Choroid is the vascular layer of the eye. Also referred to as choroid coat or choroidea, it is a thin layer of tissue which is part of the middle layer of the eye wall, found between the sclera and the retina. The choroid is filled with blood vessels, which brings nutrients and oxygen to the outer layers of the retina. The choroid of the human is said to be the thickest towards the distant extreme rear of the eye (0.2 mm). In the peripheral areas, it constricts to 0.1 mm. The choroid forms the uveal tract with the iris and ciliary body.
It’s structure can be typically divided into 4 layers –
- Haller’s layer – outermost layer of the choroid, comprising large diameter blood vessels
- Sattler’s layer – region of medium diameter blood vessels
- Choriocapillaris – this layer has capillaries
- Bruch’s membrane – it forms the choroid’s innermost layer. This transparent layer imparts a homogeneous appearance indicated by an endothelial basement membrane from the choriocapillaris layer of the capillaries
This structure covers to the interior the fibrous eye’s tunic. It makes up the posterior part of the uvea, the anterior is indicated by a thick area, the ciliary zone. The two areas are discriminated by the ora serrata at the eye’s equator, which is formed as a scalloped line.
The choroid structure exhibits two faces – the internal is concave and accommodates the retina with no adherence while the external is convex, solidarized with sclera through the ciliary nerves, vessels and the lax connective tissue. The choroid comprises 2 openings – as a demarcation, an anterior one with ora serrata and the other as a posterior one which passes through the optic nerve.
Function of Choroid
For the proper functioning of the eyes and for a clear vision, it is vital to be supplied with adequate blood supply. The choroidal circulation is said to be responsible for about 85% of the blood flow in the eye, thus making it an important structure to the function of eyes. Some other functions are –
- Regulates the temperature of the retina
- Aids in regulating the pressure in the eye
- Provides nutrients to macula, retina and optic nerve
- It absorbs light and limits reflections in the eye, which could harm vision
The vasculature of choroid is the main supply for the outer retina. The deterioration of the oxygen flow from the choroid to retina can cause age-related Macular Degeneration. The choroidal flow of blood can warm and cool the retina. Apart from its vascular functions, it comprises secretory cells, which are likely involved in the modulation of vascularization and in development of the sclera. Fluctuations in the choroidal thickness moves the retina backwards and forward, aligning the photoreceptors to the plane of focus.
Also check: What Is The Constituency Of The Choroid?
Choroid Eye Conditions
There are several reasons which can cause the choroid to develop certain conditions, such as genetic, sporadic or even trauma to the eye. Following are the conditions which could affect the choroid –
- Choroidal rupture: the retinal pigment epithelium and Bruch’s membrane which lead to an eye injury, tear in the choroid
- Choroideremia – usually seen in men, it is a condition of a hereditary and progressive impairment of the choroid
- Choroid plexus papilloma – it is a rare, benign brain tumor which develops in the choroid plexus
- Chorioretinitis – it is the inflammation of the choroid which is as a result of an autoimmune disease or an infection
- Choroidal detachment and hemorrhage – it is the separation of the choroid from the sclera. It can happen due to low or high eye pressure
Choroid forms a vital structure of the eye which could be included in several pathologies. It is of immense significance provided its functions such as thermoregulation, vascularization and even in the production of the growth factors. Choroid, histologically, exhibits 5 layers – outer pigment layer, suprachoroid; two Vascular layers, Haller (external) and Sattler (internal), choriocapillaris layer and the Bruch’s membrane.
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