The choroid plexuses are small, highly vascular tissues inside the brain’s ventricles whose primary function is producing cerebrospinal fluid (CSF). They generate roughly 500 mL of this fluid every day, enough to completely replace the brain’s entire CSF supply four to five times within 24 hours. But fluid production is only part of the story. The choroid plexuses also act as a selective barrier between blood and brain, regulate immune cell entry into the central nervous system, deliver nutrients, and help clear metabolic waste.
Where the Choroid Plexuses Are Located
Choroid plexus tissue lines nearly every ventricle in the brain. It’s present in both lateral ventricles, the third ventricle, and the fourth ventricle. The only ventricular spaces without it are the tips (frontal and occipital horns) of the lateral ventricles and the narrow cerebral aqueduct that connects the third and fourth ventricles. This widespread distribution means the choroid plexuses are positioned to influence the fluid environment of the entire brain.
How the Choroid Plexuses Are Built
Each choroid plexus has a straightforward architecture that reflects its secretory job. The outer layer is a single sheet of cube-shaped epithelial cells. These cells sit on top of a core containing connective tissue and a dense network of tiny blood vessels.
Those blood vessels are unusual. Unlike the tightly sealed capillaries found elsewhere in the brain, choroid plexus capillaries are fenestrated, meaning their walls are perforated with small openings covered by thin membranes. These openings allow water and small molecules to pass through quickly, giving the epithelial cells rapid access to the raw materials they need to manufacture CSF. The epithelial cells themselves are connected by tight junctions, protein seals that prevent substances from slipping between cells uncontrolled. This arrangement creates a two-layer system: leaky blood vessels on the inside, tightly controlled epithelial cells on the outside.
Producing Cerebrospinal Fluid
CSF production is the choroid plexuses’ headline function. The total volume of CSF in an adult brain at any moment is only about 150 mL (roughly 125 mL surrounding the brain and spinal cord, 25 mL inside the ventricles), yet the choroid plexuses secrete 400 to 600 mL of fresh fluid daily. That constant turnover keeps the fluid clean and chemically stable.
The process is active, not passive. Epithelial cells use ion pumps and transporters to move sodium, potassium, chloride, and bicarbonate from blood into the ventricles. Water follows these ions. One key pump, called Na,K-ATPase, sits on the CSF-facing surface of the epithelial cells, which is the reverse of its position in most other tissues. This unusual placement drives sodium into the ventricles, creating the osmotic pull that draws water along with it. Water channels in the cell membranes assist the flow, though the exact contribution of these channels is still debated.
The resulting CSF circulates through the ventricles, flows around the brain and spinal cord in the subarachnoid space, and eventually drains back into the bloodstream. Along the way, it cushions the brain against physical impact, distributes hormones and nutrients, and carries away waste products.
Forming the Blood-CSF Barrier
The brain is famously protected by the blood-brain barrier, but the choroid plexuses house a related and equally important checkpoint: the blood-CSF barrier. Because the capillaries here are porous, the barrier responsibility falls entirely on the epithelial cells and the tight junctions linking them.
This barrier is selective rather than absolute. The epithelial cells are studded with specialized transporters that move specific molecules in or out. Glucose, the brain’s primary fuel, enters through a transporter called GLUT1, concentrated on the blood-facing side of the cells. Transporters for vitamins, fructose, and antioxidant compounds like urate are also present. Meanwhile, other transporters actively pump potentially harmful substances in the opposite direction, from CSF back into the blood. The net effect is a gatekeeper system that lets the brain receive what it needs while blocking or ejecting what it doesn’t.
Clearing Waste From the Brain
Efficient waste removal is critical to brain health, and the choroid plexuses contribute through multiple mechanisms working simultaneously. The simplest is bulk flow: as freshly made CSF circulates through the ventricles and subarachnoid space, it collects drugs, toxins, and metabolic byproducts and carries them to drainage sites where they re-enter the bloodstream. This is a nonselective process that sweeps up whatever is floating in the fluid.
On top of this passive clearance, choroid plexus epithelial cells express a collection of membrane transporters that actively grab specific waste molecules from the CSF side and shuttle them into the blood side. These transporters handle a broad range of substrates, including pharmaceutical compounds. The epithelial cells also contain enzymes capable of chemically breaking down reactive or toxic substances before they can cause damage, functioning as a kind of on-site detoxification system.
Immune Surveillance Gateway
The choroid plexus serves as one of the main entry points for immune cells into the central nervous system. Under normal conditions, the CSF contains a small, carefully curated population of immune cells, primarily a type of T cell along with macrophages and dendritic cells. The mix of immune cells in CSF looks very different from what circulates in blood, which tells us that cells don’t wander in randomly. They are selected.
This selection happens at the epithelial layer. The choroid plexus epithelial cells display adhesion molecules and chemical signals on their surfaces that attract certain immune cell types and guide them across the barrier. Resident immune cells within the choroid plexus stroma, particularly macrophages and dendritic cells, add another layer of surveillance. Together, these systems allow the brain to maintain a low-level immune watch without triggering the kind of uncontrolled inflammation that would damage delicate neural tissue. When disease or injury occurs, this gateway can ramp up trafficking to mount a more vigorous response.
Hormone and Growth Factor Secretion
Beyond making CSF, the choroid plexuses actively secrete signaling molecules into it. One important example is a growth factor called IGF-II, which plays roles in brain development and cell survival. The choroid plexuses also produce transthyretin, a transport protein that carries thyroid hormone from the blood into the brain. Because CSF bathes nearly every surface of the brain, the choroid plexuses are well positioned to distribute these molecules widely, functioning as a kind of internal endocrine broadcaster for the central nervous system.
What Happens When the Choroid Plexuses Malfunction
Given how many critical functions the choroid plexuses perform, it’s not surprising that their dysfunction is linked to a range of neurological problems. Hydrocephalus, the dangerous buildup of CSF in the brain, can result from either excessive CSF production or blocked drainage pathways. In mouse models of Huntington’s disease, excess CSF production by the choroid plexus appears to drive the development of hydrocephalus directly. Structural problems, like impaired movement of tiny hair-like cilia on the choroid plexus surface, can also lead to enlarged ventricles.
In neurodegenerative diseases, the choroid plexuses themselves show signs of damage. In Alzheimer’s disease, pathogenic proteins such as amyloid beta, tau, and alpha-synuclein have been found accumulating in choroid plexus epithelial cells, and in some animal models this accumulation leads to epithelial cell death. A deteriorating choroid plexus could mean reduced CSF turnover, impaired waste clearance, and a less effective barrier, all of which would worsen the brain’s internal environment.
Disruptions to the choroid plexus-CSF system have also been associated with conditions that might seem unrelated at first glance, including autism spectrum disorder, bipolar disorder, and schizophrenia. Many of these conditions involve circadian rhythm disruptions, and the choroid plexus appears to play a role in regulating the daily rhythms of CSF composition. When that rhythm breaks down, the chemical environment of the brain shifts in ways that may contribute to psychiatric and developmental symptoms.