Choroid Plexus

The choroid plexus is a highly vascular specialized structure located within the ventricular system of the brain. Its principal function is the production and regulation of cerebrospinal fluid (CSF).

Choroid plexuses are present within the lateral ventricles, third ventricle, and fourth ventricle. They are not present within the cerebral aqueduct or the normal central canal of the spinal cord.

Structurally, each choroid plexus consists of a vascular connective tissue core containing fenestrated capillaries, covered by specialized choroid plexus epithelial cells. Tight junctions between these epithelial cells form the anatomical basis of the blood-CSF barrier.

Location

Choroid plexus tissue projects into the ventricular cavities at specific sites where vascular pia and ventricular lining are closely related.

The distribution differs among the lateral, third, and fourth ventricles.

Choroid Plexus of the Lateral Ventricles

Within each lateral ventricle, the choroid plexus extends through portions of the body, atrium, and temporal horn.

It is associated with the choroidal fissure, a cleft between the fornix and thalamic or fimbrial structures through which the vascular choroidal tissue is related to the ventricular cavity.

Choroid Plexus of the Third Ventricle

The choroid plexus of the third ventricle is associated with its roof.

It is related to the tela choroidea and vascular structures situated between the third ventricle and overlying deep cerebral structures.

Choroid Plexus of the Fourth Ventricle

The choroid plexus of the fourth ventricle is associated primarily with the inferior portion of its roof.

It contributes CSF directly to the fourth ventricular cavity, from which CSF can enter the subarachnoid space through the median and lateral apertures.

Ventricular Distribution

Ventricular Region	Choroid Plexus
Lateral ventricles	Present in body, atrium and temporal horn
Third ventricle	Present along roof
Cerebral aqueduct	Absent
Fourth ventricle	Present, especially along inferior roof
Central canal	Absent

Choroidal Fissure

The choroidal fissure is an important anatomical cleft associated with attachment of the choroid plexus of the lateral ventricle.

It follows a curved course related to the fornix, thalamus, fimbria, and medial temporal structures.

Tela Choroidea

The tela choroidea is a thin vascular membrane formed where pia-related vascular tissue is closely applied to the ventricular lining.

Invagination and specialization of this tissue contribute to formation of the choroid plexus.

General Structure

The choroid plexus consists of numerous branching folds and villous projections that increase its surface area.

Each projection contains a central vascular connective tissue core covered by a layer of specialized epithelial cells.

Histological Organization

The major histological components of the choroid plexus include:

• Choroid plexus epithelial cells
• Tight junctions between epithelial cells
• Basement membrane
• Connective tissue stroma
• Fenestrated capillaries

Choroid Plexus Epithelium

The ventricular surface of the choroid plexus is covered by a single layer of specialized choroid plexus epithelial cells.

These cells are modified ependymal cells and possess structural and transport characteristics suited to controlled secretion of CSF.

Apical Surface

The apical surface of choroid plexus epithelial cells faces the ventricular CSF.

Microvilli increase the surface area available for transport, while cellular transport systems regulate movement of ions, water, and other substances between blood-derived interstitial fluid and the ventricular compartment.

Tight Junctions

Adjacent choroid plexus epithelial cells are connected by tight junctions.

These junctions restrict uncontrolled movement of substances between the cells and are fundamental to the blood-CSF barrier.

Fenestrated Capillaries

The capillaries within the choroid plexus are fenestrated and relatively permeable compared with capillaries forming the blood-brain barrier.

Substances can therefore leave the capillary lumen and enter the choroid plexus stroma more readily, but passage into CSF remains regulated by the epithelial layer.

Blood-CSF Barrier

The blood-CSF barrier is formed primarily by tight junctions between choroid plexus epithelial cells.

It regulates the movement of ions, metabolites, proteins, xenobiotics, and other substances between blood and ventricular CSF.

Blood-CSF Barrier Compared with Blood-Brain Barrier

Feature	Blood-CSF Barrier	Blood-Brain Barrier
Principal barrier cells	Choroid plexus epithelial cells	Cerebral capillary endothelial cells
Site of tight junctions	Between epithelial cells	Between endothelial cells
Underlying capillaries	Fenestrated	Continuous, nonfenestrated
Primary interface	Blood and ventricular CSF	Blood and brain interstitial environment

Cerebrospinal Fluid Production

The choroid plexus produces most of the cerebrospinal fluid within the ventricular system.

CSF formation is an active secretory process involving selective transport of electrolytes across the choroid plexus epithelium, followed by movement of water into the ventricular cavity.

Mechanism of CSF Secretion

CSF production depends on coordinated transport of ions including sodium, chloride, and bicarbonate across choroid plexus epithelial cells.

Water follows the resulting osmotic gradients through transcellular pathways, producing a clear fluid with a composition distinct from blood plasma.

CSF Composition

Normal CSF contains water, electrolytes, glucose, small amounts of protein, and very few cells.

The choroid plexus contributes to maintaining this controlled composition through selective secretion, transport, and clearance mechanisms.

Functions of the Choroid Plexus

The functions of the choroid plexus extend beyond simple fluid production.

• Production of most ventricular CSF
• Formation of the blood-CSF barrier
• Regulation of CSF ionic composition
• Transport of nutrients and metabolites
• Removal or transport of selected substances from CSF
• Secretion of signaling molecules and proteins
• Participation in immune surveillance at the blood-CSF interface

CSF Circulation

After production by the choroid plexuses, CSF moves through the ventricular system and into the subarachnoid space.

A simplified pathway is:

1. Lateral ventricles
2. Interventricular foramina
3. Third ventricle
4. Cerebral aqueduct
5. Fourth ventricle
6. Median and lateral apertures
7. Cranial and spinal subarachnoid spaces

Lateral Ventricles to Third Ventricle

CSF produced within the lateral ventricles passes through the paired interventricular foramina into the third ventricle.

Additional CSF is produced by the third ventricular choroid plexus.

Third Ventricle to Fourth Ventricle

CSF passes from the third ventricle through the cerebral aqueduct into the fourth ventricle.

The cerebral aqueduct contains no choroid plexus and functions primarily as a narrow connecting channel.

Fourth Ventricle to Subarachnoid Space

CSF leaves the fourth ventricle primarily through the median aperture and paired lateral apertures.

It then circulates through the subarachnoid cisterns and around the surfaces of the brain and spinal cord.

Relationship to CSF Absorption

The choroid plexus is principally associated with CSF production rather than the major routes of CSF return to the circulation.

CSF outflow involves arachnoid granulations and additional drainage pathways associated with meningeal and lymphatic systems.

Choroid Plexus Compared with Arachnoid Granulations

Feature	Choroid Plexus	Arachnoid Granulations
Location	Within ventricles	Project from arachnoid toward dural venous sinuses
Primary association	CSF production and regulation	CSF outflow to venous circulation
Barrier function	Forms blood-CSF barrier	Participates in CSF drainage

Blood Supply

The choroid plexus has a rich arterial supply derived from choroidal arteries.

Different portions of the ventricular plexuses receive blood from branches of the internal carotid and vertebrobasilar circulations.

Anterior Choroidal Artery

The anterior choroidal artery, usually arising from the internal carotid artery, contributes to the choroid plexus of the lateral ventricle, particularly within the temporal horn.

It also supplies several important deep neural structures along its course.

Posterior Choroidal Arteries

The posterior choroidal arteries generally arise from the posterior cerebral circulation.

Medial and lateral posterior choroidal branches contribute to choroid plexuses of the third and lateral ventricles and supply adjacent deep structures.

Fourth Ventricular Choroidal Supply

The choroid plexus of the fourth ventricle receives arterial contributions from branches of the posterior inferior cerebellar artery and related posterior circulation vessels.

Venous Drainage

Venous drainage from the choroid plexuses communicates with the deep cerebral venous system.

Veins associated with the lateral and third ventricular plexuses ultimately contribute to deep venous channels such as the internal cerebral veins and related vessels.

Development

The choroid plexus develops at specialized regions where vascular mesenchyme and pia-related tissue become closely associated with the thin ventricular roof.

The epithelial covering is derived from neuroectodermal ventricular lining, while the vascular connective tissue core has mesenchymal contributions.

Development of the Ventricular Epithelium

Cells derived from the neuroepithelial lining differentiate into specialized choroid plexus epithelial cells.

These cells develop tight junctions and transport systems required for formation of the blood-CSF interface.

Choroid Plexus and Brain Homeostasis

By controlling the composition of CSF, the choroid plexus contributes to the chemical stability of the central nervous system.

Its transport systems regulate the movement of water, electrolytes, nutrients, metabolites, and selected signaling molecules.

Immune Interface

The choroid plexus also functions as an interface between the immune system, blood, and central nervous system.

Its vascular stroma can contain immune cells, while the epithelial barrier regulates movement of cells and molecules into the CSF compartment.

Imaging

The choroid plexus can be identified on CT and MRI within characteristic ventricular locations.

Because it is highly vascular, it normally enhances after administration of contrast material.

Choroid Plexus Calcification

Calcification of portions of the choroid plexus is common, particularly in adults, and is often a normal incidental imaging finding.

Calcification is frequently visible within the atria of the lateral ventricles.

Clinical Significance

Choroid Plexus Papilloma

Choroid plexus papilloma is a tumor arising from choroid plexus epithelium.

It can occur within the ventricular system and may cause hydrocephalus through excessive CSF production, obstruction of CSF pathways, or both.

Choroid Plexus Carcinoma

Choroid plexus carcinoma is a malignant choroid plexus epithelial tumor.

It is uncommon and occurs predominantly in children. Its ventricular location can produce symptoms related to increased intracranial pressure and hydrocephalus.

Hydrocephalus

Hydrocephalus results from abnormal accumulation of CSF associated with disturbed CSF circulation, absorption, or, less commonly, excessive production.

Because the choroid plexus continuously secretes CSF, obstruction of ventricular pathways can cause progressive enlargement of ventricles proximal to the obstruction.

Overproduction of CSF

Excessive CSF production is an uncommon mechanism of hydrocephalus but may occur with certain choroid plexus tumors.

More commonly, hydrocephalus results from impaired CSF flow or impaired outflow from the intracranial CSF spaces.

Intraventricular Hemorrhage

The highly vascular nature of the choroid plexus makes it relevant to intraventricular hemorrhage.

Blood entering the ventricular system can interfere with normal CSF circulation and may contribute to secondary hydrocephalus.

Choroid Plexus Cysts

Choroid plexus cysts can occur during fetal development and are often detected on prenatal ultrasonography.

Many are transient and resolve during gestation. Their clinical interpretation depends on associated findings and the broader prenatal assessment.

Choroid Plexus Inflammation

Inflammatory processes affecting the ventricular and meningeal compartments can involve the choroid plexus and alter blood-CSF barrier function.

Changes in epithelial transport and immune signaling may influence the composition of CSF during disease.

Surgical Anatomy

The choroid plexus is an important landmark during ventricular and endoscopic neurosurgical procedures.

Within the lateral ventricle, its relationship to the choroidal fissure, fornix, thalamus, and interventricular foramen helps orient the surgeon to deep ventricular anatomy.

Relationship to the Interventricular Foramen

The choroid plexus near the interventricular foramen provides a useful anatomical landmark at the junction between the lateral and third ventricles.

This region is closely related to the fornix, thalamus, septal structures, and deep cerebral veins.

Key Anatomical Features

Feature	Key Point
Primary function	Production and regulation of CSF
Locations	Lateral, third and fourth ventricles
Absent from	Cerebral aqueduct and central canal
Surface cells	Specialized choroid plexus epithelial cells
Capillaries	Fenestrated
Barrier	Blood-CSF barrier
Barrier junctions	Tight junctions between epithelial cells
Major arterial sources	Anterior and posterior choroidal arteries plus posterior circulation branches
Imaging feature	Normal contrast enhancement and frequent age-related calcification
Major clinical associations	Choroid plexus tumors and hydrocephalus

Anatomical Importance

The choroid plexus is the principal anatomical structure responsible for producing the fluid environment surrounding and supporting the central nervous system. Its highly vascular core allows close contact with the circulation, while the specialized epithelial covering carefully regulates which substances enter the ventricular CSF.

This organization creates the blood-CSF barrier, which differs structurally from the blood-brain barrier because its tight junctions are located between epithelial cells rather than between capillary endothelial cells.

By producing CSF and regulating its composition, the choroid plexus contributes to mechanical protection, chemical homeostasis, transport, waste handling, and signaling within the central nervous system. Its ventricular location also makes it an important landmark in neuroimaging and neurosurgical anatomy.