The 'Blood Brain Barrier' is a barricade of cells which acts as a sieve to prevent many substances from the blood such as drugs or chemical compounds, viruses which cause diseases, radioactive ions etc. into entering the central nervous system.
Blood Brain Barrier (BBB)
The BBB is composed of three cellular components of the brain microvasculature namely endothelial cells, astrocyte end-feet and pericytes (PCs). It is formed by a combination of special glial cells (astrocytes) with blood vessels and is critical for maintaining the homeostasis in the brain. Essential nutrients such as carbon dioxide and oxygen and substances such as steroids can pass through the BBB easily, and some such as caffeine and ethanol pass via the lipid membranes of the cells. Substances such as chloride ions, sodium and potassium need specific carrier molecules along with to reach the brain, and some substances cannot pass through at all (which can damage the brain). BBB is located in the endothelial cells of the capillaries of the brain and usually develops on the 28th day of being born. History of BBB The concept of BBB was first suggested by bacteriologist Paul Ehrlich. While conducting an experiment, he found that intravenous dyes if injected into the body stained all the tissues of the body except the brain. Later, in 1913, Edwin Goldmann directly injected the dye in the brain which stained the brain and not the rest of the body indicating a compartment between the two. This concept was further proposed by Lina Stern in 1921. Finally in 1960, some scientists used electron microscopy and electron - dense tracers to detect and confirm the existence of BBB.
The brain’s endothelial cells are attached by tight junctions of high electrical resistance which act as a barrier against molecules, whereas the endothelial cells of peripheral tissues in the body ensure a free transcellular movement of molecules.
Moreover, the capillaries of the brain are in contact with foot processes of astrocytes which segregate neurones from them. Some areas such as postrema and the posterior pituitary do not have the BBB as the pituitary has to allow some neurosecretory products to reach the blood on the brain. BBB is absent in the subfornical organ so that water balance and other homeostasis functions can be maintained.
Dysfunction of BBB
Impairment of tight junctions can cause stroke or neuroinflammatory disorders. Other diseases affecting the blood brain barrier are meningitis, epilepsy, multiple sclerosis, neuromyelitis optica, late-stage neurological trypanosomiasis (sleeping sickness), progressive multifocal leukoencephalopathy (PML), de vivo disease, alzheimer's disease and HIV encephalitis. All of these diseases cause harm as they allow unwanted substances to pass to the blood on the brain, thereby affecting BBB.