Blood-retina barriers

Abstract

Neurons of the retina are segregated from the blood stream via two different gateways: the inner and the outer blood-retina barriers (BRBs). The inner BRB consists of endothelial cells forming retinal capillaries. The inner barrier is similar to the microvascular blood-brain barrier (BBB) within other regions of the central nervous system (CNS). The outer blood-retina barrier (oBRB) is formed by the retinal pigment epithelium (RPE), which is positioned as interface between the neural retina and the nonneural choroid. In humans, only the inner two-thirds of the 250-μm thick retina is vascularized, and thus served by the inner blood barrier. The outer one-third of the retina is devoid of any vasculature and depends on nutrient/gas exchange via the RPE. Blood components reach the RPE from fenestrated, highly permeable blood vessels of the neighboring nonneuronal choroid. Despite the fact that both the endothelial and epithelial barriers have ontogenetically different origins and very diverging cellular structures, on the molecular and functional levels many similarities are evident. These include barrier characteristics based on tight junctions, carrier features with regard to transporters including P-glycoprotein (P-gp)/multidrug resistance protein, expression patterns of biotransformation enzymes, and immunological responses. © 2007 Springer-Verlag US.