Genetically engineered mice (Myf5 nLacZ/؉ , Myf5GFP-P/؉ ) allowing direct muscle satellite cell (SC) visualization indicate that, in addition to being located beneath myofiber basal laminae, SCs are strikingly close to capillaries. After GFP ؉ bone marrow transplantation, blood-borne cells occupying SC niches previously depleted by irradiation were similarly detected near vessels, thereby corroborating the anatomical stability of juxtavascular SC niches. Bromodeoxyuridine pulse-chase experiments also localize quiescent and less quiescent SCs near vessels. SCs, and to a lesser extent myonuclei, were nonrandomly associated with capillaries in humans. Significantly, they were correlated with capillarization of myofibers, regardless to their type, in normal muscle. They also varied in paradigmatic physiological and pathological situations associated with variations of capillary density, including amyopathic dermatomyositis, a unique condition in which muscle capillary loss occurs without myofiber damage, and in athletes in whom capillaries increase in number. Endothelial cell (EC) cultures specifically enhanced SC growth, through IGF-1, HGF, bFGF, PDGF-BB, and VEGF, and, accordingly, cycling SCs remained mainly juxtavascular. Conversely, differentiating myogenic cells were both proangiogenic in vitro and spatiotemporally associated with neoangiogenesis in muscular dystrophy. Thus, SCs are largely juxtavascular and reciprocally interact with ECs during differentiation to support angio-myogenesis. INTRODUCTIONMuscle tissue repair is a complex biological process that crucially involves activation of stem cells. Skeletal muscle contains two different stem cell types: 1) myogenic stem cells, so-called satellite cells (SCs), that reside beneath the basal lamina of muscle fibers (Mauro, 1961) and express both NCAM/CD56 and early myogenic cell markers such as M-cadherin, Pax7, and Myf5 (Beauchamp et al., 2000;Seale et al., 2000;Hawke and Garry, 2001; Bischoff and FranziniArmstrong, 2004) and 2) interstitial multipotent stem cells, which are extralaminal, exhibit fibroblastic morphology and do not express myogenic markers (Asakura et al., 2001;Tamaki et al., 2002). SCs are primarily quiescent in skeletal muscle, can self-renew (Collins et al., 2005) and upon activation, proliferate and further differentiate to become fusioncompetent myoblasts and ensure muscle regeneration (Hawke and Garry, 2001;Bischoff and Franzini-Armstrong, 2004). Interstitial "muscle-derived" stem cells give rise to several lineages after transplantation and in this setting, contribute to synchronized reconstitution of blood vessels (pericytes, smooth muscle cells [SMCs], and endothelial cells), peripheral nerve (Schwann cells), and muscle cells (myofibers and SCs; Tamaki et al., 2005). However, participation of multipotent interstitial stem cells in physiological muscle repair appears to be limited. Instead, it is widely accepted that sublaminal SCs represent the pre-eminent muscle stem cell type used for muscle growth, repair and regeneration (Dhawan...

Following muscle damage, fast- and slow-contracting fibers regenerate, owing to the activation of their satellite cells. In rats, crush-induced regeneration of extensor digitorum longus (EDL, a fast muscle) and soleus (a slow muscle) present different characteristics, suggesting that intrinsic differences exist among their satellite cells. An in vitro comparative study of the proliferation and differentiation capacities of satellite cells isolated from these muscles is presented there. We observed several differences between soleus and EDL satellite cell cultures plated at high density on gelatin-coated dishes. Soleus satellite cells proliferated more actively and fused into myotubes less efficiently than EDL cells. The rate of muscular creatine kinase enzyme appeared slightly lower in soleus than in EDL cultures at day 11 after plating, when many myotubes were formed, although the levels of muscular creatine kinase mRNA were similar in both cultures. In addition, soleus cultures expressed higher levels of MyoD and myogenin mRNA and of MyoD protein than EDL satellite cell cultures at day 12. A clonal analysis was also carried out on both cell populations in order to determine if distinct lineage features could be detected among satellite cells derived from EDL and soleus muscles. When plated on gelatin at clonal density, cells from both muscles yielded clones within 2 weeks, which stemmed from 3-15 mitotic cycles and were classified into three classes according to their sizes. Myotubes resulting from spontaneous fusion of cells from the progeny of one single cell were seen regardless of the clone size in the standard culture medium we used. The proportion of clones showing myotubes in each class depended on the muscle origin of the cells and was greater in EDL- than in soleus-cell cultures. In addition, soleus cells were shown to improve their differentiation capacity upon changes in the culture condition. Indeed, the proportions of clones showing myotubes, or of cells fusing into myotubes in clones, were increased by treatments with a myotube-conditioned medium, with phorbol ester, and by growth on extra-cellular matrix components (Matrigel). These results, showing differences among satellite cells from fast and slow muscles, might be of importance to muscle repair after trauma and in pathological situations.