ABSTRACT
The frequency, the distribution of fiber types, the area, the number of nuclei per fiber, the number and the morphological, metabolic and contractile characteristics of the muscle of the Masseter and Longissimus Dorsi muscles of Piau, Sorocaba and Large White breeds of pigs were studied. Superficial segments of these muscles were frozen in n-Hexane at -70 degree. For morphological studies, some cryostat sections (10µm) were stained with Hematoxylin Eosin (HE). For fiber typing, subsequent sections were reacted for NADH-TR and myofibrilar ATPase (m-ATPase) after alkaline (pH 10.4) and acid(pH 4.6) preincubations. Masseter muscle of Piau, Sorocaba and Large White pigs revealed a mosaic pattern of Slow Oxydative (SO) and Fast Oxydative Glycolytic (FOG) fibers with the following frequencies: 34.9 per cent and 64.4 per cent; 47.4 per cent and 52.6 per cent; 25,3 per cent and 74,7 per cent, respectively. The area of SO fibers was smaller than of FOG. Longissimus Dorsi muscle revealed three kinds of fibers, SO, FOG and Fast Glycolytic (FG), with frequencies of 12.5 per cent, 29.5 per cent and 57.9 per cent; 21.8 per cent, 21.8 per cent 56.4 per cent; 13.2 per cent, 17.6 per cent and 69.2 per cent, respectively. SO and FOG fibers were arranged in metabolic groups of 5-15 fibers. In Large White pigs, the frequency of SO fibers was significantly lower, while its area was larger. The number of nuclei per fiber was significantly greater in Large White and Sorocaba pigs than in the Piau.