Fiber number, area, and composition of mouse soleus muscle following enlargement.

Muscle fiber number, cross-sectional area, and composition were studied in response to enlargement produced by synergistic ablation in the mouse soleus muscle. The effect of the location of a histological section on the number of fibers that appear in the section was also studied using the mouse soleus muscle. Enlargement was produced in the soleus muscle of 15 male and 15 female mice by ablation of the ipsilateral gastrocnemius muscle. Fiber counts, using the nitric acid digestion method, revealed no difference between control and enlarged muscles in male and female mice. Mean fiber area, determined by planimetry, was 49.1 and 34.5% greater following enlargement in male and female mice, respectively. Increase in muscle weight could be totally accounted for by the increase in fiber area following enlargement. A transformation of type II to type I fibers occurred following enlargement for both sexes. Counts of fibers from histological sections revealed that there was a progressive decrease in the fiber number as the section was moved from the belly to the distal end of the muscle. The results of these studies indicate that muscle enlargement in the mouse soleus muscle is due to hypertrophy of the existing muscle fibers.

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.

The effect of reduced dietary protein on the anterior latissimus dorsi muscle fibers in the Single Comb White Leghorn pullet.

Twenty-four female Single Comb White Leghorn pullets were used to investigate the effect of two levels of dietary protein on fiber number and size of the anterior latissimus dorsi muscle (ALD). The pullets were randomly assigned to a control group of 18% dietary protein or an experimental group of 14% dietary protein at 1 day of age. They were maintained on these diets until 4 weeks of age, at which time they were sacrificed by cervical dislocation. The ALD muscle was removed and analyzed for fiber number, fiber dry weight, and fiber dry weight to muscle wet weight ratio. Muscle fiber number was determined by a direct count of all the fibers following nitric acid digestion. After counting, the muscle fibers were dried to a constant weight for determination of mean fiber dry weight and fiber dry weight to muscle wet weight ratio. Muscle weight of the control group was 39.8% greater than that of the experimental group. Fiber number was not different between control and experimental groups; whereas mean fiber dry weight was 59.6% greater in the control group (P less than .05). Fiber dry weight to muscle wet weight ratio was 7.3% greater in the control group (P less than .05). The data indicate that when growth of the ALD muscle is restricted by low dietary protein content, there is a reduced growth of the individual muscle fibers with no alteration in their number.