Synonyms in a broader sense
Vascular skin (Uvea) Medical: Choroidea English: choroid
The choroid is the rear part of the vascular skin (uvea) of the eye. It is embedded between the retina and the sclera as a central sheath. The iris and the ciliary body (corpus ciliare) also belong to the vascular skin.
With its network of blood vessels it serves to nourish neighbouring structures in the eye and itself consists of three layers. Since the choroid does not carry sensitive nerve fibres, pain always indicates the involvement of neighbouring structures with sensitive nerve fibres. The blood flow through the choroid is the strongest in the entire human body.
Structure of the choroid
The choroid belongs to the vascular skin, also called the middle eye skin (uvea). In addition to the choroid, it includes the iris and the ciliary body. It lies between the retina and the sclera. The choroid consists of the following four layers from the inside to the outside:
- Lamina basalis (connection with retina)
- Lamina choroidocapillaris (small capillaries)
- Lamina vasculosa (large arteries)
- Lamina suprachoroidea (connection with dermis)
Function of the choroid
The choroid has several functions: It contains many blood vessels and thus ensures the supply of oxygen and nutrients to parts of the eyeball (bulbus oculi), which the cells need to survive. The outer layer of the retina in particular is supplied by the blood vessels of the choroid. The retina, like the brain, has a barrier so that only selected substances can enter it: the blood-retina barrier (analogous to the blood-brain barrier).
Therefore, between the choroid and the retina lies the pigment epithelium, which anatomically belongs to the retina. The cells of the pigment epithelium are firmly connected to each other and ensure that only required substances from the blood flowing in the vessels of the choroid can penetrate the retina. Incidentally, the rich blood circulation of the choroid is the cause of the undesirable “red eye effect” when taking photographs.
It shimmers red through the eye when overexposed. Another function of the choroid is the eye’s ability to accommodate, i.e. the ability of the eye to see near or far objects in focus. The part of the choroid responsible for this function is called the rupture membrane.
The Bruch’s membrane contains many elastic fibres and is the antagonist to the ciliary muscle, which contracts the lens for near vision and makes it more spherical. Distance accommodation, on the other hand, is ensured by the passive restoring force of the elastic fibres of the rupture membrane and thus by the choroid. Finally, the choroid is also highly pigmented and, together with the above-mentioned pigment epithelium, ensures that as little as possible of the light entering the eye is reflected.
Instead, the light is completely absorbed, which is very important for seeing in different light conditions. Furthermore, the strong pigmentation of the choroid prevents the uncontrolled reflection of light within the vitreous body from causing confusing stimuli on the retina. The choroid is one of the three parts of the vascular skin (uvea) of the eye.
It lies against the retina from the outside. First, the Bruch’s membrane attaches itself externally to the cells of the retina, which receive the light impulses (photoreceptors). The Bruch’s membrane consists of connective tissue and is also called lamina elastica because of its structural proteins (collagen fibres) and the reversibly stretchable elastic fibres.
This is followed by a layer of small blood vessels (capillaries) that are branched like a network. The cells of the blood vessels have quite wide spaces (fenestrated capillaries) so that certain blood components can easily escape from the vessels. They are used for nutrition.
These windows are sealed by the cells that receive the light impulses (pigment epithelium or photoreceptors) and the rupture membrane. The last layer consists of larger vessels and lies against the layer with the small blood vessels (choriocapillaris) branched like a network from the outside. This outermost layer of the choroid carries larger blood vessels. These are mostly veins that carry blood out of the eye. The choroid is bordered on the outside by the dermis (sclera).