The choroid plexuses are small vascular structures in the brain. These regions are responsible for forming the cerebrospinal fluid, which is an essential element for the protection of the central nervous system.
Most of the cerebrospinal fluid originates in the choroid plexuses, being renewed six to seven times a day in the brains of humans. These structures stand out for being a continuation of the pia mater at the level of the ventricles. For this reason, these structures are mainly made up of modified epindymal cells.
The choroid plexuses constitute a small region of the brain that is responsible for forming the cerebrospinal fluid, an intracranial substance that runs through different regions of the brain in order to provide protection.
Characteristics of the choroid plexuses
Cerebellar medulla and cortex 1: posterior medullary velum. 2: Choroid plexus. 3: Cisterna cerebellomedullaris of the subarachnoid cavity. 4: Central channel. 5: Corpora quadrigemina. 6: Cerebral peduncle. 7: Anterior medullary veil. 8: Ependymal lining of the ventricle. 9: Cisterna pontis of the subarachnoid cavity and Arrow: Flow of cerebrospinal fluid (CSF) through the foramen of Magendie. Source: lyhana8 / Public domain
More specifically, these elements of the brain constitute vascular structures that are located on the sides of the cerebral ventricles. They are regions formed by a large number of capillaries that constitute a network and are surrounded by cells with a structure similar to an epithelium.
In this sense, the choroid plexuses lack a basal lamina and have a sharp base with extensions that join the oligondrocytes in order to use blood plasma, which is necessary to generate cerebrospinal fluid.
Together with the ependymal cells, these structures constitute a continuation of the pia mater (the internal meninx that protects the central nervous system) at the level of the ventricles.
Thus, the pia mater fulfills the same function as the choroid plexus. However, the former is carried out in the brain and spinal cord, while the latter is located in the cerebral ventricles.
Histology
Photomicrograph of the hematoxylin and eosin stained section of the normal choroid plexus. Source: Marvin_101 / Public domain
The human brain has four different choroid plexuses. Each of them is located in one of the four brain ventricles.
The choroid plexuses are formed by a layer of cuboidal epithelial cells that surround the nucleus of the capillaries and the connective tissue. The epithelial layer of the plexuses is continuous with the layer of ependymal cells, which covers the cerebral ventricles.
However, the ependymal cell layer, unlike the choroid plexuses, has a series of very tight junctions between cells. This fact prevents most substances from passing through the layer and reaching the cerebrospinal fluid.
In terms of location, the choroid plexuses are found in the upper region of the lower horn of the lateral ventricles.
They have a long structure that runs across the entire surface of the ventricle. Likewise, the choroid plexuses pass through the interventricular foramen and are present in the upper part of the third ventricle.
Furthermore, these structures can also be seen in the fourth cerebral ventricle. In this case, they are located in the section closest to the lower half of the cerebellum.
Thus, the choroid plexus is a structure that is present in all components of the ventricular system except the cerebral aqueduct, the frontal horn of the lateral ventricle, and the occipital horn of the lateral ventricle.
Functioning
The choroid plexuses configure the continuation of the pia mater at the level of the ventricles, they are formed by modified ependymal cells that have a basal lamina.
The cells of these plexuses are connected to each other through occlusive junctions, and they settle on the connective (non-nervous) tissue of the brain.
The ependymal cells of the choroid plexuses rest on the connective tissue and form a substance known as the choroidal tissue. This fabric folds into the choroid plexuses, which are characterized by having a large number of capillaries immersed in their tissue.
The plasma from these capillaries is filtered through the epithelium of the choroid plexuses and acts as a dialyzing membrane. Finally, the plasma is sent to the ventricles as cerebrospinal fluid.
Function
Coronal section of the inferior horn of the lateral ventricle. Source: Henry Vandyke Carter / Public domain
The main function of the choroid plexuses is to produce and transmit cerebrospinal fluid
Cerebrospinal fluid is a colorless substance that bathes the brain and spinal cord. It travels through the subarchnoid space, the cerebral ventricles and the ependymal canal, and has a volume of approximately 150 milliliters.
The main function of this substance is to protect the brain. Specifically, it carries out the following activities:
- It acts as a shock absorber and protects the brain regions from trauma.
- Provides hydropneumatic support to the brain to regulate local pressure.
- It helps in regulating the content of the skull.
- It fulfills the nutritional functions of the brain.
- Eliminates metabolites from the central nervous system.
- It serves as a pathway for pineal secretions to reach the pituitary gland.
Beyond the production of cerebrospinal fluid, the choroid plexuses act as a filtration system, removing metabolic waste, foreign substances, and excess neurotransmitters in the cerebrospinal fluid.
Thus, these plexuses play a very important role in adapting and maintaining the extracellular environment that the brain requires to function properly.
Related diseases
At present, the main pathology related to the choroid plexuses is tumors. Specifically, three main types have been described: choroid plexus papilloma, atypical papilloma, and carcinoma.
These alterations are quite uncommon primary brain tumors in the general population. They are derived from the epithelium of the choroid plexus and are especially prevalent during childhood.
The location of these pathologies is, in most cases, the lateral ventricles. However, they can also originate in the fourth and third ventricle.
Its most frequent clinical presentation is hydrocephalus. Likewise, it can cause leptomeningeal dissemination in cases of papilloma and carcinoma.
Overall, choroid plexus tumors represent between 0.3 and 0.6% of all brain tumors. Of the three typologies, papillomas are much more frequent, while carcinomas have a very low prevalence.
References
- Abril Alonso, Águeda et alii (2003). Biological foundations of behavior. Madrid: Sanz and Torres.
- Choroid plexus tumours. WHO Classi fi cation of Tumors of the Central Nervous System. 4th. Lyon: IARC Press; 2007. 82-5.
- Epidemiology and pathology of intraventricular tumors. Neurosurg Clin N Am. 2003; 14: 469-82.
- Hall, John (2011). Guyton and Hall textbook of medical physiology (12th ed. Ed.). Philadelphia, Pa.: Saunders / Elsevier. p. 749.
- Young, Paul A. (2007). Basic clinical neuroscience (2nd ed.). Philadelphia, Pa.: Lippincott Williams & Wilkins. p. 292.