The interphotoreceptor matrix and the interphotoreceptor space of the vertebrate retina.

The interphotoreceptor space (IPS) of the vertebrate retina is the adult corollary of the lumen within the embryonic optic vesicle. The inner limit of the IPS is formed by the intercellular junctions of Muller cells and photoreceptors forming the external limiting membrane (ELM). The apical surface of the retinal pigment epithelium (RPE) is the outer limit of the IPS. Most of the volume of the IPS is occupied by the inner and outer segments of the photoreceptors, with the microvillous processes of the RPE, and the small microvilli of the Muller cells filling lesser portions. SEM of samples fixed in glutaraldehyde containing 0.5% ruthenium red show the interstitial spaces are filled with interphotoreceptor matrix (IPM). Biochemical studies and our enzyme digestion studies suggest this extracellular material is enriched in glycoproteins and glycosaminoglycans. The carbohydrates are two-thirds sulfated, 25% sialic acid enriched, and about 10% hyaluronic acid. Most of the IPM proteins can be identified as products of adjacent cells.