Ischaemic myocardial damage is an increasing cause of heart failure in the western world and has long been considered irreversible because adult cardiomyocytes are terminally differentiated and do not proliferate.^w1 Reversal of heart failure would require replacement of damaged myocytes and restoration of blood flow. Over recent years attention has turned to the potential of stem cells to repair damaged hearts because of their ability to differentiate in vitro into cardiomyocytes, endothelial cells, and pericytes. The aim of this article is to outline the basic scientific principles of stem cell biology, to review the evidence for myocyte and vascular regeneration from stem cells in vitro and in vivo, and to discuss the potential for stem cell therapy to replace damaged myocardium and its blood supply in human coronary heart disease.

Stem cells are undifferentiated cells capable of self renewal, proliferation, and differentiation into multiple lineages permitting tissue regeneration.^w2 Haemopoietic stem cells (HSCs) were the first to be identified and exhibit all these properties. A number of types of stem cells are now recognised, as well as partially differentiated progenitor cells that are capable of proliferation and differentiation to multiple lineages.

Embryonic stem cells

Cells isolated from the embryonic blastocyst are capable of proliferating and differentiating into cells of all three embryological germ layers¹ and are capable of differentiating into any cell in the body. Embryonic stem (ES) cells have an inherent tendency to differentiate spontaneously in culture such that specific culture conditions are required to maintain them in their undifferentiated state (fig 1). Nevertheless, under defined conditions they will proliferate indefinitely while retaining their capacity to differentiate, thereby providing a potentially limitless source of cells. Human ES cells were first isolated in 1998 from the inner cell mass of embryos donated by couples who had undergone assisted reproduction.