Although pericytes were described more than 100 years ago as perivascular cells that wrap around blood capillaries, the term pericyte (peri, around; cyte, cell) was first introduced by Zimmerman in 1923. 1 Because pericytes have some morphological features of smooth muscle cells, and a smooth muscle layer is absent from capillaries and postcapillary venules, Zimmerman attributed the property of contractility to pericytes. Morphologically, pericytes possess long processes embracing the abluminal endothelium wall in precapillary arterioles, capillaries, and postcapillary venules. Long cytoplasmic processes that extend along and encircle the endothelial tube ideally position these cells for paracrine signaling with endothelial cells (EC)(Fig. 1).

Pericyte density differs with respect to the function of vessels and organs in which they are found. For example, pericytes in brain contribute to blood–brain barrier regulation, whereas in kidney they contribute to glomerular function, and in pancreas they may contribute to fibrosis. 3-5 The pericyte coverage of the abluminal endothelial surface is only partial and varies extensively (10%-40%) between capillary beds of different tissues, 6 likely reflecting specific functional features of microvessels in different organs. 7 Pericytes have an intermediate phenotype between vascular smooth muscle cells and fibroblasts with a capacity to differentiate into a myofibroblast phenotype. These varied phenotypes have translated into diverse functions, many of which are facilitated by paracrine interplay with neighboring EC and are implicated in a variety of disease processes.