Blood-Brain Barrier

Introduction & the three brain-barrier sites

• Blood-brain barrier (BBB) — a series of barriers that stringently regulate movement of ions, molecules and cells between blood and neural tissue, maintaining the tightly controlled extracellular environment required for neuronal homeostasis; it admits glucose and essential nutrients while greatly limiting entry of exogenous compounds — the central obstacle in CNS drug development.
• Three anatomical barrier sites — (1) the BBB proper — an endothelial barrier formed by blood vessels vascularizing the CNS parenchyma (most of the surface area, the key drug-delivery interface); (2) the blood–CSF barrier (BCSFB) — an epithelial barrier of choroid-plexus cells around fenestrated plexus vessels that secrete CSF; (3) the arachnoid barrier — epithelial cells separating leaky dura vessels from subarachnoid CSF.
• KDT framing — brain capillary endothelial cells have tight junctions and lack large paracellular spaces, with an investment of glial (astrocyte) processes; both BBB and blood-CSF barrier are lipoidal, admitting only lipid-soluble drugs and excluding non-lipid-soluble agents (e.g. streptomycin, neostigmine).
• Function & clinical importance — barriers insulate neurons from ionic fluctuations and protect the CNS from toxins, pathogens and the body's own immune system — critical because the CNS regenerates poorly; their disruption underlies the severe pathology of multiple sclerosis, stroke, traumatic brain injury and meningitis.
• Scale of the human BBB — a vascular network ≈600 km long, endothelial wall 200–400 nm thick, total surface area 15–25 m² — making the endothelium the primary blood–tissue interface for permeability, transport and immune-cell infiltration.