Pigmented Epithelium

Separating the choroid (the middle tunic of the globe, or
eyeball) from the retina proper is a layer of pigmented
cells, known as the pigment epithelium. This layer of the
retina acts as a restraining barrier to the indiscriminate
diffusion of material from the blood in the choroid to the
retina. The retina ends at the ora serrata, where the ciliary
body begins. The pigment epithelium continues forward
as a pigmented layer of cells covering the ciliary body; farther
forward still, the epithelium covers the posterior
surface of the iris and provides the cells that constitute
the dilator muscle of this diaphragm. Next to the pigment
epithelium of the retina is the neuroepithelium, or rods
and cones. Their continuation forward is represented by a
second layer of epithelial cells covering the ciliary body;
the term ciliary epithelium is used to describe the two layers
of cells that are the embryological equivalent of the
retinal pigment epithelium and the receptor layer (rods
and cones) of the retina. The unpigmented layer of the
ciliary epithelium is continued forward over the back of
the iris, where it acquires pigment and is called the posterior
iris epithelium.