Cellular characteristics of skeletal muscle in selected strains of pigs and mice and the unselected controls.

Skeletal muscle growth of swine differing in rate of growth and muscularity was studied by analysis of DNA, RNA, and protein in the semitendinosus muscle of fast growing domestic lean pigs (Yorkshire) and the slow-growing feral obese pigs (Ossabaw). Both lean and obese strains were fed ad libitum corn and soybean mean diet containing 14% protein. The pigs were slaughtered at 6 months of age. The Yorkshire pig had significantly greater body and muscle weights (P less than 0.005) Total DNA, RNA, and protein in semitendinosus muscles were significantly greater in the Yorkshire pigs (P less than 0.005). RNA/DNA and protein/DNA ratios were significantly different between the Yorkshire and Ossabaw pigs. These data suggest that the greater muscle growth in the Yorkshire pigs is achieved by greater cell numbers and size. The response of skeletal muscle cells and fiber characteristics to selection was studied in two strains of mice by analysis of DNA, RNA, and histological measurement of muscle fibers. Both strains were fed ad libitum on laboratory rat chow diet. They were sacrificed at 12 months of age. Leg muscle DNA and RNA were significantly greater in the mice selected for rapid growth rate as compared to the control (P less than 0.005 and P less than .10, respectively). Measurements of the semimembranosus muscle reveal that the growth strain had significantly greater muscle weight (P less than .01) and muscle length (P less than 0.005). Histological measurements of the same muscle showed that the growth strain had significantly greater fiber number (P less than 0.005) and larger fiber size (P less than 0.005) than the controls. It would appear that the greater muscle cellularity (DNA) in growth strain mice is accompanied by greater muscle length, fiber number, and fiber diameter.