Proteoglycans in the mouse interphotoreceptor matrix. I. Histochemical studies using cuprolinic blue.

Cuprolinic Blue, when applied at a critical electrolyte concentration, can be utilized for assessing the localization and structural characteristics of proteoglycans with electron microscopy. We have used this cytochemical procedure to evaluate the distribution of proteoglycan in the interphotoreceptor matrix of the mouse retina. Cuprolinic Blue-positive filaments of two distinct morphological types were present surrounding both rod and cone photoreceptors. Large filaments, 115-135 nm long and 15-25 nm in diameter, were distributed in the interphotoreceptor matrix around the outer segment and outer portion of the inner segment. These filaments appeared linked to each other to form a complex meshwork. Smaller filaments, 60-70 nm long and 5-10 nm in diameter, were principally observed around the photoreceptor inner segments. Incubation of retinas with chondroitinase AC and chondroitinase ABC eliminated Cuprolinic Blue staining of both large and small filaments, whereas hyaluronidase treatment reduced the size of the filaments but did not eliminate their staining. When retinas were washed extensively prior to fixation and staining, Cuprolinic Blue-positive filaments remained associated with the photoreceptor cell surface. These results suggest that the interphotoreceptor matrix of the mouse retina contains at least two structural types of proteoglycan, of the chondroitin sulfate-type, which are differentially distributed in this compartment. One of the proteoglycans forms a complex meshwork which surrounds the photoreceptors. Both are insoluble and appear to be firmly attached to the photoreceptor plasma membrane.