Fig. 8. Effects of increased (IMA) and decreased muscle activity (DMA) on muscle growth and differentiation. Two main fibre types exist: fast and slow fibres. Each type expresses a set of specific genes (e.g. fast or slow myosins and troponins), with some overlap (e.g. titin). IMA promotes muscle growth and promotes a fast-to-slow transition of fibre types, whereas DMA inhibits muscle growth and promotes a slow-to-fast transition. Muscle growth is promoted by IMA, partly by increased insulin-like growth factor (IGF) and myogenin signalling, and is inhibited by DMA, partly by growth and differentiation factor 8 (gdf8) signalling. Gdf8 signalling acts a.o. by downregulation of myogenin and IGF expression. Overcrowding stress inhibits gdf8 expression in zebrafish, even though it also represses muscle growth. Open arrows indicate the fast-to-slow and slow-to-fast fibre type transitions. Filled arrows indicate a positive influence, and T-shaped arrows indicate a negative influence on the process or tissue it points at. The asterisk denotes that high relative myogenin expression promotes a shift towards a slow muscle phenotype. SDHa, succinate dehydrogenase a; NADHd, diaphorase; pfk-m, phosphofructokinase in muscle.