Several cell populations with the properties of mesenchymal stem cells have been previously isolated from the skeletal muscle. One of the best characterized populations are the muscle derived stem cells (MDSCs) harvested from murine skeletal muscle [54
]. MDSCs are isolated from the muscle homogenate using a pre-plating technique, which enriches the population of MDSCs by eliminating the contaminating populations of more adherent cell types [56
]. The more slowly adherent MDSCs have demonstrated enhanced differentiation potential, and can be readily induced to become osteoblasts, adipocytes and chondrocytes in vitro
. An additional feature of this cell type is their ability to differentiate into myoblasts in vitro
and to promote muscle regeneration in vivo
Interestingly, this population of MDSCs has not been harvested from human muscle tissue solely on the basis of their adhesion characteristics. Instead, a population of cells with similar in vitro
characteristics has been identified in human skeletal muscle using FACS to isolate the cells that are positive for CD34, CD56 and CD144 [52
]. These cells, called myoendothelial cells, express surface markers of both endothelial (CD34 and CD144) and satellite cells (CD56), which are skeletal muscle stem cells. Typically, satellite cells are quiescent myoblast precursors that lie adjacent to the myofibers beneath the basal lamina, and divide asymmetrically in response to muscle injury [59
]. Although most satellite cells are committed to the myogenic lineage, the small subset of satellite cells that co-express the endothelial makers (less than 0.5% of all satellite cells) are associated with the vasculature in vivo
and capable of osteogenic, adipogenic and osteogenic differentiation [52
]. Myoendothelial cells appear to have enhanced myogenic potential in vivo
compared with satellite cells. There is also evidence that these cells can be induced to differentiate into endothelial cells under the appropriate conditions. However, these are not the only multipotent stem cell type in skeletal muscle with an anatomical affiliation to the vasculature.
Pericytes are cells intimately associated with capillaries and microvessels. They play several roles in the maintenance of the vasculature [60
], and recent experiments suggest that these cells may have other important functions for tissue regeneration. Isolated pericytes are capable of regenerating muscle tissue in vivo
], and they can differentiate into myocytes, osteoblasts, adipocytes and chondrocytes in vitro
]. Given that pericytes are present in almost every tissue in the body, it has been suggested that MSCs harvested from various tissues were in fact pericytes, or a similar cell type, which originated in the vasculature of those tissues [64
]. Recently, there has been compelling evidence to support this hypothesis by the demonstration that a sub-population of pericytes express the markers used to identify MSCs and exhibit the in vitro
differentiation characteristics of MSC populations [65
]. Based on these observations, it has been proposed that pericytes serve as a reservoir of multipotent cells that can be recruited from the vasculature as needed to repair the tissue in response to injury.
Another distinct population of multipotent cells has been harvested from skeletal muscle following traumatic injury [53
]. One important distinction that has been made about the traumatized-muscle-derived progenitor cells is that they are rapidly adherent during the harvesting procedure, as opposed to the MDSCs, which are selected on the basis of their slow adherence. The MPCs are also present in substantial numbers at the time of harvest. Multipotent cells in uninjured muscle are rare, whereas approximately one million cells per gram of tissue can be isolated from injured muscle [66
]. The debrided muscle tissue from which MPCs are harvested was in the process of wound healing and tissue remodeling in response to a traumatic injury [67
]. These observations support the hypothesis that multipotent cells are recruited from their niche following injury, and that they proliferate in the tissue to fulfill their regenerative function [64
]. As a result, the cells harvested from traumatized muscle are referred to as mesenchymal progenitor cells (MPCs) to indicate that these cells may not have been in a quiescent, stem cell state when they were harvested (). However, except that they are available in greater numbers in the tissue, there do not appear to be any major differences between the traumatized-muscle-derived MPCs and a typical MSC population [53
Origin and characteristics of muscle-derived MSCs
Traumatized muscle-derived MPCs also exhibit several of the trophic properties that are associated with MSCs. In vitro
studies have shown that MPCs can modulate local inflammatory responses [69
], promote angiogenesis by increasing the rate of endothelial cell proliferation and inhibit apoptosis of nearby cells [70
]. The MPCs also appear to be intrinsic to severely injured muscle that has compromised tissue architecture [67
]. These observations suggest that MPCs play a role in promoting the functional regeneration of skeletal muscle following traumatic injury. By providing a biochemical environment that favors tissue growth, the MPCs could augment the ability of committed myogenic progenitor cells to remodel the tissue and generate new skeletal muscle.
The origin of these proliferating cells is not currently known, although it is possible that the population of pericytes serves as a cellular reservoir for MPCs as well as the myoendothelial and satellite cell types. A model for pericyte recruitment can be constructed in which the local tissue architecture determines the pericyte fate (). In response to minor, routine muscle damage, pericytes may be recruited from their vascular niche and travel along the architecture of the skeletal muscle to the site of muscle damage where they can augment satellite cell regeneration of the myofibers and may differentiate into satellite cells after the myofiber has been repaired [52
]. Alternatively, following severe injury, the architecture of the skeletal muscle may be disrupted such that the recruited pericytes have no orientation in the tissue. In this case, they are free to proliferate in the cellular milieu, which includes inflammatory leukocytes and other wound healing cell types (e.g., fibroblasts and myofibroblasts), and assume the MPC phenotype. According to this model, skeletal muscle contains several stem cell types, which can respond to the severity of injury and repair minor muscle damage or remain as a more plastic stem cell type to promote regeneration following major tissue damage. The distinctions made between the MDSCs, myoendothelial cells, pericytes and traumatized-muscled-derived MPCs are based on their in vitro
characteristics and species of origin, which are summarized in .
Functional characteristics of pericytes
Mesenchymal stem and progenitor cells identified in skeletal muscle tissue.