Proteoglycans in the mouse interphotoreceptor matrix. V. Distribution at the apical surface of the pigment epithelium before and after retinal separation.

The distribution of chondroitin sulfate proteoglycans in the mouse interphotoreceptor matrix (IPM) proximal to the retinal pigment epithelium (RPE) was evaluated with EM histochemical techniques using Cupromeronic Blue (CmB) before and after retinal separation. Densely packed, sheet-like processes surrounding vesicle-like compartments containing CmB staining were normal constituents of the IPM at the apical surface of the RPE. Most of the vesicle-like compartments, which appeared to be isolated from the IPM in single section profiles, were found to be continuous with the IPM when three-dimensional reconstructions of serial thin sections were evaluated. Analyses of stereo image pairs of semithin sections visualized by electron spectroscopic imaging (ESI) also demonstrated that the CmB stained proteoglycans in the lumen of these pseudovesicles were in continuity with the CmB stained components present in the IPM. Moreover, ESI demonstrated that the CmB stained profiles formed an elaborate interconnecting network extending from the apical border of the RPE to the level of the outer limiting membrane of the retina. After removal of the retina, rinsing of the RPE with Ringer's solution prior to fixation eliminated proteoglycan staining near the base of the apical processes, whereas staining near the tips of these processes remained. The CmB stained filaments remaining after rinsing were thicker and shorter, and made fewer interconnections than those in the non-separated preparations. These results suggest that two types of chondroitin sulfate proteoglycans are present in the IPM which differ in distribution and in the degree of aqueous solubility. Additionally, a closely associated retina appears to be required for maintenance of the normal structure of proteoglycans associated with the RPE surface. The elaborate topography at the RPE apical surface may play a role in the delivery and/or recovery of components of the IPM.