BLOOD BRAIN BARRIER

This is a brief overview of the structure and function of the blood brain barrier. Detailed information can be found in the related websites section.

Nowhere in the body is there more need for homeostasis than in the brain. The mechanism for maintaining this barrier function lies in the capillary network supplying blood to the brain. Ion concentration levels in plasma may fluctuate abruptly. The blood- brain barrier protects the brain against surging fluctuations in ion concentrations.

The concept of the blood brain barrier was first introduced by Paul Ehrlich. He found that intravenous injection of dyes into the bloodstream stained all the tissues in most organs except the brain. Using electron microscopy and electron- dense tracers such as horseradish peroxidase (HRP) a group of scientists demonstrated that the blood- brain barrier is located in endothelial cells of capillaries of the brain.

These endothelial cells of capillaries in the brain are different to those found in peripheral tissues in various ways:

1. Brain endothelial cells are joined by tight junctions of high electrical resistance providing an effective barrier against molecules.

2. In peripheral endothelial cells there is good transcellular movement of molecules. There is no such movement in brain endothelial cells.

3. Brain capillaries are in contact with foot processes of astrocytes which essentially separate the capillaries from the neurones.

This picture illustrates the location of the blood brain barrier and the tight junction between the endothelial cells.

The blood brain barrier is both a physical barrier and a system of cellular transport mechanisms.

It maintains homeostasis by restricting the entrances of potentially harmful chemicals from the blood, and by allowing the entrance of essential nutrients.

Lipid soluble molecules, such as ethanol and caffeine are able to penetrate through the barrier relatively easily via the lipid membranes of the cells. In contrast, water soluble molecules such as sodium and potassium ions are unable to transverse the barrier without the use of specialized carrier- mediated transport mechanisms.

There are some areas of the brain that do not have a blood- brain barrier.

The area postrema and the posterior pituitary area are two regions lacking this barrier. Here the capillaries are fenestrated like those in peripheral tissues.

So why is this?

The barrier is absent in the pituitary because it allows neurosecretory products to pass into the circulation. Another area called the subfornical organ, a chemoreceptive area, the transcellular transport is required for water balance and other homeostatic functions.

There are some good websites containing further information on the blood- brain barrier. To find out more