In addition to neural and glial cells, the retina contains blood vessels with endothelial cells and pericytes. Pericytes surround the endothelial cells and are modified smooth muscle cells that play a role in autoregulation of retinal blood vessels. Endothelial cells form the blood–retina barrier; pericytes structurally support the endothelium and suppress proliferation, loss of which leads to increased permeability and development of microaneurysms.
The neuronal, glial, and vascular components of the retina together form a functional neurovascular unit. Ophthalmoscopically, the vascular components are the only visible part of the retina. The neural retina lacks pigment (except for foveal xanthophyll) and is transparent, thus allowing the passage of light through the inner retinal layers. Conditions such as diabetic retinopathy are categorized on the basis of clinically evident vascular changes. Despite the clinical emphasis on vascular changes, there is strong evidence of neuronal dysfunction early in the disease process, even prior to the detection of clinically evident disease.
Gardner TW, Antonetti DA, Barber AJ, LaNoue KF, Levison SW. Diabetic retinopathy: more than meets the eye. Surv Ophthalmol. 2002;47(Suppl 2):S253–S262.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.