What is the Blood Brain Barrier?

The blood brain barrier is formed by endothelial cells, more specifically the junctions between endothelial cells. The structure is also formed with glial cells known as astrocytes. Projections from these astrocytes surround the endothelial cells to bolster and support them.

The smallest blood vessels are called capillaries, and each one is lined with endothelial cells. Elsewhere in the human body the gaps between endothelial cells are large enough to let substances pass between the inside and outside of the vessel. However, in the blood brain barrier these spaces are much tighter, which prevents molecules from the bloodstream entering the brain.

The blood brain barrier was discovered by the German scientist Paul Ehrlich who observed that intravenous injection of dyes into the body stained almost all organs except the brain. He believed that this was because the brain just didn't take up the dyes. Then in an experiment in 1913, Edwin Goldmann, who was one of Ehrlich's students, injected dye directly into the spinal fluid and observed that the brain did stain, but the rest of the body's organs did not. This demonstrated that there was some kind of compartmentalization between the bloodstream and the cerebrospinal fluid. Although, it had not yet been observed.

It was the neurophysiologist Lina Stern who came up with the concept of the blood-brain barrier in 1921. She had been puzzled by the fact that medicines and serums injected into the body did not reach the brain, and came to the conclusion that there must be a special filter to protect the organ; she called it the "hematoencephalic barrier."

The blood brain barrier (BBB) carries out three vital functions and these are;

• Protect the brain from foreign substances that might damage it
• Maintain a constant environment for the brain
• Protect the brain from the harmful effects of some hormones

The BBB carries out its functions by preventing the passage of large molecules, and it expresses high numbers of proteins which can drive these molecules away. The structure can also slow down molecules that have a high electrical charge.

Substances that do pass through the blood brain barrier include molecules that readily dissolve in lipids. This is because the cells of the blood brain barrier have a phospholipid bilayer. Nicotine and alcohol easily dissolve in lipids and their effects can be felt very soon after they've entered the bloodstream. Other substances to be able to make it through include glucose and insulin, amino acids, oxygen, and anaesthetic drugs (lipid soluble).

The blood brain barrier provides an excellent block to bacteria, and so infections of the brain are rare. But they can occur, and are difficult to treat as antibodies cannot get across the barrier. However, some do find a way because inflammation increases blood brain barrier permeability.

The blood brain barrier does its job so well that it can make some diseases of the brain, such as cancers, very difficult to treat. This presents a huge challenge to researchers who have to work out how to design delivery methods that can get drugs through without damaging the brain or its environment.