The role of stem cells in vein graft remodelling

The vessel wall is a dynamic tissue that undergoes positive remodelling in response to altered mechanical stress. A typical example is vein graft remodelling, because veins do not develop arteriosclerosis until a vein segment is grafted on to arteries. In this process, it was observed that vascular endothelial and smooth muscle cells of vein grafts die due to suddenly elevated blood pressure. This cell death is followed by endothelial regeneration. Central to this theme is the essential role played by EPCs (endothelial progenitor cells) in regenerating the lost endothelium. The mechanisms by which EPCs attach to the vessel wall and differentiate into mature endothelial cells involve increased chemokine production and laminar shear flow stimulation on the vessel wall. It seems that neo-endothelial cells derived from EPCs lack mature cell functions and express high levels of adhesion molecules resulting in LDL (low-density lipoprotein) penetration and mononuclear cell infiltration into the sub-endothelial space. Among infiltrated mononuclear cells, there are smooth muscle progenitors that proliferate and differentiate into smooth muscle cells. Meanwhile, stem cells present in the media and adventitia may also migrate into arteriosclerotic lesions via the vasa vasorum that are abundant in the diseased vessels. However, the molecular events leading to the homing, differentiation and maturation of stem/progenitor cells still needs elucidation. The present review attempts to update the progress in stem cell research related to the pathogenesis of vein graft arteriosclerosis or remodelling, focusing on the mechanisms by which stem/progenitor cells participate in the development of lesions, and to discuss the controversial issues and the future perspectives surrounding this research area.