The micrograph (microscope photo) shows mouse myotubes (developing muscle fibers) engineered to produce GFP so they can be seen with a fluorescent microscope. These myotubes were grown in a Petri dish and exposed to a hormone that stimulates development.
Source: Michael R. Morissette, Stuart A. Cook, Cattleya Buranasombati, Michael A. Rosenberg, Anthony Rosenzweig “Myostatin inhibits IGF-I-induced myotube hypertrophy through Akt.” American Journal of Physiology—Cell Physiology. Published 1 November 2009, Vol. 297, no. 1124-1132. DOI: 10.1152/ajpcell.00043.2009
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