AUTOMATED CYTOMETRY OF FIBER SIZE AND SPATIAL-DISTRIBUTION IN THE SUPERFICIAL MASSETER MUSCLE OF THE RAT AT 3 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. Less er diameter and spatial distribution of more than 6000 muscle fibres w ere automatically measured in digital images from muscle sections stai ned 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 frequ ent (>54%). In the 21-day-old animals, females presented larger diamet ers 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 the ir fibre types. All fibre types grew in diameter with age. The coeffic ient of variation of the diameter was age-independent and remained und er 23%. The finding of an age-independent variable could have a practi cal application: an increase of the coefficient of variation (>25%) ca n be considered as pathological, even without a perfect age-matched co ntrol.