Characterization of the interstitial space: immunocytochemical and biochemical studies.

The interstitial space has been characterized using immunocytochemical and biochemical techniques. The pore size of the external limiting membrane that forms the vitread margin of this extracellular space has been probed using a series of biotinylated proteins of known Stokes' radius, followed by avidin-horseradish peroxidase histochemistry. The pore size of the zonulae adherentes that collectively comprise the external limiting membrane has been found to lie between 30 and 36A. This is sufficiently small to block passage of interstitial retinol-binding protein (IRBP, Stokes' radius = 55A) from the interstitial space vitread into the retina. IRBP has been localized to the interstitial space of bovine retina by use of indirect immunogold electron microscopy. IRBP has also been identified in cytoplasmic vesicles of rod and cone inner segments. The function of these vesicles may be for secretion and/or uptake of IRBP from the interstitial space. A postulated function of IRBP is transfer of vitamin A between the retinal pigment epithelium and photoreceptors during the visual cycle. Biochemical experiments have corroborated previous studies that IRBP becomes loaded with all-trans-retinol following bleaching illumination, while the amount of endogenous 11-cis-retinol remains unchanged. These data are consistent with the emerging concept that IRBP plays a role in transport of vitamin A among the cell types that border the interstitial space.