Final answer:
The blood-brain barrier is composed of tight junctions between the capillary endothelial cells in the brain. These junctions control the passage of substances from the blood to the brain, maintaining the CNS's protected environment and challenging drug delivery for CNS treatments.
Step-by-step explanation:
The Blood-Brain Barrier:
The blood-brain barrier is composed of tight junctions between the endothelial cells of capillaries feeding the brain. These tight junctions are crucial as they prevent the movement of nearly all substances from the bloodstream into the central nervous system (CNS). This includes blocking most molecules, pathogens, and white blood cells, which illustrates the complexity of drug delivery to the CNS and the challenges faced by pharmaceutical companies. The absence of fenestrations (pore-like structures) and pinocytotic vesicles in the cerebral capillaries, as well as the presence of a thick basement membrane and astrocyte extensions known as end feet, contribute to the highly selective nature of the blood-brain barrier.
This barrier protects the CNS from potential contaminants in the bloodstream, ensuring that substances like glucose and certain amino acids can pass through, while many other drugs and pathogens cannot. This has significant implications for treatments involving CNS infections, as medications must be designed to penetrate this barrier effectively.