The layer of the vessel wall described as consisting of a single layer of cells backed by a thin layer of elastin and collagen fibers is the

• A. Adventitia
• B. Intima
• C. Media
• D. External Elastic Lamina

Answer: (B)

The innermost lining of a vessel, the tunica intima, is made of a single layer of endothelial cells that directly contacts the blood. These endothelial cells sit on a basement membrane, and beneath that there is a thin layer of subendothelial connective tissue containing elastin and collagen fibers. This arrangement provides a smooth surface for blood flow and a small amount of elastic support without adding a bulky muscular layer. The boundary between the intima and the next layer is often marked by the internal elastic lamina, a sheet of elastic fibers, while the middle layer is the tunica media and the outer layer is the tunica adventitia.