The epithelium of the choroid plexus is continuous with the ependymal cell layer that lines the ventricles. The cells of the choroid plexus are non ciliated[2] but, unlike the ependyma, the choroid plexus epithelial layer has tight junctions[3] 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.