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UPDATED: The choroid plexuses are highly vascular structures situated in the ventricular system of the brain. They are formed by convoluted capillaries surrounded by modified ependymal cells. The choroid plexuses are responsible for the constant formation of the cerebrospinal fluid (CSF), as well as part of the blood-brain barrier, which in this case should probably be called the blood-CSF barrier.
The choroid plexuses are found in each ventricle of the brain. The accompanying image shows a dissection of a human brain where the frontal, parietal, and occipital lobes, as well as the corpus callosum have been removed to expose the lateral ventricles. The trigone has been transected and reflected posteriorly and the choroid plexuses can be seen as a cluster of grape-like longitudinal masses in each lateral ventricle.
Choroid plexuses form when three elements come in contact within the brain: pia mater, ependymal epithelium, and blood vessels. This only happens in the ventricular system of the brain.
The etymology (word origin) of the term [choroid] is a bit complicated. The suffix [-oid] means "similar to", while the root term [chor-] derives from the Greek word [χορίου] pronounced (joríu), meaning "dermis", "skin", or "membrane". The reason for the use of this term is that the Greek used the term "membrane" referring to the highly vascularized membranes that invest a fetus.
The term "plexus" means a "mesh", so the term [choroid plexuses] means "similar to the vascular membranes that invest a fetus"
Sources:
1. "Medical Meanings, A Glossary of Word Origins" Haubrich, W.S. 1997. American College of Physicians, Philadelphia, PA.
2. "Elementos de Neuroanatomia" Fernandez, J.; Miranda, EA.