The satellite cell niche and regulatory factors. (a) Fluorescence microscopic image of a mitotic satellite cell (metaphase) on a mouse muscle fiber. The satellite cell is labeled by the expression of a yellow fluorescent protein and DNA is stained in blue. (b) Schematic of the different environmental cues influencing a satellite cell in its niche. FGF, fibroblast growth factor, HGF, hepatocyte growth factor, IGF, insulin-like growth factor, MGF, mechano-growth factor, NO, nitric oxide, TGF, transforming growth factor.