Choroid plexus

At a glance

English

Choroid plexus

Latin (TA2)

Plexus chorioideus

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the choroid plexus

This is a network of folded, blood-rich tissue that projects into the ventricles of the brain. It develops where the thin roof of a ventricle, made only of ependyma and pia mater, is pushed into the cavity by vessels; this double layer is the tela choroidea, and the plexus is its fringed free edge. Each of the four ventricles has some plexus. In each lateral ventricle it runs along the floor of the body and the roof of the temporal horn, and swells into a large tuft, the glomus, in the atrium where these meet; the frontal horn and occipital horn have none. The plexus of the lateral ventricles is continuous through the interventricular foramina with two thin strips in the roof of the third ventricle. The fourth ventricle carries its own plexus in the lower part of its roof, and tufts of it poke out through the lateral apertures.

Under the microscope the plexus is a mass of villi, each a core of wide capillaries and loose connective tissue covered by a single layer of cube-shaped epithelium. This epithelium is continuous with the ependyma lining the rest of the ventricles, but it is specialised: its surface bears a brush border of microvilli, and neighbouring cells are sealed near their ventricular surface by tight junctions. The capillaries themselves are leaky, so it is the epithelium that forms the barrier between blood and cerebrospinal fluid.

Most cerebrospinal fluid is made here. Plasma passes out of the capillaries, and the epithelium pumps sodium and other ions into the ventricle, with water following by osmosis, while actively moving other substances in and out. The resulting fluid cushions and floats the brain, carries away waste products and distributes signalling molecules. The tight junctions keep most blood-borne substances out, while selected nutrients and drugs enter brain fluid mainly here. The plexus also secretes transferrin, which is involved in handling iron in the brain, and controls the entry of white blood cells into the central nervous system.

Blockage of fluid flow, or rarely overproduction by a choroid plexus papilloma, causes hydrocephalus. Choroid plexus tumours, ranging from benign papillomas to carcinomas, mainly affect young children. The glomus often calcifies with age and shows as a bright spot on CT, a useful landmark.

Function and supply

Function

Secretes most of the cerebrospinal fluid, controls its composition, forms the blood-CSF barrier, secretes transferrin and regulates the entry of immune cells into the brain.

Blood supply

In the lateral and third ventricles, the anterior and posterior choroidal arteries; in the fourth ventricle, choroidal branches of the posterior inferior cerebellar artery. Venous blood drains through the choroidal veins to the internal cerebral and basal veins.