The choroid is a component of the eye, forming part of the uvea, the middle layer of the eyeball’s wall. This vascular layer, also known as the choroidea or choroid coat, is composed of connective tissues. It plays a role in how light interacts within the eye, contributing to vision. Its structure supports the eye’s various functions.
Location and General Structure
The choroid is positioned between the retina, the light-detecting inner layer, and the sclera, the eye’s white outer wall. It extends from the ora serrata, a jagged anterior edge, to the optic nerve posteriorly, covering the rear two-thirds of the eyeball. Its thickness varies, being approximately 0.2 millimeters at the back of the eye and narrowing to about 0.1 millimeters in the outlying areas.
This pigmented layer is continuous with the ciliary body and iris, together forming the uveal tract. The choroid’s external surface is convex and loosely connected to the sclera. Its internal concave surface fits against the retina, though without adherence, allowing for a degree of movement. This structure facilitates its primary role in supporting other ocular tissues.
The Distinct Layers
The choroid is comprised of five distinct layers, moving from the outermost toward the retina. The suprachoroid, also called the lamina fusca, serves as a transitional zone between the choroid and the sclera. This layer is thought to contribute to lymphatic drainage of the eye.
Haller’s layer is the outermost vascular layer, characterized by its larger diameter blood vessels. Immediately beneath it is Sattler’s layer, which contains medium-sized arteries and arterioles. Both Haller’s and Sattler’s layers are embedded within the choroidal stroma, a connective tissue framework.
The choriocapillaris is a layer of capillaries located directly adjacent to Bruch’s membrane. This layer forms an anastomotic network, meaning its capillaries are interconnected, and they arise from the arterioles within Sattler’s layer. The innermost layer of the choroid is Bruch’s membrane, a thin connective tissue sheet that separates the choroid from the retina. It is also known by other names such as lamina basalis or lamina vitra.
Primary Functions
The choroid plays a role in maintaining the health and function of the eye, primarily through its vascular network. It is a main supplier of blood to the retina and optic nerve, which have high demands for oxygen and nutrients due to their constant signaling to the brain. The choroidal circulation accounts for a substantial portion of the blood flow within the eye, contributing to the nourishment of the outer layers of the retina, including the photoreceptors.
Beyond nutrient supply, the choroid also assists in light absorption. It contains a high concentration of melanin, a pigment that helps absorb excess light entering the eye. This absorption prevents light from reflecting within the eye, which could otherwise degrade image clarity and potentially harm vision. The choroid also contributes to thermoregulation, helping to manage the temperature of the retina.
Vascular and Nerve Supply
The choroid receives its blood supply primarily from the posterior ciliary arteries, which are branches of the ophthalmic artery. These arteries include both short and long posterior ciliary arteries.
Venous drainage from the choroid is primarily achieved through the vortex veins. These veins collect blood from the choroidal vascular network and play a role in the overall circulation of the eye. The choroid’s blood flow is also regulated by the autonomic nervous system. The vessel walls of the choroid receive sympathetic and parasympathetic innervation, influencing blood vessel diameter and, consequently, blood flow.