Fig. (2).

Molecular markers and mechanisms underlining satellite cell behavior. (A) Quiescent satellite cells are characterized by the presence of facultative heterochromatin and thus by a minimal transcriptional activity, being in a dormant state in steady-state muscles. (B) Upon proper stimulation, satellite cells become activated, express MyoD and re-engage the cell cycle. Activated cells can undergo both symmetric and asymmetric division replenishing the stem cell pool and generating committed myoblasts. Daughter cells expressing elevated levels of Pax7 and low levels of MyoD (Pax7^high/MyoD^low) maintain the stem-like phenotype, whilst cells highly inducing MyoD and down-modulating Pax7 (Pax7^low/MyoD^high) enter the differentiative program. (C) Differentiating cells express later molecular mediators of the myogenic program as myogenin and Mrf4. Myoblasts can fuse to pre-existing myofibers and express markers of the terminally differentiated phenotype as Myosin heavy chain, muscle creatine kinase and beta enolase.