The Choroid Plexus

The choroid plexus is a network of capillaries and specialized ependymal cells found in the cerebral ventricles of the brain. The choroid plexus serves two roles for the body: it produces cerebrospinal fluid and provides a toxin barrier to the brain and other central nervous system tissue. The choroid plexus and the cerebrospinal fluid that it produces are necessary for proper brain development and central nervous system function.

Location

The choroid plexus is located in the ventricular system. This series of connecting hollow spaces circulates cerebrospinal fluid. Choroid plexus structures are found within both lateral ventricles as well as the third and fourth ventricles of the brain. The choroid plexus resides within the meninges, the membrane linings that cover and protect the central nervous system.

The meninges are composed of three layers known as the dura mater, arachnoid mater, and pia mater. The choroid plexus can be found in the innermost layer of the meninges, the pia mater. The pia mater membrane shelters the cerebral cortex and spinal cord.

Structure

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The choroid plexus is composed of blood vessels and specialized epithelial tissue called ependyma. Ependymal cells contain hair-like projections called cilia which form a tissue layer that encases the choroid plexus. Ependymal cells also line the cerebral ventricles and spinal cord central canal. These altered epithelial cells are a type of nervous tissue called neuroglia that helps to produce cerebrospinal fluid.

Function

The two important functions of the choroid plexus are to aid in brain development and protection. This is accomplished through cerebrospinal fluid production and brain protection via the blood-cerebrospinal fluid barrier. Read about these below.

Cerebrospinal Fluid Production

Choroid plexus arterial blood and ependymal cells are responsible for producing cerebrospinal fluid. The clear fluid that fills cavities of cerebral ventricles—as well as the central canal of the spinal cord and subarachnoid space of the meninges—is called the cerebrospinal fluid (CSF). Ependyma tissue separates capillaries of the choroid plexus from cerebral ventricles to regulate what enters CSF. It filters water and other substances from blood and transports them across the ependymal layer into brain ventricles.

CSF keeps the brain and spinal cord safe, secure, nourished, and free of waste. As such, it is vital that the choroid plexus function properly and produce the right amount of CSF. The underproduction of CSF can stunt brain growth and overproduction can lead to the accumulation of CSF in brain ventricles, a condition known as hydrocephalus. Hydrocephalus applies excessive pressure to the brain and can cause brain damage.

Blood–Cerebrospinal Fluid Barrier

The choroid plexus also helps prevent blood and other molecules from leaking through—either leaving or entering—perforated blood vessels in the brain. The arachnoid, a largely impenetrable membrane that envelopes the spinal cord, assists the choroid plexus in this task. The protective barrier they form is called the blood-cerebrospinal fluid barrier. Together with the blood-brain barrier, the blood-cerebrospinal fluid barrier serves to block toxic blood-borne substances from entering cerebrospinal fluid and causing damage to the central nervous system.

The choroid plexus also houses and transports other defensive structures that keep the body disease-free. Numerous white blood cells can be found in the choroid plexus—including macrophages, dendritic cells, and lymphocytes—and microglia, or specialized nervous system cells, and other immune cells enter the central nervous system through the choroid plexus. These are important for preventing pathogens from making their way to the brain.

In order for viruses, bacteria, fungi, and other parasites to gain passage to the central nervous system, they must cross the blood-cerebrospinal fluid barrier. This fends off most attacks, but some microbes, such as those that cause meningitis, have developed mechanisms for crossing this barrier.

Sources

• Liddelow, Shane A. “Development of the Choroid Plexus and Blood-CSF Barrier.” Frontiers in Neuroscience, Frontiers Media S.A., 3 Mar. 2015.
• Lun, Melody P., et al. “Development and Functions of the Choroid Plexus: Cerebrospinal Fluid System.” Nature Reviews Neuroscience, U.S. National Library of Medicine, Aug. 2015.