Morphometric analysis of rat extensor digitorum longus and soleus muscles.

The ultrastructure of extensor digitorum longus (EDL) and soleus muscles from two groups of Wistar rats obtained from different colonies has been examined both qualitatively and morphometrically. Compared to soleus, EDL fibres are richer in sarcoplasmic reticulum, but contain fewer mitochondria, lipid droplets and nuclei. The quantities of these organelles and related surface volume ratios in single morphometric samples varied, reflecting the heterogeneity of fibre ultrastructure within the muscles. There were significant differences in many features quantified between the two groups of rats; these differrences present a problem in comparing different published reports but, fortunately, the EDL/soleus ratios were similar in both groups. Within each group, the fibre fractions occupied by mitochondria and sarcoplasmic reticulum found in the morphometric samples were reciprocally related. The different amounts of sarcoplasmic reticulum in the two muscles is insufficient to explain the different calcium pumping capacities observed by Briggs, Poland and Solaro (1977), suggesting a control at the level of the pump protein synthesis.