Skeletal muscle cellularity in mice selected for large body size and in controls.

Body composition and skeletal muscle weights, fiber diameters, fiber numbers, and RNA and DNA concentrations were studied in mice selected for large body size (GLW) compared to an unselected control (C) line. At 60 days of age, body weight of GLW mice was 70 to 75% greater than C mice, but % body water, fat or dry fat-free residue did not differ between the lines. In both GLW and C mice, greater muscle weight in males compared to females was due to larger diameter fibers and not to significant differences in fiber number between sexes, although in most cases, males had slightly higher mean fiber number. Fiber diameters were similar in the two lines and the greater muscle weights of GLW mice were due to more fibers than in the muscles of C mice. RNA and DNA concentrations were slightly, but not significantly, higher in GLW skeletal muscles.