The interphotoreceptor matrix, a space in sight

The interphotoreceptor matrix (IPM) has in recent years been receiving much attention due to its delicate localization between the photoreceptors and the retinal pigment epithelium (RPE). The IPM is a resilient, structure forming and hydrophilic matrix composed of large glycoproteins and proteoglycans, which occupies the subretinal space between the photoreceptors. The IPM is most likely assembled with components synthesized by all the surrounding cell types: the photoreceptor cells, the RPE cells, and the Müller cells. It has been implied to be involved in the development and maintenance of photoreceptors, and as a major factor in retinal adhesion. Therefore, it has been thoroughly studied also in several models of photoreceptor degeneration. Comparative studies have revealed some remarkably consistent features between different species, such as the presence of the rod and cone specific matrix domains. Studies made in the IPM of several species have measured large fluctuations in ion concentrations as a result of changes in illumination. In some species, these ionic fluctuations coincide with the intriguing dynamic redistributions of IPM constituents that can be visualized with histochemical techniques. It can be hypothesized that because of the intensive biochemical activity and the frequent changes in metabolic states of rods and cones the IPM may act as a kind of "buffer." These studies have brought a new extracellular aspect to photoreceptor studies and a new perspective to photoreceptor-RPE research.