Endothelium is the cover term for the blood-oriented cells in the innermost layer of the vascular wall (intima). Functioning as physiological barrier to tissue, the CD 31- and vWF-positive cells express nitrogen oxide (NO) which plays a role in regulating the tonus of the vascular musculature. Damage to the endothelial cell layer has been assumed to be possibly a causative factor in the development of arteriosclerosis. Endothelium plays an equally important role in inflammatory processes and may be locally activated by various endogenous or microbial substances.