The choroid, also known as the choroidea or choroid coat, is a part of the
uvea
The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye.
History and etymolog ...
, the
vascular
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...
layer of the
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
, 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 then ...
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 human eye containing mainly collagen and some crucial elastic fiber. In humans, and som ...
. 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 as wel ...
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 then ...
. 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
Iris most often refers to:
*Iris (anatomy), part of the eye
*Iris (mythology), a Greek goddess
* ''Iris'' (plant), a genus of flowering plants
*Iris (color), an ambiguous color term
Iris or IRIS may also refer to:
Arts and media
Fictional enti ...
, the choroid forms the uveal tract
The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye.
History and etymology ...
.
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
Sattler's layer, named after Hubert Sattler, an Austrian ophthalmologist, is one of five (or six) layers of medium-diameter blood vessels of the choroid, and a layer of the eye. It is situated between the Bruch's membrane, choriocapillaris belo ...
- 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
Bruch's membrane is the innermost layer of the choroid of the eye. It is also called the ''vitreous lamina'' or ''Membrane vitriae'', because of its glassy microscopic appearance.
It is 2–4 μm thick.
Layers
Bruch's membrane consists of five l ...
(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 uvea
The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye.
History and etymolog ...
l, supplied in humans by posterior ciliary arteries, originating from the ophthalmic artery
The ophthalmic artery (OA) is an artery of the head. It is the first branch of the internal carotid artery distal to the cavernous sinus. Branches of the ophthalmic artery supply all the structures in the orbit around the eye, as well as some st ...
(a branch of the internal carotid artery
The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these b ...
). The arteries of the uvea
The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye.
History and etymolog ...
l circulation, supplying the uvea
The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye.
History and etymolog ...
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 normal ...
. 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 avas ...
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
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 medical ...
, and blood flow can be revealed by Indocyanine green angiography
Indocyanine green angiography (ICGA) is a diagnostic procedure used to examine choroidal blood flow and associated pathology. Indocyanine green (ICG) is a water soluble cyanine dye which shows fluorescence in near-infrared (790–805 nm) range, wi ...
, and laser Doppler imaging
Laser Doppler imaging (LDI) is an imaging method that uses a laser beam to scan live tissue. When the laser light reaches the tissue, the moving blood cells generate doppler components in the reflected ( backscattered) light. The light that comes ...
.
In bony fish
Teleosts
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Teleo ...
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
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
, 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 pink or blue eyes. Individuals with the condition are referred to as albino.
Varied use and interpretation of the term ...
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 vi ...
. 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 ''tapetum lucidum'' ( ; ; ) 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 reflects visible light back through the retina, increasing the light a ...
, 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
The ''tapetum lucidum'' ( ; ; ) 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 reflects visible light back through the retina, increasing the light a ...
(see Tapetum lucidum
The ''tapetum lucidum'' ( ; ; ) 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 reflects visible light back through the retina, increasing the light a ...
).
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
Common Era (CE) and Before the Common Era (BCE) are year notations for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Common Era and Before the Common Era are alternatives to the or ...
) around 400 BCE, calling it the "chitoon
malista somphos" (more spongy tunic [than 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 human eye containing mainly collagen and some crucial elastic fiber. In humans, and som ...
]).[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
Herophilos (; grc-gre, Ἡρόφιλος; 335–280 BC), sometimes Latinised Herophilus, was a Greek physician regarded as one of the earliest anatomists. Born in Chalcedon, he spent the majority of his life in Alexandria. He was the first sci ...
(c. 335 – 280 BCE) also described the choroid from his dissections on eyes of cadaver
A cadaver or corpse is a dead human body that is used by medical students, physicians and other scientists to study anatomy, identify disease sites, determine causes of death, and provide tissue to repair a defect in a living human being. Stud ...
s.[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 (anatomy), iris, choroid, and ciliary body, but can also be divided into class I (low metastatic risk) and class II (high metasta ...
, where the latter is a primary tumour arising from the choroid itself.
See also
*Chorioretinitis
Chorioretinitis is an inflammation of the choroid (thin pigmented vascular coat of the eye) and retina of the eye. It is a form of posterior uveitis. If only the choroid is inflamed, not the retina, the condition is termed choroiditis. The ophthalm ...
*Uveitis
Uveitis () is inflammation of the uvea, the pigmented layer of the eye between the inner retina and the outer fibrous layer composed of the sclera and cornea. The uvea consists of the middle layer of pigmented vascular structures of the eye and ...
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
Laser Doppler imaging (LDI) is an imaging method that uses a laser beam to scan live tissue. When the laser light reaches the tissue, the moving blood cells generate doppler components in the reflected ( backscattered) light. The light that comes ...
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