Choroid Choroid The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissues, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye (at 0.2 mm), while in the outlying areas it narrows to 0.1 mm. The choroid provides oxygen and nourishment to the outer layers of the retina. Along with the ciliary body and iris, the choroid forms the uveal tract. The structure of the choroid is generally divided into four layers (classified in order of furthest away from the
Retinal pigment epithelium The RPE is composed of a single layer of hexagonal cells that are densely packed with pigment granules. At the ora serrata, the RPE continues as a membrane passing over the ciliary body and continuing as the back surface of the iris. This generates the fibers of the dilator. Directly beneath this epithelium is the neuroepithelium (i.e., rods and cones) passes jointly with the RPE. Both, combined, are understood to be the ciliary epithelium of the embryo. The front end continuation of the retina is the posterior iris epithelium, which takes on pigment when it enters the iris. When viewed