CNS barriers. (A) The BBB is composed of endothelial cells with tight junctions fenced by the glia limitans, contributing to the true nature of this system. SHH: sonic hedgehog. (B) The BLMB is composed of the dura mater, the arachnoid mater, subarachnoid space, and the innermost layer pia mater. (C) The BCSFB, which is composed of the ChP and CSF, lies in the brain ventricles and has its own CSF and stromal immune cell niche.