Choroid

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Structure of the eyeball seen in a transverse section.

The choroid forms part of the vascular layer of the eyeball, along with the ciliary body and iris. It is a thin, pigmented vascular connective tissue layer of the eyeball that extends from the ora serrata to the optic nerve (CN II). It is located between the fibrous outer sclera and inner retina, extending from the optic nerve posteriorly.

The choroid is approximately 200μm thick at birth and decreases in thickness with age (decreasing to about 80μm at 90 years of age). This vascular tissue structure of the eye is formed of five main layers (from internal to external): bruch's membrane, the choroiocapillaris, the two vascular layers (Haller's and Sattler's), and the suprachoroidea. 

Bruch's membrane forms the innermost layer of the choroid. It is a thin connective tissue sheet that separates the retina from the choroid and supplies oxygen and nutrition to the outer retina.

The choroiocapillaris layer is immediately adjacent to bruch's membrane and is formed by an anastomotic network of capillaries. These capillaries arise from the arterioles of Sattler's layer of the choroid. The choroiocapillaris provides nourishment to the retinal epithelium and photoreceptors. It also filters waste products from the outer retina and functions in thermoregulation of the back of the eye. 

The highly vascularized region of the choroid consists of the outer Haller's layer and the inner Sattler's layer embedded in choroidal stroma. Haller's layer is formed of large blood vessels, while Sattler's layer is composed of smaller arteries and arterioles. Surrounding stroma is formed by collagen, elastic fibers, fibroblasts, non-muscular smooth muscle cells and melanocytes, as well as macrophages, mast cells and lymphocytes.

The final layer outer layer of the choroid is the suprachoroidea, which is a transitional zone between the choroid and adjacent sclera. It is composed of collagen fibers, fibroblasts and melanocytes. This thin outer layer contains epithelium lined spaces which empty into veins and are thought to play a role in lymphatic drainage of the eye. 

The choroid receives its rich vascular supply from the short and long posterior ciliary arteries and is drained by the vorticose veins. The vessel walls of the choroid receive parasympathetic innervation from the pterygopalatine ganglion and sympathetic innervation from the superior cervical ganglion.

The main function of the choroid is to provide oxygen and nourishment to the outer retina. However, it is also important in thermoregulation of the posterior eye, the modulation of intraocular pressure and in the drainage of aqueous humor from the anterior chamber.