The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the

vascular Vascular can refer to: * blood vessels, the vascular system in animals * vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue ...

layer of the eye. It contains

connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...

s, and lies between the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

and the

sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...

. 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
/ref> The choroid provides

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and nourishment to the outer layers of the

. Along with the ciliary body 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 – 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 internal carotid artery is an artery in the neck which supplies the anterior cerebral artery, anterior and middle cerebral artery, middle cerebral circulation. In human anatomy, the internal and external carotid artery, external carotid ari ...

). 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 (, : 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, leaf veins, or streams. Such a connection may be ...

. This is clinically significant for diseases affecting choroidal blood supply. The macula 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 system, visual information from the retina to the brain. In humans, the optic nerve i ...

are dependent on choroidal blood supply. 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. LDHvsICG

In bony fish

Teleosts bear a body of capillaries 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 (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

, 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 Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers and simians ( monkeys and apes). Primates arose 74–63 ...

, melanin occurs throughout the choroid. In

albino Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos. Varied use and interpretation of ...

humans, frequently melanin is absent and vision is low. In many animals, however, the partial absence of melanin contributes to superior

night vision Night vision is the ability to see in low-light conditions, either naturally with scotopic vision or through a night-vision device. Night vision requires both sufficient spectral range and sufficient intensity range. Humans have poor night v ...

. 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 The ; ; : tapeta lucida) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It Reflection (physics), reflects visible light back through the retina, increas ...

, 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 eyes. It occurs when using a photographic flash at low lighting or at night. When a flash passes through the eyes and rebounds at the back of the ...

on photos, whereas the controlled reflection of light from the tapetum lucidum produces eyeshine (see

Tapetum lucidum The ; ; : tapeta lucida) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It Reflection (physics), reflects visible light back through the retina, increas ...

History

The choroid was first described by

Democritus Democritus (, ; , ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greece, Ancient Greek Pre-Socratic philosophy, pre-Socratic philosopher from Abdera, Thrace, Abdera, primarily remembered today for his formulation of an ...

(c. 460 – c. 370 BCE) around 400 BCE, calling it the "chitoon malista somphos" (more spongy tunic [than the

]).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. 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

cadaver A cadaver, often known as a corpse, is a Death, dead human body. Cadavers are used by medical students, physicians and other scientists to study anatomy, identify disease sites, determine causes of death, and provide tissue (biology), tissue to ...

s.Reverón, R. (2015). Herophilos, the great anatomist of antiquity. Anatomy, 9(2), 108–111. 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 choroidal metastases are usually due to breast cancer, while unilateral metastasis is due to lung cancer. Choroidal metastases should be differentiated from uveal melanoma, where the latter is a primary tumour arising from the choroid itself.

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

* {{Authority control Human eye anatomy Visual system