Endothelial cells comprise the lining of the blood
vessels.  They play a fundamental role in overall health due to their ability
to: a) act as a selective barrier between circulation and tissue, allowing
certain cells to enter tissue from the blood stream while not allowing others;
b) keeping the blood vessels free of clotting; and c) controlling contraction of
the blood vessels.  Diseases such as atherosclerosis are believed to occur
because of injury to the endothelial cells.  The Ross classic paradigm states
that seemingly disparate cardiovascular risk factors act on a final common
pathway that culminates in endothelial-cell injury (Ross. The pathogenesis of
atherosclerosis: a perspective for the 1990s. Nature 1993;362:801-9).

The body has naturally occurring stem cells that regenerate
the endothelium called