[1] Regions of the choroid plexus produce and secrete most of the cerebrospinal fluid (CSF) of the central nervous system.

[2][3] The choroid plexus consists of modified ependymal cells surrounding a core of capillaries and loose connective tissue.

The choroid plexus consists of a layer of cuboidal epithelial cells surrounding a core of capillaries and loose connective tissue.

[6][7] Unlike the ependyma, the choroid plexus epithelial layer has tight junctions[8] 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 cerebrospinal fluid (CSF); thus the choroid plexus acts as a blood–CSF barrier.

Choroid plexus epithelial cells actively transport sodium ions into the ventricles and water follows the resulting osmotic gradient.

The choroid plexus regulates the production and composition of cerebrospinal fluid (CSF), that provides the protective buoyancy for the brain.

[10][11] In this way the choroid plexus has a very important role in helping to maintain the delicate extracellular environment required by the brain to function optimally.

The choroid plexus is also a major source of transferrin secretion that plays a part in iron homeostasis in the brain.

[14][16] 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.