Journal of Neuroscience, Vol 7, 4176-4194, Copyright © 1987 by Society for Neuroscience
Reinnervation of muscle fiber types in the newborn rat soleus
LC Soileau, L Silberstein, HM Blau and WJ Thompson
Department of Zoology, University of Texas, Austin 78712.
We have examined the selectivity of reinnervation of fiber types in rat
soleus muscle denervated by crush of the soleus nerve 2 d after birth. The
fibers innervated by single, regenerated motor axons were identified by use
of glycogen depletion approximately 2 weeks following denervation. The
types of fibers were determined by immunohistochemistry employing
anti-myosin antibodies and, in some cases, by myofibrillar ATPase staining.
Two distinct types of fibers are present in soleus at 2 d and through the
next 16 d of normal postnatal development. These fiber types are retained
in a denervated muscle for the period of time required for reinnervation.
Although 40% of the fibers are lost from the muscle during reinnervation,
we find no evidence for interconversion of muscle fiber types. Nonetheless,
10 of the 12 single motor units examined had fiber type compositions that
were markedly biased toward one or the other of these 2 types; the bias in
these units could not be explained by chance reinnervation. On the basis of
the topographical distribution of the muscle fibers in each of these units,
the motor axons reinnervated a novel set of fibers. We interpret these
findings to mean that neonatal soleus motor neurons reinnervate fiber types
in a selective manner. This selective innervation may explain the bias in
the fiber type composition of normal motor units during early postnatal
development.