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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.
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