Intrinsic Regulation of Skeletal Muscle Fiber Type Specification in Embryonic Myoblast Cell Lineages

Muscle fibers are multinucleated cells that are the smallest functional unit of a skeletal muscle. A single muscle generally contains a heterogeneous population of muscle fibers. The varied, multiple functions of a muscle are a direct reflection of the patterning of fibers within it. For instance, quick, precise movements of the eye require a muscle composed of mainly fast fibers, while muscles of the back are mixed with slow and fast fibers to allow for slow, sustained, postural movements in addition to fast, directed movements. Fibers can differ by expression of myosin heavy chain (MyHC) isoforms, morphology, contractile properties, and metabolic profile. The MyHC isoform composition of a muscle fiber determines the fiber's rate of contraction and is also the major factor determining fiber type. Avian muscle has two distinct fiber types, fast and fast/slow. The slow MyHC2 gene is the predominant adult slow isoform and it governs fiber type in the adult chicken.