Comparison of photoreceptor-specific matrix domains in the cat and monkey retinas

Two lectins, wheat germ agglutinin (WGA) and peanut agglutinin (PNA), were used to compare domains within the interphotoreceptor matrices (IPM) of the cat and monkey, two species where the morphological relationship between the retinal pigment epithelium (RPE) and photoreceptors is distinctly different. In the monkey, PNA labeling was heaviest over the cone outer segments and a discrete region of the interphotoreceptor matrix bordering the cone inner and outer segments--a region which has been termed the cone matrix sheath. Near the apical border of the RPE, the outer margin of the PNA-labeled matrix is surrounded by a circular array of apical microvilli. In the cat retina, PNA labeling was highest over a region of the IPM lying between the outer margin of the cone sheath processes and surrounding rod matrix. In contrast, intracellular labeling of cone inner and outer segments was sparse. The RPE apical processes forming the cone sheath were not labeled. In the monkey retina, WGA preferentially labeled rod outer segments and the region of the IPM around rod inner and outer segments. The cone matrix sheath was not preferentially labeled using this lectin. Rod inner segments and cone inner and outer segments were labeled moderately. In the cat retina, WGA labeling was dense over both rod outer segments and cone outer segments as well as the cone sheath. Rod and cone inner segments, as well as the IPM around both rods and cones, were moderately labeled. These observations suggest that the specialized processes arising from the apical surface of retinal pigment epithelial cells, together with photoreceptor-specific extracellular matrix domains, contribute to the formation of specific microenvironments around rod and cone photoreceptor cells.