CELLS OF BLOOD-BRAIN BARRIER: a endothelium b. pericytes c. ependimocytes d. astrocytes e. microglia

The correct answer is a. endothelium.The blood-brain barrier (BBB) is a highly selective and protective barrier that separates the blood vessels in the brain from the brain's extracellular fluid. The endothelium, which is the inner lining of the blood vessels, plays a crucial role in the formation and maintenance of the BBB. The endothelial cells form the physical barrier that restricts the passage of substances from the bloodstream into the brain, ensuring that only essential nutrients and ions can enter while preventing potentially harmful substances from crossing.The other options listed, such as pericytes, ependimocytes, astrocytes, and microglia, are all important components of the brain's anatomy and function, but they do not directly contribute to the formation of the blood-brain barrier.