlational analysis revealed that changes in gap and tight junctions were significantly related to each other and to the increase in background particle density. The diameter of background particles within the normal and post-breakdown dystrophic junctions was measured in order to see whether the dispersal of gap and tight junctional particles (8-10 nm) into the surrounding membrane contributes to the increased particle density. These measures showed that background particles in all size ranges were more numerous in the dystrophic RPE, but that the largest increase was in the smallest diameter particles (6-7 nm). Thus, while gap and tight junctional sized particles contribute to the increase, particles from other sources may also be involved. Particle density of apical and basal membranes in the normal and in the 10 week and older dystrophic RPE was analyzed to study the effects of tight junctional breakdown on the distribution of intramembrane particles. These measures showed that particle density was greater basally than apically in the normal RPE and that particle density in both membranes decreased slightly in the dystrophic RPE, but that their ratio remained unchanged. It has been shown previously that even a single intact tight junctional strand is sufficient to maintain differences in particle density between apical and basal surfaces [14, 15] and in the majority of abnormal dystrophic junctional complexes at least one tight junctional strand remains intact. Thus, while an influx of particles from apical and basal surfaces may contribute to the increase in background particles in junctional complex membrane, a large scale redistribution of intramembrane particles does not occur during tight junctional breakdown in the dystrophic RPE.