Choroid

The choroid forms part of the vascular layer of the eyeball, along with the ciliary body and iris. It is a thin, pigmented vascular connective tissue layer of the eyeball. It is located between the fibrous outer sclera  (the whites of the eyes) and inner retina (the light-sensitive tissue in the back of the eye). The choroid is approximately 200μm thick at birth and decreases in thickness with age (decreasing to about 80μm at 90 years of age). This thin layer of tissue is made up almost entirely of blood vessels. These blood vessels supply oxygen and nutrients to the outer part of the retina. In short, the choroid is the life source that keeps the retina healthy and functioning.

1.0Anatomy

• The choroid is part of the uvea, which also consists of the iris and the ciliary body. The iris and ciliary body are located in the front of the eye and work together to dilate and constrict the pupils.
• The choroid layer begins in the peripheral edges of the eyeball and lines the entire back of it, sandwiched between the sclera and the retina. The thickness of the choroid varies depending on what part of the eye it’s lining. For example, it’s the thickest in the back of the eye (approximately 0.2 mm) and narrows to approximately 0.1mm as it gets to the peripheral part of the eyeball.
• There are four different layers of the choroid:
• Bruch’s membrane– Thin layer of tissue located on the innermost part of the choroid.
• Choriocapillaris– Layer made up of capillaries (tiny blood vessels that connect arteries to veins).
• Sattler’s layer– Layer of medium blood vessels.
• Haller’s layer– Outermost layer of the choroid that contains large blood vessels

2.0Function of the Choroid

The eyes and clear vision depend on sufficient blood supply in order to function. Choroidal circulation accounts for 85% of blood flow within the eye, making it a vital structure to the function of your eyes.

Other important functions of the choroid include:

• Providing nutrients for the retina, macula and optic nerve.
• Regulating the temperature of the retina.
• Helping control pressure within the eye.
• Absorbing light and limiting reflections within the eye that could harm vision. This part of the choroid is what causes “red eyes” when a photograph is taken using the flash unit of a camera.

3.0Conditions that may affect the choroid

• Choroid eye conditions can develop sporadically, genetically or as a result of eye trauma. 
• Each of the following conditions can be evaluated by an eye doctor, who will then determine and prescribe proper treatment.

1. Choroidal detachment

• A “serous choroidal detachment” occurs when fluid fills between the sclera and choroid. 
• It can occur due to injury, medications or some medical conditions, but it most typically occurs due to low pressure after eye surgery. 

2. Hemorrhagic choroidal detachment

• A “hemorrhagic choroidal detachment” occurs when blood fills the space between the sclera and choroid, such as when a blood vessel bursts. 
• It is associated with high pressure in the eyes and can occur during surgery. 
• It is usually more painful than a serous detachment.

3. Choroidal rupture

• A tear in the choroid, Bruch’s membrane and the retinal pigment epithelium (RPE) that result from an eye injury.

4. Choroid plexus papilloma

• Rare, benign brain tumor that develops in the choroid plexus (tissue that makes cerebrospinal fluid).

5. Chorioretinitis

• Inflammation of the choroid caused by infection or an autoimmune disease.

6. Choroideremia

• A hereditary, progressive deterioration of the choroid; this condition primarily affects men. 

4.0Choroid Plexus

• The choroid plexus is a complex network of capillaries lined by specialized cells and has various functions. 
• One of the primary functions is to produce cerebrospinal fluid (CSF) via the ependymal cells that line the ventricles of the brain. 
• Secondly, the choroid plexus serves as a barrier in the brain separating the blood from the CSF, known as the blood-CSF barrier.

5.0Structure and Function

1. CSF Production

• The brain is composed of three layers of meninges known as the dura mater, arachnoid mater, and the pia mater. 
• The choroid plexus resides in the innermost layer of the meninges (pia mater) which is in close contact with the cerebral cortex and spinal cord. It is a highly organized tissue that lines the ventricles of the brain. 
• The choroid plexus has a lining of specialized epithelial tissue known as ependyma. Ependymal cells are glial cells with a ciliated simple columnar form that line the ventricles and central canal of the spinal cord. 
• Apical surfaces have a covering of hair-like projections known as cilia (which circulate CSF) and microvilli (which help in CSF absorption). Microvilli perform this function via their brush border, which significantly increases the surface area of the choroid plexus, permitting increased CSF absorption. 
• Ependymal cells are essential in the production of CSF as the choroid plexus may secrete up to 500ml of CSF per day in the adult human brain.

2. Blood-CSF Barrier

• The choroid plexus forms the blood-CSF barrier along with the arachnoid mater to create a pair of membranes that separate the blood from the CSF. 
• This barrier is composed of a combination of ependymal (choroid epithelial) cells with tight junctions on its apical surface (the side facing the ventricles), and a core of fenestrated capillaries surrounded by connective tissue.