 Experiments using electron-dense markers have discovered that functional components of the blood–brain barrier are the endothelial cells that compose the vessel itself.These endothelial cells contain highly impermeable tight junctions that cause the blood vessels of the brain to exhibit none of the “leakiness” found in arteries and veins elsewhere in the body.Through both in vivo and in vitro experiments the astrocytic foot processes of the glia limitans were shown to induce the formation of the tight junctions of the endothelial cells during brain development.