The choroid plexus is a small, ruffled tissue inside the brain’s fluid-filled chambers (ventricles) that produces cerebrospinal fluid, the clear liquid surrounding and cushioning the brain and spinal cord. It also acts as a selective filter between the bloodstream and the brain, controlling which substances get in and which stay out. Despite its small size, it plays an outsized role in brain health.
Where It Sits in the Brain
The brain contains four interconnected cavities called ventricles, and the choroid plexus is found in all four of them. Each piece of choroid plexus looks like a small, frilly curtain attached to the ventricle wall. That ruffled shape isn’t decorative. It dramatically increases the surface area available for producing fluid and filtering blood, much like the villi lining your intestines maximize nutrient absorption.
Structurally, the choroid plexus has two main layers. The outer layer is a sheet of specialized cube-shaped epithelial cells. Beneath that sits a core of connective tissue packed with tiny blood vessels. Blood flows through these vessels, and the epithelial cells on top do the actual work of filtering that blood and secreting cerebrospinal fluid into the ventricles.
How It Produces Cerebrospinal Fluid
The choroid plexus is the brain’s primary source of cerebrospinal fluid (CSF). The process is active, not passive. Rather than simply leaking plasma into the ventricles, the epithelial cells selectively pull water, salts, and specific nutrients from the blood and secrete them into the ventricular space in a carefully controlled composition. The resulting fluid circulates through the ventricles, flows around the brain and spinal cord, and is eventually reabsorbed into the bloodstream.
CSF does more than cushion the brain against impacts. It carries away metabolic waste products, delivers certain nutrients, and helps maintain stable chemical conditions around nerve cells. When CSF production, flow, or absorption is disrupted, fluid can build up in the ventricles, a condition called hydrocephalus, which increases pressure inside the skull.
The Blood-CSF Barrier
You may have heard of the blood-brain barrier, the tight seal that prevents most substances in the bloodstream from entering brain tissue. The choroid plexus operates a related but distinct checkpoint called the blood-CSF barrier. The epithelial cells lining the choroid plexus are linked together by tight junctions, protein structures that seal the gaps between neighboring cells. This forces nearly everything to pass through the cells rather than between them, giving the tissue precise control over what enters the CSF.
The barrier is selective in both directions. On the blood-facing side, transporter proteins pull in substances the brain needs, like thyroid hormones (carried by specific transport proteins called MCT8 and OATP1C1). On the ventricle-facing side, different transporters actively pump out waste products and potentially harmful compounds, including certain drugs and toxins. The choroid plexus even contains drug-metabolizing enzymes that can break down foreign molecules before they reach the brain.
This two-way filtering system means the choroid plexus isn’t just a fluid factory. It’s a gatekeeper that simultaneously feeds the brain what it needs and removes what it doesn’t.
What Choroid Plexus Cysts Mean on an Ultrasound
One of the most common reasons people search for the choroid plexus is because it appeared on a prenatal ultrasound. Choroid plexus cysts, small fluid-filled pockets within the tissue, show up in about 1 in 50 fetuses at 20 weeks of gestation. They form when a small amount of CSF gets trapped within the folds of the developing choroid plexus.
In the vast majority of cases, these cysts are harmless and resolve on their own before birth. They are, however, associated with a slightly increased risk of trisomy 18, a serious chromosomal condition. The key word is “slightly.” When a choroid plexus cyst is the only unusual finding on the ultrasound, with no other structural abnormalities visible, the risk remains very low. Doctors typically consider the cyst alongside other markers and maternal screening results rather than treating it as an alarm on its own.
Calcification in Adults
As people age, the choroid plexus commonly develops small calcium deposits. This calcification is so frequent that it’s often considered a normal finding on CT scans of the brain, particularly in adults over 40. For years, researchers suspected these calcifications might signal ongoing inflammation or declining choroid plexus function. A population study of over 800 adults, however, found no meaningful link between calcification volume and markers of active systemic inflammation. The calcifications may instead reflect a past inflammatory event that has already resolved, rather than any current problem.
Separately, research has found that enlargement of the choroid plexus (distinct from simple calcification) may impair the brain’s waste-clearance system, known as the glymphatic pathway, and worsen white matter damage over time. This is an area where the choroid plexus’s role in long-term brain health is becoming clearer.
Choroid Plexus Tumors
Tumors can arise directly from the choroid plexus, though they are rare. They are classified into three grades. Grade 1 tumors, called choroid plexus papillomas, are benign. Grade 2 tumors, called atypical papillomas, are also benign but have some features that warrant closer monitoring. Grade 3 tumors, choroid plexus carcinomas, are cancerous.
These tumors are more common in children than adults and most often appear during the first year of life. Because the choroid plexus produces CSF, tumors growing from it can cause overproduction of fluid or block its normal flow, leading to hydrocephalus. Symptoms in infants often include a rapidly enlarging head, irritability, and vomiting. In older children and adults, headaches, vision changes, and balance problems are more typical. Grade 1 and 2 tumors generally have a favorable outlook after surgical removal, while grade 3 carcinomas are more aggressive and harder to treat.