Overview of RPE Structure
The RPE is a monolayer of neuroectoderm-derived epithelial cells, located between the highly vascular choriocapillaris and the photoreceptor outer segments (Figs 13-1, 13-2). Embryologically, it is derived from the same neural anlage as the neurosensory retina. The retina and RPE develop as an invagination of the embryonic optic cup that folds the neuroectodermal layer into apex-to-apex contact with itself. The outer layer forms the RPE and the inner layer forms the neurosensory retina. The intervening area remains throughout life as a potential space and is the plane of separation for retinal detachment.
In humans, there are approximately 4–6 million RPE cells per eye. On the apical surface of RPE cells are long microvilli that interdigitate with the outer segments of photoreceptor cells (see Figs 13-1, Fig 13-2). These cells are joined near their apical side by tight junctions that establish polarity, block the passage of water and ions, and constitute the outer blood–retina barrier. The RPE basal surface, which is adjacent to the Bruch membrane (an extracellular matrix between the RPE and the choriocapillaris), has many infoldings that increase the surface area available for the exchange of solutes (see Fig 13-1).
In addition to the organelles found in most cells (eg, the nucleus, Golgi apparatus, smooth and rough endoplasmic reticulum, and mitochondria), RPE cells contain melanin granules and phagosomes, reflecting their role in light absorption and phagocytosis (discussed later in the chapter). The RPE is particularly rich in microperoxisomes, suggesting that it is active in detoxifying the large number of free radicals and oxidized lipids generated in this highly oxidative and light-rich environment.
Figure 13-1 The retinal pigment epithelium (RPE) and Bruch membrane. A, The Bruch membrane separates the RPE from the choriocapillaris. Note the interdigitation of the apical processes of the RPE with the photoreceptor outer segments as well as the infoldings of the basal surface. B, The thickness of the different layers of Bruch membrane is demonstrated (starting from the top): basement membrane of the RPE, inner collagenous layer, elastic layer, outer collagenous layer, and basement membrane of the choriocapillaris. APRPE = apical processes of the RPE.
(Part A modified courtesy of University of Rochester; part B illustration by Daniel Casper, MD, PhD.)
Figure 13-2 RPE. A, Monolayer of cultured RPE cells grown to confluence shows the polygonal appearance of the RPE cells (phase microscopy, ×25). B, Scanning electron micrograph of RPE cells demonstrates the apical microvilli.
(Parts A and B reproduced with permission from Handa JT. Cell biology of the retinal pigment epithelium. In: Schachat AP, ed.
6th ed. Edinburgh: Elsevier; 2017: Figs 18.1 and 18.4.)
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.