choroid n : a highly vascular membrane in the eye between the retina and the sclera [syn: choroid coat]

English

Etymology

From sc=polytonic + sc=polytonic.

Pronunciation

• /ˈkəʊrɔɪd/, a US /ˈkoʊrɔɪd/

Noun

1. The vascular layer of the eye lying between the retina and the sclera.

Related terms

• chorioid
• choroidea
• choroid coat

Translations

The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina http://www.eyeglossary.net/#C. Along with the ciliary body and iris, the choroid forms the uveal tract.

Layers

The structure of the choroid is generally divided into four layers:

• Haller's layer - outermost layer of the choroid consisting of larger diameter blood vessels
• Sattler's layer - layer of medium diameter blood vessels
• Choriocapillaris - layer of capillaries
• Bruch's membrane - innermost layer of the choroid

Blood supply

The choroid is supplied in humans by posterior ciliary arteries branching in a segmental distribution to the end arterioles and not anastomoses. This is clinically significant for diseases affecting choroidal blood supply. The macula responsible for central vision and the anterior part of the optic nerve are dependant on choroidal blood supply.

Mechanism

Melanin, a darkly 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 other primates, melanin occurs throughout the choroid. In albino humans, frequently melanin is absent and vision is low. In many animals, however, the partial absence of melanin contributes to superior night vision. In these animals, melanin is absent from a section of the choroid and within that section a layer of highly reflective tissue, the tapetum lucidum, helps to collect light by reflecting it in a controlled manner. The uncontrolled reflection of light from dark choroid produces the photographic red-eye effect on photos, whereas the controlled reflection of light from the tapetum lucidum produces eyeshine (see Tapetum lucidum).

See also

• Chorioretinitis
• Uveitis

Additional images

References

External links