Iris pigment epithelium

Anatomy

The iris pigment epithelium (IPE) is a two-cell thick layer of cuboidal cells lying behind the iris. Both layers are jet black due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, the IPE terminates at the pupillary margin. Peripherally, the IPE is continuous with the ciliary epithelium, also double-layered but pigmented in only one layer. The ciliary epithelium is continuous in its turn with the multilayered retina, which has an outermost pigmented layer, the retinal pigment epithelium (RPE). Despite their very different functions and histological appearances, these regions have a common origin from the two layers of the embryological optic cup, which is derived from the neural crest. The melanosomes of the IPE are distinctive, being larger, blacker and rounder than those in the ciliary epithelium or RPE.

Physiology

All of these pigmented layers, including the IPE, help to optimise visual contrast by reducing internal reflected light, just as the black paint does inside a camera. The RPE and ciliary epithelium also have well-understood metabolic roles. No analogous functions for the IPE are known so far, but its intimate contact with the aqueous humour and with the iris stroma make it likely that significant metabolic functions will be found. It can be stimulated in vivo to adopt some of the functions of the RPE, including the abilities to metabolise retinol and to engulf photoreceptor outer segments. This has led to interest in the IPE as a tissue which is much more accessible than RPE, and which, perhaps, can be substituted for RPE cells in research on outer retinal diseases.

Pathology

The IPE is affected by glaucoma, diabetes, and iris inflammations and atrophies of various kinds, generally responding by showing patchy thinning and depigmentation. In complete ocular albinism it is nonpigmented and translucent, which contributes to the photophobia of that condition. However, diseases specific to the IPE itself are almost non-existent, and it seems to be a highly differentiated, stable cell type that almost never gives rise to malignancies, unlike the melanocytes of the iris stroma. Reports of adenomas and adenocarcinomas of the IPE exist, but some of these are debatable.