The choroid, also known as the choroidea or choroid coat, is a part of the
uvea, the
vascular layer of the
eye, and contains
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
s, and lies between the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
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.
[MRCOphth Sacs questions](_blank)
/ref> The choroid provides oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
and nourishment to the outer layers of the retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
. Along with the ciliary body
The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...
and iris, the choroid forms the uveal tract.
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;
* Sattler's layer - layer of medium diameter blood vessels;
*Choriocapillaris
The capillary lamina of choroid or choriocapillaris is a layer of capillaries that is immediately adjacent to Bruch's membrane in the choroid. The choriocapillaris was first described in man by Hovius in 1702, although it was not so named until 1 ...
- layer of capillaries; and
* Bruch's membrane (synonyms: Lamina basalis, Complexus basalis, Lamina vitra) - innermost layer of the choroid.
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 (a branch of the internal carotid artery). 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.["Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 174 Encyclopædia Britannica, 1987] They branch in a segmental distribution to end arterioles and not anastomoses
An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be norm ...
. This is clinically significant for diseases affecting choroidal blood supply. The macula
The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal av ...
responsible for central vision and the anterior part of the optic nerve
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...
are dependent on choroidal blood supply. The structure of choroidal vessels can be revealed by optical coherence tomography
Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media (e.g., biological tissue). It is used for medica ...
, and blood flow can be revealed by Indocyanine green angiography, and laser Doppler imaging.
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.
Mechanism
Melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
, 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
The red-eye effect in photography is the common appearance of red pupils in color photographs of the eyes of humans and several other animals. It occurs when using a photographic flash that is very close to the camera lens (as with most compact ...
on photos, whereas the controlled reflection of light from the tapetum lucidum produces eyeshine (see Tapetum lucidum).
History
The choroid was first described by Democritus
Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. No ...
(c. 460 – c. 370 BCE) around 400 BCE, calling it the "chitoon
malista somphos" (more spongy tunic sclera.html" ;"title="han the sclera">han the sclera.[Dolz-Marco, R., Gallego-Pinazo, R., Dansingani, K. K., & Yannuzzi, L. A. (2017). The history of the choroid. In J. Chhablani & J. Ruiz-Medrano (Eds.), Choroidal Disorders (Vol. 1–5, pp. 1-5). Academic Press. https://doi.org/0.1016/b978-0-12-805313-3.00001-6] Democritus likely saw the choroid from dissections of animal eyes.
About 100 years later, Herophilos (c. 335 – 280 BCE) also described the choroid from his dissections on eyes of cadavers.[Reverón, R. (2015). Herophilos, the great anatomist of antiquity. Anatomy, 9(2), 108-111. https://doi.org/10.2399/ana.15.003 https://dergipark.org.tr/en/download/article-file/371071]
Clinical significance
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 choroial metastatses are usually due to breast cancer, Meanwhile, unilateral metastasis is due to lung cancer. Choroidal metastases should be differentiated from uveal melanoma
Uveal melanoma is a type of eye cancer in the uvea of the eye. It is traditionally classed as originating in the iris, choroid, and ciliary body, but can also be divided into class I (low metastatic risk) and class II (high metastatic risk). S ...
, where the latter is a primary tumour arising from the choroid itself.
See also
* Chorioretinitis
* Uveitis
Additional images
Schematic diagram of the human eye en.svg, Schematic cross section of the human eye; choroid is shown in purple.
File:Laser_Doppler_holography_of_retinal_and_choroidal_blood_flow.jpg, Laser Doppler imaging of retinal and choroidal blood flow
File:Gray878.png, Iris, front view
File:Gray880.png, The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section
File:Gray1206.png, The interior of the posterior half of the left eyeball
File:Three Main Layers of the Eye.png, Structures of the eye labeled
File:Three Internal chambers of the Eye.svg, This image shows another labeled view of the structures of the eye
File:Calf-Eye-Posterior-With-Retina-Detached-2005-Oct-13.jpg, Calf's eye dissected to expose the choroid: its tapetum lucidum is iridescent blue
References
External links
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Human eye anatomy
Visual system