Vascular wall-resident stem cells.

New vessels in the adult have been considered to be formed not only by angiogenesis, but also by postnatal vasculogenesis via endothelial progenitor cells (EPCs). However, it is still a matter of debate as to what extent the EPCs contribute to new vessel formation in the adult. While the role of the circulating and bone marrow-derived EPCs has intensively been studied, the contribution of the vascular wall itself was neglected for a long time. Evidence published in the last few years strongly suggests the existence of different stem and progenitor cell types in the vascular wall. Particularly, the presence of EPCs and smooth muscle progenitor cells (SMPCs) in distinct zones of the vascular wall supports the hypothesis that not only BM- or C-EPCs, but also vascular wall-resident stem cells (VW-SCs) might contribute to new vessel formation and vascular wall morphogenesis. However, the differentiation potential of the VW-SCs, e.g. whether a VW-SC is able to give rise to a complete hierarchy of vascular progenitors still remains to be studied. This review will provide a survey about the VW-SCs and their potential impact in vascular biology.