Developmental patterns of rat muscle histochemistry.

A developmental progression of histochemical fiber typing is demonstrated in normal rat soleus muscle. By utilizing the acid lability of ATPase, the type II fibres are subdivided into type IIA, IIB and IIC. No IIB fibers are present in the newborn or adult rat soleus. In the 1-day-old animal 90% of the fibers can be differentiated into type I and II with all of the type II's demonstrating IIC characteristics. Only type I and IIC fibers are present until 18 days, when poorly differentiated IIA fibers first appear and gradually become the predominant type II fiber at the expense of IIC's. Further analysis documents the sequence of disappearance of myotubues from up to 80% of all fibers in the 1-day-old to less than 1% between 15 and 18 days. An analysis of the progression of fiber size according to type is presented.