Growth factors controlling muscle development

The enlarged muscles of certain breeds of cattle, such as the Belgian Blue, have been shown to result from a marked increase in the number of normal s ized muscle fibers. Originally insulin-like growth factors (IGFs) were impl icated in this myofiber hyperplasia, as IGFs have been shown to stimulate m yoblast proliferation as well as maintain fiber differentiation. Recently i t has been reported that mice lacking a myostatin gene, a member of the TGF beta superfamily, have enhanced skeletal mass resulting from increased mus cle fiber number and size. Mutations in this gene have been found in double -muscled cattle, indicating that myostatin is an inhibitor of muscle growth . Myostatin is expressed early in gestation and then maintained to adulthoo d in certain muscles. Myostatin expression in bovine muscle is highest duri ng gestation when muscle fibers are forming and some of the myogenic regula tory factors have elevated expression over the same period as myostatin. Mo lecular expression of the IGF axis does not differ between Belgian Blue and normal muscled cattle, and IGF-II mRNA is increased throughout formation o f secondary fibers in both breeds. However, myostatin and MyoD expression i n muscle differ between normal and hypertrophied muscle cattle breeds. This evidence strongly suggests that lack of myostatin is associated with an in crease in fiber number which then results in a marked increase in potential muscle mass in double-muscled cattle. (C) 1999 Elsevier Science Inc. All r ights reserved.