The blood-retina barrier in health and disease

Abstract

The blood–retina barrier (BRB) is the term used to define the properties ofthe retinal capillaries and the retinal pigment epithelium (RPE), which sep-arate the systemic circulation from the retina. More specifically, the innerblood–retina barrier (iBRB) is used to describe the properties of the endo-thelial cells that line the microvasculature of the inner retina, while theouter blood–retina barrier (oBRB) refers to the properties of the RPE cellsthat separate the fenestrated choriocapillaris from the retina. The BRB isnot a fixed structure; rather, it is dynamic, with its components makingunique contributions to its function and structural integrity, and thereforethe retina. For example, while tight junction (TJ) proteins between retinalendothelial cells are the key molecular structures in the maintenance of theiBRB, other cell types surrounding endothelial cells are also important. Infact, this overall structure is termed the neurovascular unit (NVU). Theintegrity of the BRB is crucial in the maintenance of a ‘dry’, tightly regu-lated retinal microenvironment through the regulation of transcellular andparacellular transport. Specifically, breakdown of TJs can result in oedemaformation, a hallmark feature of many retinal diseases. Here, we willdescribe the oBRB briefly, with a more in-depth focus on the structure andfunction of the iBRB in health and diseased states. Finally, the contribu-tion of the BRB to the pathophysiology of age-related macular degenera-tion (AMD), diabetic retinopathy (DR) and other rarer retinal diseases willbe discussed.