The choroid plexus is a delicate tissue network found deep within the brain, playing an active role in brain health. This structure functions as a specialized gland and a selective barrier that maintains the brain’s unique internal environment. Its continuous activity is fundamental to the central nervous system’s proper function, providing mechanical support and a highly regulated chemical milieu. Without this specialized tissue, the brain would be unprotected and unable to manage its own waste and nutrient requirements.
Anatomy and Location
The choroid plexus (CP) is strategically located within the ventricles, the four interconnected, fluid-filled cavities at the center of the brain. A plexus is found in each of the two large lateral ventricles, the third ventricle, and the fourth ventricle. The tissue has a highly vascular, frond-like or cauliflower-like appearance, which dramatically increases its surface area for secretion.
The structure consists of a core of capillaries and loose connective tissue covered by a single layer of specialized cells called choroid epithelial cells. These cuboidal epithelial cells are modified ependymal cells and serve as the functional units of the plexus, sitting between the blood supply and the ventricular space. This rich blood supply, fed by the choroidal arteries, is crucial for the tissue’s primary function.
Primary Role: Cerebrospinal Fluid Production
The most recognized function of the choroid plexus is the production of Cerebrospinal Fluid (CSF), the clear, colorless fluid that bathes and protects the entire central nervous system. In humans, the CP produces approximately 500 milliliters of CSF daily, completely replacing the total volume several times each day. This process is not passive filtration of blood, but rather an energy-intensive, active secretory process performed by the choroid epithelial cells.
The formation of CSF begins with the movement of ions, particularly sodium, chloride, and bicarbonate, from the blood across the epithelial cells and into the ventricular space. This unidirectional movement is achieved by a complex set of ion transport proteins and channels, such as the \(Na^+-K^+\) ATPase pump. This active pumping of ions creates an osmotic gradient that draws water across the epithelium, primarily through water channels called aquaporin 1, thus forming the CSF.
The CSF provides mechanical protection against shock and movement for the delicate brain tissue. The fluid’s buoyancy effectively reduces the brain’s weight from about 1,400 grams to a mere 50 grams, preventing it from crushing itself against the skull. Furthermore, CSF acts as a circulatory system, removing metabolic waste products and delivering essential nutrients, hormones, and signaling molecules to the surrounding neural tissue.
The Protective Blood-CSF Barrier
The choroid plexus is the primary site of the Blood-Cerebrospinal Fluid (Blood-CSF) Barrier, a protective interface that strictly regulates the passage of substances from the blood into the CSF. This barrier is formed by tight junctions that link the individual choroid epithelial cells together on the side facing the ventricle. These junctions create a selective seal, preventing most molecules, immune cells, and pathogens from passing freely between the cells.
This barrier differs fundamentally from the Blood-Brain Barrier (BBB), which is established by the tight junctions of the endothelial cells lining blood vessels within the brain tissue. Capillaries within the choroid plexus are fenestrated, allowing free movement of small molecules into the tissue core. The selectivity of the Blood-CSF Barrier is located at the epithelial cell layer, where transport mechanisms actively control what enters the CSF.
The barrier is highly selective, using specific carrier proteins to transport necessary nutrients and peptides into the CSF. It also actively pumps out harmful substances and metabolic waste, providing bi-directional control fundamental to brain homeostasis and immune surveillance. The CP also secretes various molecules, including the hormone-carrying protein transthyretin.
When Things Go Wrong: Clinical Relevance
Dysfunction of the choroid plexus is directly linked to several clinical conditions, most commonly involving an imbalance in CSF dynamics. The most frequent issue is hydrocephalus, characterized by an excessive accumulation of CSF leading to ventricular enlargement and increased pressure. While hydrocephalus is usually caused by impaired CSF drainage, it can sometimes be linked to fluid overproduction, though this is rare.
Choroid plexus tumors are a rare but serious pathology, typically classified as either slow-growing papillomas or malignant carcinomas. A low-grade papilloma can cause hydrocephalus by overproducing CSF, overwhelming the brain’s ability to drain the fluid. Carcinomas are more aggressive, fast-growing, and can spread tumor cells throughout the central nervous system via the circulating CSF.
Another finding, particularly in prenatal ultrasounds, is the presence of choroid plexus cysts. These cysts are common in developing fetuses and are generally considered harmless, frequently resolving on their own before or shortly after birth. Although often benign, their presence can sometimes be associated with chromosomal abnormalities, making them a point of clinical interest during prenatal screening.