Final answer:
The blood-brain barrier is composed of capillary endothelial cells that form tight junctions, which prevent the movement of nearly all substances between the blood and the CNS. The correct answer is C. Tight junctions.
Step-by-step explanation:
The blood-brain barrier is a highly selective membrane that separates the circulating blood from the extracellular fluid in the brain and spinal cord. It's an essential mechanism that helps maintain the central nervous system's protected environment, allowing for the selective transfer of substances required for brain function while blocking harmful substances. The blood-brain barrier is formed by the endothelial cells of brain capillaries which are linked together by tight junctions. These junctions create a watertight seal, supported by a thick basement membrane and astrocyte extensions, known as end feet. The correct answer to your question is C. Tight junctions. Tight junctions in the brain capillaries are crucial because, unlike continuous capillaries in other parts of the body, there are no intercellular clefts or fenestrations. These tight junctions stop nearly all substances from moving between the blood plasma and the interstitial fluid of the CNS. It is due to this highly selective permeability that harmful substances and pathogens are largely prevented from entering brain tissue.