What composition primarily makes up the blood-brain barrier?

The blood-brain barrier is primarily composed of tightly packed endothelial cells that line the capillaries in the brain. This unique structure is crucial for maintaining the brain's environment by selectively regulating the passage of substances from the bloodstream into the brain's interstitial fluid. The tight junctions between these endothelial cells significantly restrict the movement of potentially harmful substances and pathogens, thereby protecting neural tissue while still allowing essential nutrients and gases, like glucose and oxygen, to pass through.

In addition to tight junctions, the blood-brain barrier is supported by other cell types, including astrocytes and pericytes, which provide additional structural support and help regulate the barrier's permeability. However, the defining feature of the blood-brain barrier is indeed the compact arrangement of the endothelial cells, which is what enables its selective permeability and overall functionality in brain protection.