Fig. 5. Model of primary fiber-type patterning in avian hypaxial muscles. An intrinsic commitment to either a fast or a slow fiber-type lineage occurs in myogenic precursors while still within the somite. Myogenic precursors migrate into the limb stroma where extrinsic signals operate on fiber-type-committed myoblasts. One mechanism to establish the size and fiber-type pattern of individual anatomic muscles is through expansion of the appropriate myoblast population. In the case of the pectoralis muscle shown here, myoblasts committed to form fast fibers are preferentially amplified in the chicken, while in the quail, both fast- and slow-fiber-committed myoblast populations are expanded.