It is found in the superior part of the inferior horn of the lateral ventricles. It follows up along this boundary, continuous with the inferior of the body of the lateral ventricles. It passes into the interventricular foramen, and is present at the top of the third ventricle.
There are four choroid plexus in the brain, one in each of the ventricles. The CP consist of a layer of cuboidal epithelial cells surrounding a core of capillaries and loose connective tissue. The CP epithelial layer is continuous with the ependymal cell layer that lines the ventricles, but unlike the ependyma the CP epithelial layer has tight junctions in between the cells on the side facing the ventricle (apical surface). These tight junctions prevent the majority of substances from crossing the cell layer into the CSF; thus the CP acts as a blood-CSF barrier. The CP folds into many villi around each capillary, creating frond-like processes that project into the ventricles. The villi, along with a brush border of microvilli, greatly increases the surface area of the CP. CSF is formed as plasma is filtered from the blood through the epithelial cells. CP epithelial cells actively transport sodium, chloride and biocarbonate ions into the ventricles and water follows the resulting osmotic gradient.
In addition to CSF production, the CP act as a filtration system, removing metabolic waste, foreign substances, and excess neurotransmitters from the CSF. In this way the CP have a very important role in helping to maintain the delicate extracellular environment required by the brain to function optimally.
CPCs typically disappear later during pregnancy, and are considered soft markers. They are likely harmless, and studies have shown that they have no effect on infant and early childhood development.
Strazielle and Ghersi-Egea. "Choroid Plexus in the Central Nervous System: BIology and Physiopathology". 2000. Jouranal of Neuropathology and Experimental Neurology 59(7): 561-574.