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

The correct answer is a. endothelium.<br /><br />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 endothelium, which is the inner lining of blood vessels, plays a crucial role in the formation and function of the BBB. The endothelial cells form the physical barrier that restricts the passage of substances from the bloodstream into the CNS.<br /><br />Pericytes, ependimocytes, astrocytes, and microglia are other types of cells found in the CNS, but they do not directly contribute to the formation of the BBB. Pericytes are contractile cells that wrap around the endothelial cells of blood vessels and capillaries, providing structural support and regulating blood flow. Ependimocytes are glial cells that line the ventricles of the brain and spinal cord, while astrocytes are star-shaped glial cells that provide support and nutrition to neurons. Microglia are the resident immune cells of the CNS, responsible for detecting and responding to injury or infection.