Putative myogenic and endothelial (myo-endothelial) cell progenitors were identified in the interstitial spaces of murine skeletal muscle by immunohistochemistry and immunoelectron microscopy using CD34 antigen. Enzymatically isolated cells were characterized by fluorescence-activated cell sorting on the basis of cell surface antigen expression, and were sorted as a CD34+ and CD45− fraction. Cells in this fraction were ∼94% positive for Sca-1, and mostly negative (<3% positive) for CD14, 31, 49, 144, c-kit, and FLK-1. The CD34+/45− cells formed colonies in clonal cell cultures and colony-forming units displayed the potential to differentiate into adipocytes, endothelial, and myogenic cells. The CD34+/45− cells fully differentiated into vascular endothelial cells and skeletal muscle fibers in vivo after transplantation. Immediately after sorting, CD34+/45− cells expressed only c-met mRNA, and did not express any other myogenic cell-related markers such as MyoD, myf-5, myf-6, myogenin, M-cadherin, Pax-3, and Pax-7. However, after 3 d of culture, these cells expressed mRNA for all myogenic markers. CD34+/45− cells were distinct from satellite cells, as they expressed Bcrp1/ABCG2 gene mRNA (Zhou et al., 2001). These findings suggest that myo-endothelial progenitors reside in the interstitial spaces of mammalian skeletal muscles, and that they can potentially contribute to postnatal skeletal muscle growth.

Background-Recent studies have shown that skeletal muscle-derived stem cells (MDSCs) can give rise to several cell lineages after transplantation. However, the potential therapeutic uses of MDSCs, the functional significance of the transplanted tissue, and vasculogenesis, myogenesis, and reconstitution of other tissues have yet to be investigated in detail. In addition, the relationship between MDSCs and mesenchymal bone marrow cells is of interest. Methods and Results-We developed a severe-damage model of mouse tibialis anterior muscle with a large deficit of nerve fibers, muscle fibers, and blood vessels. We investigated the potential therapeutic use of freshly isolated CD34 ϩ /45 Ϫ (Sk-34) cells. Results showed that, after transplantation, implanted cells give rise to myogenic, vascular (pericytes, vascular smooth muscle cells, and endothelial cells), and neural (Schwann) cells, as well as contributing to the synchronized reconstitution of blood vessels, muscle fibers, and peripheral nerves, with significant recovery of both mass and contractile function after transplantation. Investigation of Sk-34 cell transplantation to the renal capsule (nonmuscle tissue) and fluorescence in situ hybridization analysis for the transplanted muscle detecting the Y chromosome revealed the intrinsic plasticity of the Sk-34 cell population. In addition, there were no donor-derived Sk-34 cells in the muscle of lethally irradiated bone marrow-transplanted animals, indicating that the Sk-34 cells were not derived from bone marrow. Conclusions-These

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In this communication, we demonstrate the sequential expression of endogenous molecules, including immediate early genes (IEGs), cytokines, neurotrophins, and neurotrophin receptors in the injured spinal cord. In the acute phase, expression of IEGs and cytokines mRNAs were rapidly upregulated within 1 h in nonneuronal cells in the lesioned sites and the surrounding spinal white and gray matter. Maximal expression was observed at 1 h for c-fos and TNF-alpha mRNAs, at 3 h for c-jun and IL-6 mRNAs, and at 6 h for IL-1 beta mRNA, and these signals were virtually nondetectable after 6-12 h from the onset of the injury. Some of these genes products may promote the degeneration of damaged cells and tissues, while others may be involved in the subsequent repair processes. In the subacute phase, expression of NGF, BDNF, NT-3, p75LNGFR and Trk B mRNAs began to increase in the nonneuronal cells and neuronal cells from 6 h, and peaked at 24-72 h in the area where expression of mRNAs for IEGs and cytokines overlapped. Signals for IL-6 mRNA were also observed in motoneurons at 24-72 h after the injury, with the suggestion that these molecules may be involved in promoting axonal sprouting in the injured spinal cord. Of further interest was the finding that this upregulation of IL-1 beta, BDNF, and NT-3 mRNAs in injured spinal cord was attenuated by treatment with high dose glucocorticoids, with the suggestion that the downregulation of BDNF and NT-3 might be disadvantageous to survival and axonal sprouting of spinal neurons.

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