The Anatomy of the Choroid Plexus

Complex Secretory Tissue That Makes Cerebrospinal Fluid

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The choroid plexus is a complex tissue configuration made up of epithelial cells, capillaries (tiny blood vessels), and connective tissue that lines the ventricles of the brain. Its function first and foremost is to secrete cerebrospinal fluid (CSF), a clear fluid that protects the brain and spinal cord. It has other important functions as well.

Anatomy

The meninges are membranes that surround the brain. They consist of three layersthe dura mater, arachnoid mater, and the pia mater. The choroid plexus is located within the pia mater which is the innermost layer (closest to the brain). It is found within all the ventricles of the brain with the exception of the frontal/occipital horn of the lateral ventricles and the cerebral aqueduct.

Composition

The tissue of the choroid plexus consists of both columnar cuboidal epithelial cells and specialized cells called ependyma. The ependyma are ciliated, meaning they have tiny hair-like projections on the outside of the cell as well as microvilli (projections which work to absorb CSF).

The ependyma are largely responsible for the production of CSF. These cells surround tiny capillaries that contain pores that allow for the exchange of molecules functioning to filter the plasma. The cells and capillaries have tight junctions (small spaces between them) which forms the blood-cerebrospinal fluid barrier.

Development

The choroid plexus arises from the neural tube beginning with the fourth ventricle of the brain during the ninth week of pregnancy. It then develops in the lateral and third ventricle.

Blood Supply

The choroid plexus is supplied with blood via several different arteries depending on location including the choroidal artery, the internal carotid artery, and the anterior/posterior inferior cerebellar arteries.

Function

The two main functions of the choroid plexus include the production of cerebrospinal fluid and the formation of the blood-cerebrospinal fluid barrier.

Production of CSF

The cerebrospinal fluid serves as a cushion that protects the brain in the event of trauma or jarring of the head preventing the brain from being damaged by impacting the skull. It also provides nourishment to the brain and spinal cord and functions to remove waste from these areas. 

The ependymal cells of the choroid plexus produce up to 500 milliliters (ml) a day of CSF in the average adult.

The composition of the CSF itself is similar to blood plasma with an altered concentration of electrolytes such as a higher amount of sodium, magnesium, and chloride with lower amounts of potassium and calcium. It may contain small numbers of white blood cells.

This fluid is filtered by the fenestrated capillaries that help to make up the choroid plexus. The composition of CSF is further regulated by the microvilli which reside on the surface of cells that make up the choroid plexus and act to absorb CSF when necessary.

The Blood-CSF Barrier

The blood-CSF barrier is formed by the epithelial cells of the choroid plexus which are linked together with tight junctions and envelop a network of fenestrated capillaries. The epithelial cells face the ventricles of the brain. The other portion of the blood-CSF barrier is the arachnoid membrane, one of the meninges that surround the brain.

This blood-CSF barrier, not to be confused with the blood-brain barrier, functions to separate the blood from CSF. The blood-CSF barrier prevents harmful substances from entering the brain while also allowing harmful waste to exit back into the bloodstream. Water is allowed to pass the blood-CSF barrier and is used to manufacture cerebrospinal fluid.

Associated Conditions

Meningitis

Impairment of the blood-CSF barrier can allow the entrance of harmful microbes into the CSF leading to dangerous inflammation in the meninges surrounding the brain and spinal cord.

Several different types of germs can cause meningitis including viruses, bacteria, fungi, and parasites. Non-infectious meningitis can be related to conditions such as certain types of cancers or lupus.

Hydrocephalus

Hydrocephalus is a condition where there is too much CSF in the brain either due to a blockage, (such as a growth or tumor), preventing proper flow of CSF, the overproduction of CSF, or a problem with the way CSF is normally absorbed.

This extra fluid on the brain can cause symptoms such as headache, convulsions, vision disturbances, and mental difficulties, long term problems can result if left untreated. Hydrocephalus is normally treated using a shunt, which diverts the extra fluid away from the brain to a location where it can be reabsorbed.

CSF Leak

Cerebrospinal fluid leaks are the result of a tear or hole in the dura surrounding the brain which allows CSF to leave the subarachnoid space. This can be caused by a traumatic injury (usually to the head or face), rarely as a complication of sinus surgery, as a complication of lumbar puncture or epidural anesthesia, or they can be congenital.

Spontaneous CSF leaks are rare. They may be due to an underlying condition or lack an identifiable or obvious cause.

A CSF leak can cause clear watery fluid to run from your nose or ears, a headache which may get worse or improve when you change positions and ringing in your ears (tinnitus).

They can lead to meningitis since the hole in the dura creates a pathway for microbes to enter the subarachnoid space. Standard treatment for CSF leak may be as simple as bedrest for very small leaks resulting in few symptoms or may require an epidural blood patch, surgical repair, or other treatments to address the underlying cause.

Tests

While medical imaging such as magnetic resonance imaging (MRI) or computerized tomography (CT) scans may be used, the most specific test to evaluate the function of the choroid plexus, CSF, and associated conditions is a lumbar puncture (spinal tap).

Lumbar puncture involves inserting a needle into the subarachnoid space at the level of the lumbar spine and aspirating a small amount of CSF, which can then be analyzed for composition or the presence of harmful microbes. Local anesthesia is used to prevent discomfort. While rare, this procedure can result in complications including CSF leak.

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