Figure 8. Summary of proposed roles of characterized SC subpopulations in myogenesis.

In this study, two distinct SC subpopulations (P40/50 and P50/70) were isolated from muscle of 4-day-old piglets. Freshly isolated cells of the P40/50 subpopulation highly expressed Myf5 and MyoG and constituted a fast-proliferating phenotype. During in vitro cultivation, the P40/50 cells show a constantly high oxidative capacity, an increased differentiation potential, and fusion rates higher than the P50/70 cells. From these results, we assume that these cells by being a source of new myonuclei are the main contributors to hypertrophic growth of existing myofibers (a). Freshly isolated P50/70 cells showed considerably slower proliferation and expressed high amounts of markers for terminal differentiation (Desmin, eMyH). This leads us to suppose that some of the P50/70 cells are involved in tertiary fiber formation occurring with the highest intensity during the first week of pig postnatal muscle development (b). During culture, at least some of the P50/70 cells pass through a reversible period of low mitochondrial activity; this is associated with the appearance of higher numbers of Pax7⁺ cells and reduction of the differentiation potential/fusion rate compared with P40/50 cells whereas the proliferation rate equalized. Based on these results, we hypothesize that a subpopulation of P50/70 is withdrawn from differentiation to form and maintain a pool of slowly cycling, more immature precursor/reserve cells (c) at existing muscle fibers (d) gradual transformation to quiescent adult SC) that also can give rise to fast-proliferating, more committed (P40/50) cells (e) to prevent their exhaustion during intensive growth.