The Behavior of Four Fiber Types in Developing and Reinnervated Muscle

The behavior of type I, type IIA, IIB, and IIC muscle fibers was examined in the newborn rat and in successively older animals. In addition, cross reinnervation experiments were carried out in the adult rat. The nerve to the anterior tibial group of muscles was used to cross reinnervate the denervated soleus and gastrocnemius. In neonatal rats, the first differentiation of muscle fiber types is into type I and type IIC fibers. Later differentiation into type IIB and then IIA fibers is associated with a diminution of type IIC fibers. This suggests the possibility that the type IIC fibers may be a relatively primitive form of type II fiber which is capable of differentiation into type IIA or B. Homogeneous groups of type I, IIA, and IIB fibers were produced by cross reinnervation. This suggests that these fiber types are independently innervated. Type IIC fibers were never seen in a homogeneous group but were found with other fiber types and perhaps represented transitional fibers. There was also a lack of correlation between the adenosine triphosphatase reactions and the oxidative enzyme reactions in reinnervated muscle.