Effect of denervation at birth on the development of skeletal muscle cell types in the rat.

In the newborn rat all cells of soleus, extensor digitorum longus (EDL), and tibialis anterior (TA) muscles stained for fast troponin I. A proportion of the cells, that was much higher in the soleus, also stained for slow troponin I. Fast and slow troponin I were segregated in different cell types in all three muscles 10 to 12 days after birth. No subsequent changes in the distribution of the two forms of troponin I occurred with further growth of EDL and TA muscles. The number of type I cells in soleus steadily increased with increasing age to 24 weeks. Three weeks after denervation at birth, almost all cells in soleus muscle stained for fast troponin I but less than 5% stained significantly dark for slow troponin I. All cells stained for myosin ATPase after alkaline preincubation, but very few after acid preincubation. Three weeks after denervation of EDL and to a lesser extent with TA muscle, fast and slow troponin I were still segregated in different cells. After alkaline preincubation all cells stained equally dark for myosin ATPase but only those positive for slow troponin I stained for myosin ATPase after acid preincubation.