Endothelium--an organized monolayer of highly specialized cells.

Research on endothelial cells has expanded in near exponential fashion during the last few years and has become a noteworthy field of both clinical and experimental investigation. Endothelium is indisputably a highly specialized tissue which mediates and controls many physiological and pathological processes. It can no longer be regarded as a passive semipermeable barrier between blood and tissues. The endothelial cell--the structural unit of endothelium--is, consequently, a metabolically highly active cell with many unique characteristics.