The choroid plexuses are specialized structures within the brain. These networks of cells produce cerebrospinal fluid, a clear liquid that surrounds and protects the brain and spinal cord. Their operation is necessary for maintaining a stable environment within the central nervous system.
Locating the Choroid Plexuses
The choroid plexuses are networks of blood vessels and specialized epithelial cells. These structures reside within the brain’s fluid-filled spaces, known as ventricles. The brain contains four ventricles—two lateral, a third, and a fourth—and choroid plexuses are present in all of them.
These plexuses appear as frond-like or cauliflower-like masses, folding into many villi that extend into the ventricles. The epithelial cells lining the capillaries within the choroid plexus have microvilli on their surface, which significantly increases the surface area for fluid production.
Producing Cerebrospinal Fluid
The primary function of the choroid plexuses is the continuous production of cerebrospinal fluid (CSF). This process begins with the filtration of blood plasma from fenestrated capillaries within the choroid plexus into an interstitial space. Unlike typical brain capillaries, these are permeable, allowing water, ions, and small molecules to pass through.
Specialized ependymal cells, which form the epithelial lining of the choroid plexus, then actively transport specific ions like sodium, chloride, and bicarbonate from this interstitial fluid into the ventricles. This active transport creates an osmotic gradient, drawing water along with the ions into the ventricular spaces to form CSF. An adult human brain typically produces about 500 milliliters of CSF per day, maintaining a constant volume of approximately 125 milliliters within the central nervous system.
The Importance of Cerebrospinal Fluid
Cerebrospinal fluid provides multiple functions for the health of the brain and spinal cord. A primary role is to act as a physical cushion, protecting these delicate organs from shocks and trauma. The buoyancy provided by CSF reduces the effective weight of the brain by approximately 10 to 15 times, preventing it from compressing under its own weight against the skull.
Beyond physical protection, CSF serves as a medium for delivering nutrients and hormones to brain tissue. It circulates through the ventricles and around the brain and spinal cord, distributing necessary substances for neuronal function. The fluid also continuously removes metabolic waste products, such as excess ions and cellular byproducts, from the central nervous system, helping to maintain a clean and stable chemical environment. This continuous circulation and exchange helps regulate intracranial pressure and supports brain function.
Other Key Contributions
Beyond CSF production, the choroid plexuses contribute to brain health in other ways, including the formation of the blood-CSF barrier. This barrier is composed of tight junctions between the epithelial cells lining the choroid plexus. It meticulously controls the passage of substances from the blood into the brain’s fluid environment, acting as a selective filter. This prevents harmful substances and pathogens from entering the central nervous system while allowing beneficial molecules to pass.
The choroid plexuses also participate in the brain’s immune surveillance, acting as an interface between the peripheral immune system and the central nervous system. They contain various immune cells, such as macrophages and dendritic cells, and can secrete signaling molecules called cytokines and chemokines. These actions allow the choroid plexus to detect pathogens and respond to inflammation, orchestrating the recruitment of immune cells to protect the brain from infection and injury. Additionally, the choroid plexus may play a role in neurogenesis and contribute to repair mechanisms following brain damage.
When Choroid Plexuses Malfunction
Dysfunction of the choroid plexuses can lead to serious neurological conditions, primarily related to imbalances in cerebrospinal fluid production or circulation. The most recognized condition is hydrocephalus, often called “water on the brain,” which involves an abnormal accumulation of CSF within the brain’s ventricles. This buildup can occur if the choroid plexus produces too much CSF, or more commonly, if the fluid’s absorption or flow pathways are obstructed.
The excess fluid increases pressure within the skull, potentially damaging brain tissue and impairing brain function. Symptoms vary depending on age and severity, ranging from headaches and cognitive difficulties to more severe neurological impairments. In some instances, surgical interventions like choroid plexus cauterization are used to reduce CSF production and manage hydrocephalus.