Choroid

The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissues, and lying between the retina and the sclera. 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 provides oxygen and nourishment to the outer layers of the retina. Along with the ciliary body and iris, the choroid forms the uveal tract.

Choroid
Cross-section of human 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
TA	A15.2.03.002
FMA	58298
Anatomical terminology

The structure of the choroid is generally divided into four layers (classified in order of furthest away from the retina to closest):

• Haller's layer - outermost layer of the choroid consisting of larger diameter blood vessels;[1]
• Sattler's layer - layer of medium diameter blood vessels;[1]
• Choriocapillaris - layer of capillaries;[1] and
• Bruch's membrane (synonyms: Lamina basalis, Complexus basalis, Lamina vitra) - innermost layer of the choroid.[1]

Blood supply

There are two circulations of the eye: the retinal (in the retina) and uveal, supplied in humans by posterior ciliary arteries, originating from the ophthalmic artery that derived from the internal carotid artery.[2] The arteries of the uveal circulation, supplying the uvea 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 are 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 dependent on choroidal blood supply.[4]. The structure of choroidal vessels can be revealed by optical coherence tomography, and blood flow can be revealed by Indocyanine green angiography, and laser Doppler imaging [5].

Choroidal blood flow revealed with ICG-angiography (Spectralis, Heidelberg) and laser Doppler imaging[6]

In bony fish

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]

Mechanism

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 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

• 
  Schematic cross section of the human eye; choroid is shown in purple.
• 
  Laser Doppler imaging of retinal and choroidal blood flow
• 
  Iris, front view
• 
  The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section
• 
  The interior of the posterior half of the left eyeball
• 
  Structures of the eye labeled
• 
  This image shows another labeled view of the structures of the eye
• 
  Calf's eye dissected to expose the choroid: its tapetum lucidum is iridescent blue

References

1. MRCOphth Sacs questions
2. The Ocular Circulation
3. "Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 174 Encyclopædia Britannica, 1987
4. Hayreh S (November 1975). "Segmental nature of the choroidal vasculature". Br J Ophthalmol. 59 (11): 631–48. doi:10.1136/bjo.59.11.631. PMC 1017426. PMID 812547.
5. Puyo, Léo, Michel Paques, Mathias Fink, José-Alain Sahel, and Michael Atlan. "Choroidal vasculature imaging with laser Doppler holography." Biomedical optics express 10, no. 2 (2019): 995-1012.
6. Léo Puyo, Michel Paques, and Michael Atlan, "Spatio-temporal filtering in laser Doppler holography for retinal blood flow imaging," Biomed. Opt. Express 11, 3274-3287 (2020)
7. "Eye (Vertebrate)" McGraw-Hill Encyclopedia of Science and Technology, vol. 6, 2007.

External links

• Histology image: 08008loa – Histology Learning System at Boston University