What is the choroid?

The choroid is that part of the uveal tract extending from the edge of the optic nerve head to the ora serrata. The choroid underlies the retinal pigment epithelium and is continuous anteriorly at the ora serrata with the ciliary body.

ATTACHMENTS:

The choroid is part of the uveal tract which includes the iris and ciliary body as well. The uveal tract is attached at 3 sites:

1. the scleral spur

2. internal scleral exit channels of the vortex veins

3. optic nerve

The usually heavily pigmented posterior or choroidal portion of the uveal tract is loosely adherent to the overlying sclera. This plane of loose attachment is a zone of potential separation known as the suprachoroidal space that is common to both the choroidal and ciliary portions of the uveal tract. The suprachoroidal space has relevance for massive choroidal edema, clinically referred to as "choroidal detachments" or in local parlance as "choroidals". Attachment of the longitudinal (meridional) ciliary muscle to the scleral spur limits the space . The enlargement is limited posteriorly by attachment of the choroid to the sclera, augmented by the outward passage of the vortex veins, by the perforating short posterior ciliary arteries and by border tissue at the scleral aperture for the optic nerve.

VASCULATURE: The choroid is richly vascular and provides nutrients for the outer portion of the retina including the photoreceptors and the retinal pigment epithelium . It has an extremely rapid blood flow which is provided by the choriocapillaris, inner vascular layer and outer vascular layer. The capillary layer of the choroid, the choriocapillaris, lies directly under Bruch’s membrane and is critical to supply retinal photoreceptors. The larger arteries are found most readily in the outer layers of the posterior choroidal stroma. The long posterior ciliary arteries and their corresponding long ciliary nerves lie within the suprachoroidal space in the horizontal plane, encased by collagenous tissue. Branches from the nerves to the adjacent choroid form small net-like arrangements where large ganglion cells may be observed. The venous drainage system is seen as four vortex systems each located in a posterior quadrant. Each system converges to form a single vestibule, the ampulla, which then exits through the sclera by a vortex vein.
STROMA: The choroid contains flattened or interconnecting collagen lamellae that give the melanocytes a spindle shaped appearance. These melanocytes have a stellate shape and contain pigment granules. The cells contain small oval nucleoli. They are usually accompanied by the fibrovascular stroma of the choroid. The melanocytic cells cells are considered to be the source for the most common primary malignant neoplasm of the eye- melanoma.

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