Current investigations pertaining to the treatment of major vascular complications are based on progenitor cells. Most of the vascular progenitor cells have the potential to differentiate themselves into their respective mature cells and contribute to vascular remodeling and repair. Endothelial Progenitor Cells (EPCs), the recently discovered bone-marrow derived circulating progenitor cells, show high potential for post-natal vasculogenesis (neo-vascularization) in adults. They constitute a circulating pool of cells able to form a cellular patch at sites of endothelial injury, thus contributing directly to the homeostasis and repair of the endothelial layer (re-endothelialization). They also differentiate into mature endothelial cells with high functional ability to incorporate into neo-vessels.

Current research in the field of vascular biology investigating therapy for treating various vascular diseases (CVD, CAD, PVD) has now explored the potential of progenitor cells. Vascular progenitor cells are a subset of primitive bone marrow-derived cells that have the capacity to proliferate, migrate and differentiate into various mature cell types (Iwata and Sata, 2007; and Szmitko et al., 2003). These cells possess the ability to restore vascular function and are capable of homing and differentiating into mature endothelial or smooth muscle like cells, contributing to vascular remodeling and repair. Needless to say, discovery of vascular progenitor cells (EPC, SMPC, MPC) has revolutionized the use of cells as medicines. Therapy based on such progenitor cells is studied under a new emerging branch termed as `Progenitor Biology' (Metharom and Caplice, 2007).

Among the vascular progenitor cells, endothelial progenitors contribute to restore vascular homeostasis, and smooth muscle progenitors contribute to pathological changes (Jevon et al., 2008). Notably, concepts of vascular regeneration and repair were previously considered limited to the differentiated cells within vascular tissue (Caplice and Doyle, 2005). However, the recent discovery of EPCs with high potential for post-natal vasculogenesis and vascular homeostasis in adults has revolutionized therapeutic neo-vascularization. The formation of collateral blood vessel is a regulated process involving the proliferation, migration and remodeling of endothelial cells from pre-existing vessel (angiogenesis) or from differentiation of endothelial precursor cells (neo-vascularization) (Faulkes and Peters, 2005). In analogy with vessel development in the embryo, the de novo formation of new blood vessels from EPCs has been termed as vasculogenesis. The coronary collateral vessels formation constitutes a compensatory bypass adaptation to ischemia. Migration and homing of EPCs to ischemic regions lead to de novo formation of vascular structures (Leri and Kajstura, 2005). This vascular response increases blood supply to the myocardium, reducing the area of hypoxic damage and ultimately the infarct size. The recruitment of collateral vessels within the coronary circulation is favored by multiple episodes of ischemia, which provides the stimuli necessary for the growth of vascular structures and increased blood flow. Needless to say this potential of EPCs is being employed to promote therapeutic neo-vascularization within the ischemic tissues (Szmitko et al., 2003).

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