Vascular disease: A new progenitor biology

Vascular disease is primarily the result of atherosclerosis which affects all layers of the adult vessel wall. Our understanding of atherosclerosis has evolved over the past three decades; from initial hypotheses based on lipid deposition and fibrocellular proliferation within the intima of the vessel wall to a more complex interplay between conventional risk factors, inflammation and the immune system implicating pan-vascular biologic processes. More recently circulating progenitor cells have been shown to possess diverse differentiation capacity within the remodelling vessel wall. Current investigation of atherosclerosis therefore encompasses an expanding field of biological science; from molecular genetics, classical vascular biology, and immunology to stem cell biology and vasculogenesis. However, a decade after their initial description, scientists still know little about the proximate relationship between vascular progenitor cells and atherosclerosis progression or stability. In recent years, the discovery of progenitor cells of myeloid origin has offered the exciting prospect of merging classical concepts of myeloid cell biology in atherosclerosis with evolving concepts of myeloid cell plasticity and endothelial/ smooth muscle cells differentiation within the injured vessel wall. In this context, early stage atherosclerosis associated with vascular injury may involve neovascularisation and re-endothelialisation in which a significant contribution comes from bone marrow-derived vascular progenitor cells. For this review, emphasis will be on endothelial progenitor cells (EPC), smooth muscle progenitor cells (SPC) and putative myeloid precursors.