Anatomy of the Choroid Plexus

Brain Lining That Produces Cerebrospinal Fluid

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The choroid plexus is a thin structure that lines most of the the ventricles of the brain. It is a protective barrier that produces cerebrospinal fluid (CSF), a fluid that provides nourishment and cushioning for the brain and spinal cord.

Cysts or tumors can form in the choroid plexus, and the cysts are not usually as dangerous as the tumors. Nevertheless, a choroid plexus cyst or tumor can cause problems, such as hydrocephalus (fluid accumulation) and pressure in the brain, potentially requiring surgical intervention. 

The choroid plexus lines the brain and produces CSF

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Anatomy 

The choroid plexus is part of the pia mater, which is the deepest layer of the three layers of meninges that surround and protect the central nervous system (CNS). The CNS is composed of the brain and spinal cord.

The pia mater and the choroid plexus are directly adherent to the brain tissue, while there is a small space between the brain and the other layers of the meninges (dura mater and arachnoid mater). The pia mater covers the whole CNS, but the choroid plexus is only present in some of the regions of the pia mater.

The choroid plexus-produced CSF flows around the surface of the whole CNS.

Structure 

The choroid plexus is composed of highly vascular secretory epithelial tissue. Epithelial tissue is the type of tissue that lines most of the structures of the body.

The choroid plexus is embedded with capillaries, which are very thin and somewhat permeable blood vessels. The structure of the choroid plexus makes it moderately porous, allowing selected material to flow between the CSF and the blood. 

The lining of the choroid plexus also has villi, which are small hairlike structures that help direct the material of the CSF.

Location 

The choroid plexus lines the inner surface of the ventricles of the brain. The ventricles in the brain are fluid filled cavities, located deep in the cerebral cortex (the upper portion of the brain). The choroid plexus lines the left and right lateral ventricles and the third and fourth ventricles. It is not present in the frontal horn or the cerebral aqueduct.

Anatomical Variations

Variations in the function or structure of the choroid plexus can be associated with cysts and other congenital (from birth) malformations. If they block CSF flow, choroid plexus cysts can lead to hydrocephalus and other brain malformations.

These issues can be detected before a child’s birth or during early childhood. The most visible effects are an enlarged head and lethargy.

Function 

The choroid plexus has several functions. It creates a barrier. It produces and secretes CSF and regulates the composition of the CSF. The choroid plexus also contains immune cells that help fight infections in the CNS. 

Roles of the choroid plexus include:

  • CSF production: The CSF flows throughout the cavities of the brain and spinal cord, and well as along the outer layers of the CNS. It is a fluid cushion that provides nourishment to the CNS and helps remove waste material. 
  • CSF regulation: The choroid plexus maintains the composition of fluid, electrolytes, and immune cells in the CSF.
  • Blood-CSF barrier: The blood-CSF barrier, which is created by the choroid plexus and the meninges, helps protect the brain from infectious organisms and helps maintain control of the nourishment and waste in and out of the brain. The permeability of this structure affects the ability of medications, drugs, and other substances to enter the brain.

Associated Conditions

Several structural issues can arise from the choroid plexus. And a number of neurological conditions affect and are impacted by the choroid plexus and/or CSF flow. 

Cysts

Cysts of the choroid plexus are small growths, and they are usually benign (noncancerous). They may be seen incidentally on brain imaging tests, and generally don’t cause symptoms or complications, although they can lead to hydrocephalus. Cysts also have a small risk of bleeding.

There may be an increased incidence of choroid plexus cysts among newborns who have other birth defects. The cysts can often be detected before birth with a fetal ultrasound.

Cancer

Tumors can develop from the choroid plexus, potentially growing, spreading to other areas of the CNS and/or obstructing the ventricular flow of CSF, causing fluid accumulation. Choroid plexus tumors include papilloma and carcinoma.

These tumors are more common in very young children and can sometimes be diagnosed in utero with a maternal ultrasound. They are treated with surgery, radiation, and/or chemotherapy.

Infections

The CSF and the choroid plexus help prevent CNS infections, and while CNS infections aren't common, they may occur. Infections of the CNS include meningitis (infection of the meninges) and encephalitis (infection of the brain).

Inflammation

Inflammatory disease of the CNS is characterized by an excess of immune cells, which can lead to obstruction of CSF flow.

CSF Obstruction

When the flow of CSF is obstructed by a cyst, tumor, infection, or inflammation, there can be a buildup of fluid in and around the brain. Minor obstruction may lead to neurological symptoms, such as lethargy and head pain. Severe obstruction is life-threatening.

Hydrocephalus

A buildup of fluid around the brain can lead to enlargement of the ventricles, with possible changes of consciousness in adults. Very young children may develop an enlarged head because their skull bones are not yet fully fused.

Hydrocephalus is a medical emergency that requires surgical removal of fluid and/or placement of a ventricular shunt.  

Tests

The choroid plexus can be examined with brain imaging tests, and alterations in CSF volume or flow might be seen with imaging tests as well. The CSF can be sampled with an invasive lumbar puncture (LP).

A tumor in the choroid plexus can be examined with a microscopic examination of a biopsy sample, which is obtained during a surgical procedure.

Imaging

Problems such as inflammation, cysts, and tumors can be visualized with diagnostic tests such as brain magnetic resonance imaging (MRI). Obstructions in CSF flow and hydrocephalus may be associated with enlarged ventricles. Sometimes areas of blockage in the ventricles can be visualized with brain imaging tests.

Invasive Testing

The CSF pressure and composition can be directly examined with an LP (also called a spinal tap), which is an invasive procedure. This test is safe and it must be done by a doctor, who would insert a needle into the lower back, below the level of the spinal cord.

The fluid drips from the needle and the fluid pressure can be measured. The CSF can be sent to a laboratory for examination—which may show signs of infection, inflammation, or cancer. 

A tumor may need to be directly examined with a biopsy. This involves an operation for sampling the tissue to determine the tumor type. The treatment would be tailored to the type and grade of the tumor.

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