What is the choroid?
The choroid bears the medical title Choroidea or Chorioidea. Together with the iris and the ciliary body (corpus ciliare), it forms the middle eye skin (tunica media bulbi or uvea), which has the largest share of this structure.
The choroid lies directly on the dermis inside the eye and has a dark, brown-black pigmentation. It is the middle layer between the dermis (Sclera) and the retina (Retina) and encloses except for a small portion in the front area of the eye almost the entire glass body. The choroid has its name because it is traversed by numerous small vessels that make it the most well-perfused structure in the body.
Anatomy & Construction
The choroid of the eye is composed of four different layers in humans: on the far side lies the supra-thoracic lamina, which consists of pigmented connective tissue. A similar structure is found in the lamina vasculosa, in which the connective tissue is traversed by the larger arterial and venous blood vessels of the choroid.
On the other hand, an extensive network of very fine capillaries penetrates the deeper layer of the choroid, the lamina choroidocapillaris. Immediately adjacent to the pigmented layer of the retina is the basal lamina, also referred to as the lamina vitra or complexus basalis. It makes a direct connection with the retina via the Bruch's membrane, which ensures the supply of the retina.
This is crucial for the nutrition of this important structure of the eye. In addition to its extensive range of blood vessels in various sizes, the choroid consists of fibrocytes and collagen, which form the connective tissue of the skin, as well as melanocytes, which form the basis for pigmentation.
Function & Tasks
The main task of the choroid lies in the care of the eye with the focus on the retina. Because of its position inside the eye, it is dependent on a continuous and sufficient supply of blood and oxygen. The choroid can do this optimally, having a dense network of larger and smaller blood vessels. These lead the arterial, oxygen-enriched blood to the retina and transport the venous blood back.
Through the blood, the retina gets all the nutrients it needs for optimal function. Because the retina is in daily use on a daily basis, it needs a consistent supply of nutrients, which is why the supplying choroid is for good reason the area in the body that is the most supplied with blood. The second important function of the choroid derives from its many melanocytes and the resulting strong pigmentation: The black-brown protection is effectively able to prevent the ingress of stray light into the interior of the eye. Among other things, stray light has the unpleasant effect that objects that are in poor contrast to each other are poorly recognized.
This especially affects the eyesight at dusk and at night, especially when added by oncoming traffic while driving an additional dazzling effect. Thus, the pigmented choroid comes to a protective function with far-reaching effect.
A typical disease of the back of the eye, where most of the choroid is located, is inflammation. If the inflammatory reaction concerns only the choroid, it is called choroiditis, and at the same time the veins and the retina are affected, it is chorioretinitis.
Often, these inflammations develop on the basis of another disease such as toxoplasmosis, but it is always a bacterial cause to be considered. The inflammation of the choroid and retina may or may not cause any discomfort. Disturbances of vision are characteristic, which can lead to complete blindness - namely, when the retina can no longer fulfill its function due to degenerative changes. Uveitis is the inflammation of the entire middle eye skin, which can also spread to the retina and the vitreous body.
Possible signs, which also depend on the respective localization of the inflammation in the front or rear eye area, include a blurred vision, a watery eye, foreign body sensation and photosensitivity. Again, the causes may be a systemic disease or the ingress of bacteria. In children, the disease is often associated with rheumatism. In addition to inflammation, the choroid may also be affected by trauma and have, for example, bruises.
A malignant disease that affects one person per 100, 000 people per year is the choroidal melanoma that results from degeneration of the skin's melanocytes. The disease can be detected by ultrasound or fluorescence angiography. It is a serious disease because it tends to form metastases and in these cases is often fatal. Therapeutic options include not only surgery but also laser therapy and - also in combination - radiotherapy.