| Cell transplantation offers the chance to repopulate myocardial infarcts with new muscle tissue. Although transplanted cardiomyocytes can integrate into normal or injured myocardium, donor cardiomyocytes are not readily available for transplantation. Embryonic stem (ES) cells present an attractive source of cells for cardiac repair. ES cells are pluripotent and capable of spontaneous differentiation into cells of all three germ lineages, including cardiomyocytes. However, spontaneous differentiation of ES cells in embryoid bodies produces only a low yield of cardiomyocytes. Here we investigated whether the heart provides an instructive environment to guide differentiation of ES cells into cardiomyocytes in vivo. Undifferentiated mouse ES cells, derived from the 129 and C57Bl/6 mouse strains were injected into the normal left ventricular myocardium of immunodeficient nude mice and immunocompetent C57Bl/6 mice. Subsequently infarcted C57Bl/6 mice received C57-ESC injection into the border zone of the infarct. At 3 weeks after cell injection, teratomas were observed in the hearts of all nude mice, independent of the ES cell type injected. Immunocytochemistry revealed cells from endoderm lineage, mesoderm lineage and ectoderm lineage. A dose response curve indicated no viable grafts when fewer than 100 000 cells were implanted whereas teratomas were consistently seen at doses of ≥250 000 cells. In all teratomas, only a very small number of ES cell-derived cardiomyocytes could be detected by sarcomeric actin staining. No difference in the number of sarcomeric actin positive cardiomyocytes could be observed in extra-cardiac (hamstring muscle) versus intracardiac locations. Allogeneic ES cells also formed viable grafts in immunocompetent hosts, but these grafts showed significant infiltration by leukocytes. All infarcted C57Bl/6 animals showed tumor formation 3 weeks post transplantation. These data suggest that undifferentiated ES cells are not guided to form new myocardium when injected into the normal and injured heart. Allogeneic and syngeneic ES cell grafts form tumors in the host, although allogeneic ES cell-derived teratomas seem to be subject to the host immune response 3 weeks post transplantation. Any cardiac therapies involving ES cell derivatives will need to avoid undifferentiated ES cells, and may need immune modulation to be successful prior to transplantation. |
