A Brief Study of Within Litter and Within Strain Variation in Skeletal Muscle Fiber Number in Three Lines of Laboratory Rodents

The purpose of this study was to describe the soleus muscle fiber number variation within litter and within strain of three lines of laboratory rodents in an effort to determine the value of these lines in muscle fiber number research. The three lines of laboratory rodents used for the study were inbred mice (18 litters, 113 animals), outbred mice (8 litters, 97 animals), and inbred rats (14 litters, 126 animals). The animals were sacrificed at approximately 7 weeks of age and the soleus muscle from the right leg was removed for fiber number determination. Fiber number was determined by the nitric acid digestion method. The mean percent difference between the animals with the highest and lowest fiber number within litters was 14.4%, 13.0%, and 18.3% for inbred mice, outbred mice, and inbred rats, respectively. This was substantially less than the 38.2%, 27.8%, and 49.1% differences, respectively, that existed within strain. The results of this study indicate that fiber number variation within litter and within strain in the three lines of laboratory rodents used in this study would make comparison of fiber number between animals difficult to interpret.