Automated cytometry of fibre size and spatial distribution in the superficial masseter muscle of the rat at three ages.

An image cytometry program was applied to sections of the superficial masseter muscle of female and male 21-, 42- and 105-day-old rats. Lesser diameter and spatial distribution of more than 6000 muscle fibres were automatically measured in digital images from muscle sections stained for myofibrillar ATPase activity. In this muscle, only type 2A, 2B and 2C fibres were observed. At the three ages and in both sexes, 2A fibres were found to have the largest diameter and were the most frequent (> 54%). In the 21-day-old animals, females presented larger diameters than did males; in contrast, in the 105-day-old animals, the three fibre types were larger in males than in females. At all ages and in both sexes, type 2A occupied 32 to 80% more cross-sectional area than type 2B. Most images (98%) showed a random spatial distribution of their fibre types. All fibre types grew in diameter with age. The coefficient of variation of the diameter was age-independent and remained under 23%. The finding of an age-independent variable could have a practical application: an increase of the coefficient of variation (> 25%) can be considered as pathological, even without a perfect age-matched control.