| Choroid | |
|---|---|
 Cross-section ofhuman eye, with choroid labeled at top. | |
 Interior of anterior half of bulb of eye. (Choroid labeled at right, second from the bottom.) | |
| Details | |
| Artery | Short posterior ciliary arteries,long posterior ciliary arteries |
| Identifiers | |
| Latin | choroidea |
| MeSH | D002829 |
| TA98 | A15.2.03.002 |
| TA2 | 6774 |
| FMA | 58298 |
| Anatomical terminology | |
Thechoroid, also known as thechoroidea orchoroid coat, is a part of theuvea, thevascular layer of theeye. It containsconnective tissues, and lies between theretina and thesclera. 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.[1] The choroid providesoxygen and nourishment to the outer layers of theretina. Along with theciliary body andiris, the choroid forms theuveal tract.
The structure of the choroid is generally divided into four layers (classified in order of furthest away from the retina to closest):
There are two circulations of the eye: the retinal (in the retina) anduveal, supplied in humans byposterior ciliary arteries, originating from theophthalmic artery (a branch of theinternal carotid artery).[2] The arteries of theuveal circulation, supplying theuvea and outer and middle layers of the retina, are branches of the ophthalmic artery and enter the eyeball without passing with the optic nerve. The retinal circulation, on the other hand, derives its circulation from the central retinal artery, also a branch of the ophthalmic artery, but passing in conjunction with the optic nerve.[3] They branch in a segmental distribution to end arterioles and notanastomoses. This is clinically significant for diseases affecting choroidal blood supply. Themacula responsible for central vision and the anterior part of theoptic nerve are dependent on choroidal blood supply.[4] The structure of choroidal vessels can be revealed byoptical coherence tomography, and blood flow can be revealed byIndocyanine green angiography, andlaser Doppler imaging.[5]
Teleosts bear a body of capillary adjacent to the optic nerve called the choroidal gland. Though its function is not known, it is believed to be a supplemental oxygen carrier.[7]
Melanin, a dark colored pigment, helps the choroid limit uncontrolled reflection within the eye that would potentially result in the perception of confusing images.
In humans and most otherprimates, melanin occurs throughout the choroid. Inalbino humans, frequently melanin is absent andvision is low. In many animals, however, the partial absence of melanin contributes to superiornight vision. In these animals, melanin is absent from a section of the choroid and within that section a layer of highly reflective tissue, thetapetum lucidum, helps to collect light by reflecting it in a controlled manner. The uncontrolled reflection of light from dark choroid produces the photographicred-eye effect on photos, whereas the controlled reflection of light from the tapetum lucidum produceseyeshine (seeTapetum lucidum).
The choroid was first described byDemocritus (c. 460 – c. 370BCE) around 400 BCE, calling it the "chitoonmalista somphos" (more spongy tunic [than thesclera]).[8] Democritus likely saw the choroid from dissections of animal eyes.[9]
About 100 years later,Herophilos (c. 335 – 280 BCE) also described the choroid from his dissections on eyes ofcadavers.[10][11]
Choroid is the most common site for metastasis in the eye due to its extensive vascular supply. The origin of the metastases are usually from breast cancer, lung cancer, gastrointestinal cancer, and kidney cancer. Bilateral choroidal metastases are usually due to breast cancer, while unilateral metastasis is due to lung cancer. Choroidal metastases should be differentiated fromuveal melanoma, where the latter is a primary tumour arising from the choroid itself.[12]
| Fibrous tunic (outer) |
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|---|---|---|---|---|---|---|---|
| Uvea / vascular tunic (middle) |
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| Retina (inner) |
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| Anatomical regions of the eye |
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| Other | |||||||
