Histochemical characterization of the interphotoreceptor matrix in the retina of Octopus bimaculoides

Proteoglycans, located in the interphotoreceptor matrix (IPM) of vertebrate retinas, mediate interactions between the photoreceptors and retinal pigment epithelium. Molluscan retinas also have an IPM located between apposing rhabdomeres. Like the cone matrix sheath of the vertebrate IPM, the octopus IPM is labeled by peanut agglutinin (PNA) and contains retinoid-binding-like proteins. In this study we demonstrate further similarities of the vertebrate/invertebrate IPM and identify specific molecular components in this extracellular compartment of the octopus retina. For light microscopy, paraffin-embedded sections of octopus retinas were stained with dyes specific for acid mucopolysacharides including Alcian blue and colloidal iron. In addition, sections were digested with enzymes specific for hyaluronan, chondroitin sulfate, and sialoglycoconjugates. Digestion of sections with these enzymes and subsequent staining with Alcian blue or colloidal iron demonstrated the presence of chondroitin sulfate and sialoglycoconjugates in the octopus IPM as well as other retinal layers and cells. At the electron-microscope level we treated retinal tissue with Cuprolinic Blue and observed the distribution of sulfated glycosaminoglycans along the rhabdomere edges facing the IPM and in a more central area of the IPM where microvillous processes of supportive cells are located. The octopus IPM may have importance in retinal structure and may be a scaffolding on which molecular components of the IPM are located.