Participation of photoreceptor cells in retrieval and degradation of components in the interphotoreceptor matrix.

Evidence for endocytosis of components of the interphotoreceptor matrix by photoreceptor inner segments is presented. Photoreceptors cultured in the presence of horseradish peroxidase (HRP) rapidly interiorized this tracer as evidenced by the presence of HRP-reaction product within primary endocytotic vesicles in the inner segment cytoplasm. The tracer was also observed in multivesicular bodies. Retinas were incubated in a suspension of interstitial retinol-binding protein (IRBP) coated colloidal gold (CG). IRBP-CG was interiorized by photoreceptors where it was found in endocytotic vesicles and in multivesicular bodies in both rod and cone inner segments. Acid phosphatase reaction product co-localized with IRBP-CG in multivesicular bodies suggesting that IRBP is degraded following endocytosis and is not recycled.