Hyperplasia is a distinctive component of the hypertrophy that occurs in the overloaded and stretched avian wing; a large number of embryonic myosin positive fibers appear in intra- and interfascicular locations. The number of fibers formed decreases as a function of increasing age. This proposal examines two potential mechanisms of nascent fiber formation: 1) from activation, proliferation and fusion of satellite cells, and 2) from existing fibers through a process referred to as "fiber splitting", where a fiber splits along its longitudinal axis, producing first a branched fiber, and ultimately two independent fibers. Mature and immature chickens will be infused continuously with bromodeoxyuridine, a thymidine analogue and marker of cell division, during the overload period in order to distinguish whether one or both processes are involved in nascent fiber formation. Virtually all myonuclei of nascent fibers formed through proliferation and fusion of satellite cells will be labelled, while nascent fibers formed from pre-existing fibers will contain few labelled nuclei. Satellite cells involved in formation of new fibers in interfascicular spaces would be required to migrate from their fibers into these areas. The location of activated and proliferating satellite cells will be examined during overload in order to determine if satellite cells located on fibers at the periphery of fascicles are preferentially activated. Finally, Dr. Schultz will test directly if environments in overloaded muscles promote interstitial fiber formation by implanting myoblasts into control and overloaded muscles and comparing their fusion behavior with the electron microscope. These studies should provide fundamental additional information to our understanding of the plasticity of skeletal muscle during a response to increased functional demand.
