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First published online September 3, 2008
doi: 10.1242/10.1242/jcs.019661

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All muscle satellite cells are equal, but are some more equal than others?

King's College London, Randall Division of Cell and Molecular Biophysics, New Hunt's House, Guy's Campus, London SE1 1UL, UK

View larger version (28K): [in this window] [in a new window]   Fig. 1. Possible origins of satellite cells. Self-renewing somitic stem cells can be identified by the expression of Pax3 and Pax7, but lack expression of myogenic regulatory factors (MRFs), such as Myf5 and MyoD. These stem cells generate embryonic and foetal myoblasts, which proliferate before differentiating and fusing to produce myotubes. Somitic stem cells may become satellite cells directly if they are partitioned beneath the basal lamina as it forms. The satellite-cell niche might also be transiently occupied by foetal myoblasts, which differentiate to contribute myonuclei for the initial phases of post-natal growth. Such foetal myoblasts might also be specified by the niche to become satellite cells. Satellite cells can self-renew, and produce satellite-cell-derived myoblasts that proliferate and differentiate to contribute myonuclei to growing muscle.

View larger version (80K): [in this window] [in a new window]   Fig. 2. Two possible modes of satellite-cell self-renewal. (A) Quiescent Pax7-positive satellite cells (green nucleus) co-express MyoD (green and red nucleus) when they are activated following muscle damage. After proliferating as Pax7-positive and MyoD-positive satellite-cell-derived myoblasts, most cells downregulate Pax7, maintain MyoD expression (red) and differentiate to replace lost myonuclei (black). In other satellite-cell progeny, however, MyoD expression is lost and Pax7 expression is maintained (green nucleus) as the cell self-renews. (B) There are two types of Pax7-positive satellite cell, those that have expressed Myf5 (green and yellow nucleus) and those that have not (green nucleus). The Pax7-positive and Myf5-positive satellite cells (green and yellow nucleus) are essentially transit-amplifying cells that are able to give rise to Pax7-positive and MyoD-positive myoblasts (green and red nucleus) that undergo limited proliferation before downregulating Pax7 expression (red nucleus) and differentiating. By contrast, the Pax7-positive, Myf5-negative satellite `stem' cell (green nucleus) can divide asymmetrically, giving rise both to more satellite-stem-cell progeny for the maintenance of the stem-cell pool, and to Pax7-positive and Myf5-positive satellite cells (green and yellow nucleus) to replace myonuclei.

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