Myostatin (formerly known as growth differentiation factor 8)'s a growth factor that limits muscle tissue growth, for example higher concentrations of myostatin in the body may cause the individual to have less developed muscles. The myostatin protein's produced primarily in skeletal muscle cells, circulates in the blood and lymph and acts on muscle tissue, apparently by slowing down the development of muscle stem cells. The precise mechanism remains unknown. Myostatin has been found in humans… (More on Myostatin)