Plasticity and physiological role of stem cells derived from skeletal muscle interstitium: contribution to muscle fiber hyperplasia and therapeutic use.

Stem cells other than satellite cells that can give rise to primary myoblasts, which are able to form additional new fibers postnatally, are present in the interstitial spaces of skeletal muscle. These cells are sorted into CD34(+)/45(-) (Sk-34) and CD34(-)/45(-) (Sk-DN) cell fractions, and they are wholly (>99%) negative for Pax7 at initial isolation. Colony-forming units of these cells typically include non-adherent type myogenic cells, while satellite cells are known to be adherent in cell culture. In addition, both Pax7(-) and Pax7(+) cells are produced, depending on asymmetric cell division. A large number of myotubes are also formed in each colony, thus suggesting that putative Pax7(+) satellite cells also present in each colony. Interestingly, interstitial myogenic cells show basal lamina formation at early stages of myogenesis in response to various types of stimulation in compensatory enlarged muscle, a property that satellite cells do not possess in the parent fiber basal lamina cylinder. Basal lamina formation and production of satellite cells are essential before muscle fiber establishment in vivo. It is therefore likely that myogenic cells in skeletal muscle can be divided into two populations: 1) basal lamina-producing myogenic cells; and 2) basal lamina-non-producing myogenic cells. The latter population may be Pax7(+) satellite cells showing adherent capacity in cell culture, while the lamina-producing myogenic population derived from interstitial multipotent stem cells, which is predominant among Sk-34 and Sk-DN cells, plays a role in primary myoblast generation and shows non-adherent behavior in culture. Therefore, the physiological role of interstitial myogenic cells is as a source for new postnatal muscle fiber formation, and multinucleated muscle fibers (cells) are potentially formed clonally.