Haematopoietic stem cells participate in muscle regeneration

It has previously been shown that bone marrow cells contribute to skeletal muscle regeneration, but the nature of marrow cell(s) involved in this process is unknown. We used an immunocompetent and an immunocompromised model of bone marrow transplantation to characterize the type of marrow cells participating regenerating skeletal muscle fibres. Animals were transplanted with different populations of marrow cells from Green Fluorescent Protein (GFP) transgenic mice and the presence of GFP(+) muscle fibres were evaluated in the cardiotoxin-injured tibialis anterior muscles. GFP(+) muscle fibres were found mostly in animals that received either CD45(-), lineage(-), c-Kit(+), Sca-1(+) or Flk-2(+) populations of marrow cells, suggesting that haematopoietic stem cells (HSC) rather than mesenchymal cells or more differentiated haematopoietic cells are responsible for the formation of GFP(+) muscle fibres. Mac-1 positive population of marrow cells was also associated with the emergence of GFP(+) skeletal muscle fibres. However, most of this activity was limited to either Mac-1(+) Sca(+) or Mac-1(+)c-Kit(+) cells with long-term haematopoietic repopulation capabilities, indicating a stem cell phenotype for these cells. Experiments in the immunocompromised transplant model showed that participation of HSC in the skeletal muscle fibre formation could occur without haematopoietic chimerism.