Choroid

The Choroid, also referred to as the choroid coat or choroidea, is the vascular layer of the eye. 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. This layer is filled with blood vessels, bringing nutrients and oxygen to the outer layers of the retina. In humans, the choroid is thickest towards the distant rear of the eye (0.2 mm) and thinnest in the peripheral areas (0.1 mm). The choroid, together with the iris and ciliary body, forms the uveal tract.

![The Choroid]()

Choroid Structure

Its structure can typically be divided into four 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 contains capillaries.

Bruch’s membrane forms the innermost layer of the choroid and is transparent. It gives the choroid a homogeneous appearance due to the endothelial basement membrane from the capillaries of the choriocapillaris layer.

Choroid Anatomy

The ciliary zone makes up the anterior part of the uvea, while the posterior is covered by a fibrous eye’s tunic. The two areas are separated 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

The choroid is a layer of tissue that is located between the retina and the sclera in the eye. Its main function is to provide oxygen and nourishment to the outer layers of the eye, as well as to absorb excess light. It also helps to regulate the temperature of the eye.

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 approximately 85% of the blood flow in the eye, making it a critical structure for the functioning of the eyes. Additionally, the choroid performs other important functions, such as:

Regulates the temperature of the retina

Helps maintain the pressure in the eye

Provides nutrients to the macula, retina, and optic nerve

It absorbs light and reduces reflections in the eye, thus protecting vision.

The vasculature of choroid is the main supply for the outer retina. 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 the development of the sclera. Fluctuations in the choroidal thickness moves the retina backwards and forward, aligning the photoreceptors to the plane of focus.

Check out: What is the constituency of the choroid?

Choroid Eye Conditions

The choroid can develop certain conditions for a variety of reasons, such as genetic, sporadic, or trauma to the eye. Below are some of the conditions that can affect the choroid:

Choroidal Rupture: A tear in the choroid which is caused by an eye injury, resulting from the retinal pigment epithelium and Bruch’s membrane being damaged.

Choroideremia - Usually seen in men, this is a hereditary and progressive condition which affects the choroid.

Choroid Plexus Papilloma - It is a rare, benign brain tumor that develops in the Choroid Plexus.

Chorioretinitis - It is the inflammation of the choroid, which can be caused by an autoimmune disease or an infection.

Choroidal Detachment and Hemorrhage - It is the separation of the choroid from the sclera which can occur due to either low or high eye pressure.

Summary: This statement is a summary of the given text.

The Choroid is a vitally important structure of the eye, as it is involved in several pathologies. It is of great significance due to its functions, including thermoregulation, vascularization, and the production of growth factors. Histologically, the Choroid is composed of five layers: the outer pigment layer, the suprachoroid; two Vascular layers, Haller (external) and Sattler (internal); the choriocapillaris layer; and the Bruch’s membrane.

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