Embryonic stem (ES) cells have taken centre stage in the scientific debate on potentially useful progenitor cell populations for post-infarct cardiac repair. Whereas effective cardiac specification of adult bone marrow-derived progenitor cells remains a matter of intense debate, much of the appeal of pluripotent ES cells stems from their intrinsic capacity to transdifferentiate into endoderm-derived cardiomyocytes1^–3 and the characterisation of well-defined protocols for in vitro expansion (scalable for potential human application).4 The successful generation of expandable, fully differentiated force-generating cardiomyocytes, capable of excitation–contraction coupling,3 renders them a particularly attractive cell type for post-infarction repair. However, irrespective of ethical considerations, major technical hurdles continue to concern researchers and doctors alike, including the risk of uncontrolled neoplastic transformation, potential development of teratomas, immune responses and disruptive growth within the host tissue primed with paracrine signals from ES cell-derived cytokines.8

In this issue of Heart, Leor and coworkers9 report an elaborate number of xenotransplantation experiments, in which they investigate the fate of transplanted human ES cells in normal and infarcted myocardium of immuno-compromised nude rats (see article on page 1278). The authors use two different NIH-approved human ES cell lines to investigate in a first set of experiments the integration, survival and eventual transdifferentiation of these cells …