Résumé :
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Communication n° 666. Obtaining an effective regeneration of adult skeletal muscle constitutes a challenge, in part because of a poor understanding of the nature and origin of skeletal progenitor cells. Several groups have examined the possibility of using bone marrow (BM) or hematopoietic stem cells (HSC) as a source of cells capable to engraft and to regenerate skeletal muscle in mice. Hematopoietic cells migrate to skeletal muscle where they occasionally fuse with the myofibers in response to inflammatory signals. Whether or not BM grafts produce functional myogenic precursor cells is unknown. To address this question in murine models, we first developed a method to identify myogenic progenitor cells in skeletal muscle. Second, we tested if BM or HSC displayed myogenic activity or gave rise in vivo, to myogenic cells. Using flow cytometry we defined that CD45- Sca1- CD34+ mono-nuclear cells of hind limb muscles were highly enriched in myogenic activity. This was defined by in vitro differentiation criteria including fusion indices and frequency of myotube-forming cells. In contrast, we failed to detect myogenic activity from freshly-isolated CD45+ cell fractions of muscle or BM cells including BM CD45+ stem cell-enriched fractions. We transplanted lethally irradiated C57Bl6 mice or mdxCV4 mice with total BM or with HSC obtained from GFP transgenic mice. BM-derived or HSC-derived cells predominantly regenerated the hematopoietic compartment of muscle as determined by co-expression of GFP and CD45 markers. However, a small percentage of donor-derived cells was identified in the CD45- Sca1- muscle compartment. Yet, when isolated, these cells consistently failed to grow or to differentiate into muscle either in standard myogenic culture conditions or in co-cultures with myoblasts. We conclude that BM or HSC are not myogenic and do not contribute to a functional pool of myogenic precursor cells in muscle after transplantation. However, the prospective identification of resident muscle progenitor cells should be useful to further study the cellular and molecular mechanisms of muscle regeneration.
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