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OBJECTIVE - Skeletal muscle-derived cells have the potential to repopulate the major peripheral blood lineages of lethally irradiated mice and thus behave like hematopoietic stem cells (HSC). We have recently shown that muscle cells with HSC activity (ms-HSC) express CD45 and Sca-1, suggesting a hematopoietic origin. Here we sought to clarify contradictions in the literature regarding the phenotype of ms-HSC and precisely define the hematopoietic origin of these cells.
METHODS - Skeletal muscle-derived cells fractionated based on the expression of CD45 and c-kit and efflux of Hoechst 33342 and were examined for HSC activity in vivo. WBM HSC expressing beta-galactosidase were transplanted into lethally irradiated recipients, whose ms-HSC compartment was later analyzed for beta-galactosidase activity to determine if ms-HSC were derived from WBM HSC.
RESULTS - Muscle-derived HSC fall exclusively in the c-kit(dim)CD45(pos) compartment of the muscle side population (msSP). Furthermore, the CD45(pos) msSP compartment of skeletal muscle is derived from WBM HSC. CD45(pos)c-kit(dim) msSP are about 22-fold less potent in HSC activity than WBM HSC cells in competitive repopulation assays and express low levels of c-kit relative to WBM HSC.
CONCLUSIONS - In our transplantation experiments, WBM HSC gave rise to ms-HSC, suggesting that WBM HSC and ms-HSC likely represent the same stem cell population in distinct environments. However, these two related populations are both functionally distinct in their ability to repopulate the peripheral blood of irradiated mice and phenotypically distinct.