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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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