Enhanced reappearance of fast fibers in regenerating crayfish claw closer muscles

In the pristine claws of adult crayfish the muscle fibers of the closer are all of slow type as judged by sarcomere lengths of greater than 6 micron, and a uniform degree of myofibrillar ATPase activity. In regenerating claws of mature and immature crayfish, the muscle has a central band of fast type fibers as characterized by shorter sarcomeres (less than 6 micron) and a higher degree of ATPase activity than the surrounding slow fibers. During primary development, the closer muscle has a fiber composition similar to that of the regenerating muscle except for a smaller proportion of fast fibers. Thus the reappearance of fast fibers during regeneration recapitulates ontogeny while their enhanced proportions may reflect epigenetic influences such as restriction of nerve-mediated muscle activity in the limb bud.