Blood-brain-barrier

A selective permeability barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system.

The blood-brain barrier (BBB) is a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). The BBB is formed by brain endothelial cells, which are connected by tight junctions with an extremely high electrical resistivity. This barrier restricts the passage of solutes from the bloodstream much more than endothelial cells in capillaries elsewhere in the body.

Structure

The blood-brain barrier is composed of endothelial cells, astrocyte end-feet, and pericytes. The endothelial cells form the walls of the capillaries in the brain and are tightly joined together by tight junctions, which prevent the passage of most substances. Astrocytes, a type of glial cell, envelop the capillaries and provide biochemical support to the endothelial cells. Pericytes are embedded in the basement membrane and contribute to the regulation of blood flow and the stability of the blood-brain barrier.

Function

The primary function of the blood-brain barrier is to protect the brain from potentially harmful chemicals while allowing essential nutrients to pass through. It achieves this by:

• Restricting the passage of pathogens and large or hydrophilic molecules.
• Allowing the passage of small or lipophilic molecules, such as oxygen, carbon dioxide, and hormones.
• Facilitating the transport of essential nutrients like glucose and amino acids through specific transport mechanisms.

Clinical Significance

The integrity of the blood-brain barrier is crucial for maintaining the homeostasis of the CNS. Disruption of the BBB can lead to neurological diseases such as multiple sclerosis, Alzheimer's disease, and stroke. In these conditions, the barrier becomes more permeable, allowing potentially harmful substances to enter the brain tissue.

Research and Therapeutic Implications

Research into the blood-brain barrier is ongoing, with a focus on understanding its role in disease and developing methods to deliver drugs across the barrier. Techniques such as focused ultrasound and nanoparticle carriers are being explored to temporarily disrupt the BBB to allow therapeutic agents to reach the brain.

Also see

• Central nervous system
• Endothelial cell
• Astrocyte
• Pericyte
• Tight junction
• Neurovascular unit