Choroid plexus cysts are usually an isolated finding. The finding is relatively common, with a prevalence of ~1%. These fluid-filled cysts can be detected by a detailed second trimester ultrasound. Clinical significance Choroid plexus cysts ĭuring fetal development, some choroid plexus cysts may form. This cellular trafficking has implications both in normal brain homeostasis and in neuroinflammatory processes. The choroid plexus cells secrete cytokines that recruit monocyte-derived macrophages, among other cells, to the brain. The blood–cerebrospinal fluid barrier has also been shown to modulate the entry of leukocytes from the blood to the central nervous system. For a number of substances, the BCSFB is the primary site of entry into brain tissue. Despite the similar function between the BBB and BCSFB, each facilitates the transport of different substances into the brain due to the distinctive structural characteristics of each of the two barrier systems. Similar to the blood–brain barrier, the blood–CSF barrier functions to prevent the passage of most blood-borne substances into the brain, while selectively permitting the passage of specific substances into the brain and facilitating the removal of brain metabolites and metabolic products into the blood. There is a CSF-brain barrier at the level of the pia mater, but only in the embryo. The blood–CSF boundary at the choroid plexus is a membrane composed of epithelial cells and tight junctions that link them. The blood–cerebrospinal fluid barrier (BCSFB) is a fluid–brain barrier that is composed of a pair of membranes that separate blood from CSF at the capillary level and CSF from brain tissue. There is also much active transport of substances into, and out of, the CSF as it is made. Fluid filters through these cells from blood to become cerebrospinal fluid.

The choroid plexus consists of many capillaries, separated from the ventricles by choroid epithelial cells. Choroid plexus epithelial cells actively transport sodium ions into the ventricles and water follows the resulting osmotic gradient. CSF is formed as plasma is filtered from the blood through the epithelial cells. The villi, along with a brush border of microvilli, greatly increase the surface area of the choroid plexus.

The choroid plexus folds into many villi around each capillary, creating frond-like processes that project into the ventricles.

These tight junctions prevent the majority of substances from crossing the cell layer into the cerebrospinal fluid (CSF) thus the choroid plexus acts as a blood–CSF barrier. Unlike the ependyma, the choroid plexus epithelial layer has tight junctions between the cells on the side facing the ventricle (apical surface). Progenitor ependymal cells are monociliated but they differentiate into multiciliated ependymal cells. The epithelium of the choroid plexus is continuous with the ependymal cell layer (ventricular layer) that lines the ventricular system. The choroid plexus consists of a layer of cuboidal epithelial cells surrounding a core of capillaries and loose connective tissue. A choroid plexus is in part of the roof of the fourth ventricle. In the third ventricle there is a small amount in the roof that is continuous with that in the body, via the interventricular foramina, the channels that connect the lateral ventricles with the third ventricle. There is no choroid plexus in the anterior horn. In the lateral ventricles it is found in the body, and continued in an enlarged amount in the atrium. There is a choroid plexus in each of the four ventricles.

Blood brain barrier definition