Anatomy

Abstract

The name “uvea” is derived from the Latin uva, or grape as the eye, peeled of sclera hanging on the “stalk” of the optic nerve, reminded early anatomists of a black grape. The uvea, the middle coat of the eye, consists of a highly pigmented vascular layer composed of heterogeneous types of mesenchymal cells including fibroblasts, melanocytes, and vascular supporting cells that arise mostly from the neural crest (mesectoderm) but also from the mesoderm, including vascular endothelial cells. The uveal tract is divided into three distinct portions: the iris, the ciliary body, and the choroid. It performs several functions: the anterior uvea, the iris, is the “diaphragm” of the eye, regulating light that enters the eye through the pupil. The middle portion, the ciliary body, is essential for lens accommodation and aqueous humor formation. The most posterior, and largest, portion of the uvea, the choroid, provides nourishment for the outer retina, especially in the region of the macula, and regulates ocular temperature. Microvessels in the uveal tract bear basic similarities to the capillaries or vasculature of other tissues and organs and have a similar ultrastructure and organization, although the lumen of choriocapillaris vessels is of unusually wide caliber compared to other capillary beds. However, the capillaries in the ciliary processes are large and fenestrated as are those in the choriocapillaris, whereas in other zones of the uveal tract (iris, ciliary body stroma), the capillaries are non-fenestrated. There are no lymphatics in the mammalian uveal tract; however, the uveal tract is richly endowed with populations of immune cells including macrophages, dendritic cells, and connective mast cells. Understanding the phenotype, distribution, and function of these key resident players in immune regulation is critical to unraveling the complex nature of the variety of intraocular inflammatory responses. The biology of these cells in the uvea is described in chapter 3 and detailed reviews are available.