<P> The blood--brain barrier (BBB) is a highly selective semipermeable membrane barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). The blood--brain barrier is formed by brain endothelial cells and it allows the passage of water, some gases, and lipid - soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function . Furthermore, it prevents the entry of lipophilic potential neurotoxins by way of an active transport mechanism mediated by P - glycoprotein . Astrocytes have been claimed to be necessary to create the blood--brain barrier . A few regions in the brain, including the circumventricular organs, do not have a blood--brain barrier . </P> <P> The blood--brain barrier occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation . Endothelial cells restrict the diffusion of microscopic objects (e.g., bacteria) and large or hydrophilic molecules into the cerebrospinal fluid (CSF), while allowing the diffusion of hydrophobic molecules (O, CO, hormones) and likely also small polar molecules . Cells of the barrier actively transport metabolic products such as glucose across the barrier with specific proteins . This barrier also includes a thick basement membrane and astrocytic endfeet . </P> <P> This "barrier" results from the selectivity of the tight junctions between endothelial cells in CNS vessels, which restricts the passage of solutes . At the interface between blood and the brain, endothelial cells are stitched together by these tight junctions, which are composed of smaller subunits, frequently biochemical dimers, that are transmembrane proteins such as occludin, claudins, junctional adhesion molecule (JAM), or ESAM, for example . Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex that includes ZO - 1 and associated proteins . </P> <P> The blood--brain barrier is composed of high - density cells restricting passage of substances from the bloodstream much more than do the endothelial cells in capillaries elsewhere in the body . Astrocyte cell projections called astrocytic feet (also known as "glia limitans") surround the endothelial cells of the BBB, providing biochemical support to those cells . The BBB is distinct from the quite similar blood--cerebrospinal fluid barrier, which is a function of the choroidal cells of the choroid plexus, and from the blood--retinal barrier, which can be considered a part of the whole realm of such barriers . </P>

Which of the following is most likely able to cross the blood-brain barrier