The iris pigment epithelium (IPE) is a one cell thick layer of cuboidal cells lying behind the iris. The epithelial cells are highly pigmented 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 inner, non-pigmented layer of the ciliary epithelium. The iris dilator muscle is strictly attached to the anterior side of the iris pigmented epithelium and represents the anterior continuation of the pigmented ciliary epithelium. The ciliary epithelia represent the anterior continuation of the multilayered retina, whose retinal pigmented epithelium (RPE) corresponds to the pigmented ciliary epithelium, while the multilayered sensory retina fades into the non-pigmented ciliary epithelium. Despite their very different functions and histological appearances, these regions have a common origin from the two layers of the embryological optic cup. The melanosomes of the IPE are distinctive, being larger, blacker and rounder than those in the ciliary epithelium or RPE.
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👉 Iris pigment epithelium in the context of Eye color
Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.
In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky. Neither blue nor green pigments are present in the human iris or vitreous humour. This is an example of structural color, which depends on the lighting conditions, especially for lighter-colored eyes.