Effect of neonatal neurectomy on differentiation of fiber types in rat skeletal muscle

Abstract

The differentiation of fiber types in normal and denervated muscles [gastrocnemius, extensor digitorum longus (EDL) and soleus] of the rat was compared by histochemical and electron microscopic techniques. By myosin ATPase test with acid (pH 4.35) and alkali (pH 10.4) preincubations the muscle fibers of the newborn rat reacted equally strongly and did not show selective staining characteristic of type I and type II fibers. By electron microscopy also there was no evidence of fiber type differentiation; the thickness of Z bands and amounts of sarcoplasmic reticulum and mitochondria appeared to be comparable in all fibers. Fiber types were well differentiated in control muscles at 2–3 weeks after birth, but not in the denervated muscles. In the latter ATPase test with acid preincubation showed fibers with weak and strong staining but there was no reciprocal relationship with the staining after alkali preincubation. By electron microscopy there was some difference in the amount of sarcoplasmic reticulum in EDL and soleus fibers but not in respect to Z bands and mitochondria. It was concluded that differentiation of typical fiber type characteristics did not occur following neonatal denervation.