Retinal pigment epithelium

The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells.

History

The RPE was referred to in the 18th and 19th centuries as the pigmentum nigrum, reflecting the observation that the RPE is dark (black in many animals, brown in humans); and as the tapetum nigrum, referring to the observation that in animals with a tapetum lucidum, in the region of the tapetum lucidum the RPE is not pigmented.

Anatomy

The RPE is composed of a single layer of hexagonal cells that are densely packed with pigment granules.

When viewed from the outer surface, these cells are smooth and hexagonal in shape. When seen in section, each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends as a series of straight thread-like processes between the rods, this being especially the case when the eye is exposed to light.

The size and morphology of RPE cells vary with both anatomical location and age. From the central macula (~12–18 μm in height and ~10 μm in width) toward the peripheral retina, the cells gradually become broader, flatter, and more heterogeneous in shape. Ageing is associated with a reduction in RPE cell number, particularly in central regions. Local cell loss may be partially compensated by the lateral migration of adjacent cells, with remaining cells expanding or elongating to cover the affected areas.

Development

The RPE co-differentiates from the neural retina and both layers can mutually affect the other’s development, as in gene mutations in a layer that more severely affects the opposite layer than itself.