In this article we will analyze the various aspects of the choroid plexus.
Many of us will have heard of cerebrospinal fluid (CSF) and are familiar with its important role in providing protection for the brain and spinal cord. But where is this colorless fluid found? And where is it produced?
Our brain’s ventricular system is responsible for the production of CSF. This system is made up of four interconnecting cavities called ventricles. On the other hand, we have 3 membranous layers called the meninges that provide protection to the brain and spinal cord.
The innermost layer of the meninges, called the pia mater, forms invaginations in some parts of the ventricles. These vascularized invaginations, are lined by a plexus of specialised cells that produce our CSF. This plexus of cells is called the choroid plexus.
- The ventricular system
- Anatomy of the choroid plexus
- Histology of the choroid plexus
- Clinical notes
The ventricular system
The ventricular system consists of:
- two lateral ventricles
- a third ventricle
- cerebral aqueduct
- a fourth ventricle
These ventricles are lined by a specialized type of glial cell called ependymal cells, or the ependyma. The choroid plexus is formed by these vascularized invaginations, bordered by the ependyma.
The two lateral ventricles are located in the brain parenchyma, one in each cerebral hemisphere. The lateral ventricles are roughly C shaped and consist of a body with an anterior, a posterior (occipital) and an inferior (temporal) horn.
The inferior horn resides in the temporal lobe, while the body of the ventricle passes through the parietal lobe and into the frontal lobe. The anterior lobe extends even further into the frontal lobe, while the posterior horn projects backwards into the occipital lobe.
The anterior end of the body of the lateral ventricle (before the beginning of the anterior horn) is demarcated by a twin opening called the interventricular foramen (foramen of Monro), which both connect and open into the third ventricle.
The third ventricle is a narrow (unpaired) cavity that lies between the cerebral hemispheres. Specifically, this ventricle lies in the diencephalon where it is enclosed laterally by the hypothalamus and thalamus.
The third ventricle is connected to the fourth ventricle posteroinferiorly through the cerebral aqueduct (aqueduct of Sylvius). The cerebral aqueduct is a narrow tube-shaped channel that travels the length of the midbrain towards the pons.
The fourth ventricle is diamond-shaped and lies at the level of the brainstem, specifically within the pons or in the superior portion of the medulla. Lateral apertures (foramina of Luschka), and a median aperture (foramen of Magendie) in the roof of the fourth ventricle facilitates the exiting flow of CSF.
Cerebrospinal fluid circulation
While each ventricle produces CSF, it also receives CSF from the ventricle upstream. Therefore, CSF produced in the lateral ventricles travels to the third ventricle, while CSF produced in the third ventricle travels to the fourth ventricle. A combination of CSF produced by the lateral, third and fourth ventricles flow out to the subarachnoid spaces in order to protect the entire central nervous system.
Anatomy of the choroid plexus
Choroid plexus in the lateral ventricles
The body, posterior horn, and inferior horn of each lateral ventricle join in a triangular area called the atrium (or collateral trigone). The choroid plexus of the lateral ventricles is found in the superomedial portion of the inferior horn and the anteromedial portion of the body. The plexus found in these areas also continue into the atrium.
At the junction of the body and inferior horn in the atrium, the choroid plexus becomes enlarged and more prominent, forming tufts called choroid glomus. In the lateral ventricles, the choroid plexus is always bound to a thin cleft called the choroid fissure.
The choroid plexus of the lateral ventricles are supplied by the anterior choroidal arteries (branch of internal carotid artery) and the lateral posterior choroidal arteries (branch of the posterior cerebral artery).
Choroid Plexus of the third ventricle
As previously mentioned, the lateral ventricles are connected to the third ventricle by a twin opening called the interventricular foramen (or foramen of Monro). At the junction of the anterior horn and the inferior portion of the body of the lateral ventricles, the choroid plexus continues along the interventricular foramen on both sides.
The path of choroid plexus from each lateral ventricle joins along the roof of the third ventricle and projects into the superior part of the ventricle. The choroid plexus of the third ventricle is supplied by the medial posterior choroidal arteries (branch of posterior cerebral artery).
Choroid plexus of the fourth ventricle
The third ventricle is connected to the fourth ventricle through the cerebral aqueduct. The cerebral aqueduct is void of choroid plexus. The choroid plexus is located in the posterior medullary velum which partially forms the roof of the fourth ventricle. The choroid plexus is supplied by the branches of the posterior inferior cerebellar arteries.
The ventricular system is derived from the neural canal, which describes the lumen of the neural tube. By the fourth gestational week, three swellings or vesicles form around the neural canal. These swellings represent the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). Between the fourth and sixth weeks of gestation, the central nervous system is said to be in a five vesicle stage, as the prosencephalon is now divided into the telencephalon and the diencephalon, while the rhombencephalon is now divided into the metencephalon and the myelencephalon. The lateral ventricles are derived from the telencephalon, while the third ventricle is derived from the diencephalon. The ventricles in the rhombencephalon become the fourth ventricle, while the cerebral aqueduct is derived from the mesencephalon.
The ventricles, along with the epithelial cells of the choroid plexus, are derived from the neural tube. However, the endothelial cells of all the choroid plexuses, and the mesenchymal stem cells (cells that differentiate into other cell types) of the choroid plexus of the fourth ventricle are derived from the cephalic mesoderm. On the other hand, mesenchymal cells of the lateral and third ventricles are derived from the mesencephalic neural crest. Signs of choroid plexus development of the fourth ventricle are evident around the 6th or 7th week of gestation, with the choroid plexus of the lateral ventricles developing at the same time, or shortly after in week 7. The choroid plexus of the third ventricle generally begins to develop a bit later in week 8. Over a 4 stage process, the epithelial cells of the choroid plexus and ventricles alternate in size and continue to develop right up until week 40 of gestation.
Histology of the choroid plexus
As mentioned above, if we were to look closely at the medial wall of the lateral ventricle or the roof of the third or fourth ventricle, small invaginations would be visible. The regions of the ventricles that contain invaginations of the pia mater are known as tela choroidea.
The deepest component of the choroid plexus is a layer of simple cuboidal epithelial cells, collectively known as the ependyma. This ependymal layer, with hair like projections, is composed of ependymal cells. These ependymal cells are specialized glial cells (derived from neural stem cells) that have the ability to produce cerebrospinal fluid, which is released into the ventricles. The apical side of the ependymal cells hosts many small processes called microvilli. The microvilli are responsible for increasing the surface area of the ependyma. There is also evidence of some larger villi on the apical surface which assist in the stirring of the CSF flow.
On the lateral surface of the ependymal cells, near the apical pole, desmosomes (junctions that are specialized for cell to cell adhesion) and tight junctions are responsible for holding the cells together, making up the blood-CSF barrier and preventing the passage of larger molecules.
On its basal side, the ependyma is surrounded by a basement membrane, which is anchored to the vascularized pia mater. The pia mater is composed of loose fibrous connective tissue. The proximal portion of this connective tissue houses fenestrated capillaries, while larger arterioles are evident at the base.
The choroid plexus receives sympathetic and parasympathetic innervation. Sympathetic fibers from the superior cervical ganglion control blood flow to the choroid plexus, while parasympathetic fibers reduce CSF production.
Choroid plexus papilloma
Choroid plexus papilloma describes the development of a rare benign tumor in the choroid plexus epithelium. This condition is most commonly seen in the lateral ventricle of children, with over 85% of cases occurring in children under the age of 5. Tumors can also develop in adulthood, however, they are most likely to develop in the fourth ventricle at this point.
Patients generally present with hydrocephalus, which describes an increase in cerebral ventricle volume as a result of increased CSF volume. A solid mass, with some calcifications, is generally evident on imaging. The most common treatment for choroid plexus papilloma is complete surgical excision.
Antenatal choroid plexus cysts
Antenatal choroid plexus cysts are benign cysts that occur due to an infolding of the neuroepithelium in a fetus, most commonly in the lateral ventricles. Ranging from a few mm to 1-2cm in size, these cysts generally occur in approximately 2% of all pregnancies. They are described as pockets or bubbles of choroid plexus filled with CSF and cellular material. These cysts are typically visible during ultrasound in the 2nd trimester. Generally, the cysts disappear later in the pregnancy with no repercussions, however, the fetus should be monitored for other karyotypic abnormalities, particularly if there are large, multiple cysts.