The Myogenic Lineage: Evidence for Multiple Cellular Precursors during Avian Limb Development

Abstract

Conclusions

These studies indicate that there are no generic myoblasts that fuse with other genetic myoblasts to form generic skeletal muscle fibers which then can be molded by the central nervous system into fibers of a variety of types. Experiments on limbs developing aneurally (35, 36) or in which neuromuscular transmission was blocked by curare (3, 37) clearly demonstrated in both instances that muscle fiber diversity proceeded normally. This is most consistent with the diversity of the primary muscle fibers of the limb musculature being rooted in diversity of the myoblast population that produces the first muscle fibers. This is not to say that there is no role for the central nervous system in defining the definitive phenotype of a muscle fiber. Cross-reinnervation experiments as well as analysis of secondary fiber formation in the fetus showed that innervation can modify myosin isoform expression (3, 38, 39). Additional influences such as activity and thyroid hormone exposure can affect the phenotype of a muscle fiber as well (25, 40–43). Thus, there are both intrinsic and extrinsic components that define the definitive characteristics of all skeletal muscle fibers (14). Intrinsic components are important in early limb development when the primary fibers are first formed; where, especially in the bird, endocrine organs have not yet appeared and axonal outgrowth has not reached the dorsal and ventral muscle masses. Subsequently, all fibers formed from fetal myoblasts and satellite cells are subject to a variety of external influences that can produce a variety of fiber phenotypes.