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Histopathological Observation of Stress Myopathy in M. Longissimus in the Pig and Relationships with Meat Quality, Fattening and Slaughter Traits¹

Academy of Agriculture² 61-637, Poznan, Poland

Abstract

The characteristic myopathic features revealed by histological observations included strong proliferation of connective and fatty tissue, perivascular infiltrations and necrosis of muscle fibers with phagocytosis to the lesser extent. In the myopathic muscle, as well as in giant fibers, histochemical techniques showed a reduction in succinate dehydrogenase and lactate dehydrogenase activity in type ßR (slow-twitch, oxidative) and R (fast-twitch, oxidative and glycolytic). Magnesium-activated adenosine triphosphatase reaction ranged from diffuse to negative in ßR, R and W (fast-twitch, glycolytic) fiber types. Diffuse reaction for acid phosphatase and total loss of glycogen content were observed. The micrographs of the myopathic muscle indicated enlarged mitochondria with atrophy or complete destruction of cristae. Many myofibrils were hypercontracted. Giant fibers possessed mitochondria enlarged to an even greater extent and many of the myofibrils had loss of continuity, were narrow, depleted and were also hypercontracted. Significant differences between myopathic and normal groups were found in number of ßR fibers (lower in the myopathic group), number of R fibers and percent of R and W fibers (higher in the myopathic group). Differences (P<.01) existed between meat pH₁ value in the myopathic group (mean value of 5.95) and the normal group (mean value of 6.29). Meat from the myopathic group of pigs also had a lower (P<.01) pH₂₄ value and reduced water-holding capacity (P<.01) relative to the meat of the normal pigs. The lack of difference of fattening and slaughter traits between the groups suggested that the White Zlotnicka pigs is of particular value because it is possible to improve the production traits without increasing the incidence of these syndromes within the breed. Negative correlations (P<.05) between number of giant fibers and percent of W fibers, and between percent of giant fibers and percent of W fibers indicate that W fibers can undergo degeneration and be transformed into giant fibers. Therefore, it it suggested that giant fibers should be treated as muscular, pathological results of past stresses and not as an additional type of normal muscle cells.

¹ The help of Dr. R. G. Cassens of the Muscle Biol. Lab. Univ. of Wisconsin-Madison is gratefully acknowledged.

² Dept. of Anim. Origin Products.