Morphometric analysis of skeletal muscle growth in the high growth mouse.

The high growth phenotype in the mouse results from a single autosomal recessive locus (hg) mapped to chromosome 10, that produces a marked increase in growth rate and body size compared to controls. Using morphometric methods, we quantified skeletal muscle growth in newborn male and female high growth mice, and at 3, 6, and 9 weeks of age. Adult male high growth mouse body weights increased by 42% and muscle weights increased 39-43% compared to controls. Differences between high growth and control muscle weights were removed after normalization to body weight. Similar changes in muscle and body weights were found in female high growth mice. Muscle cross-sectional area was significantly (P < 0.05) increased in high growth mouse gastrocnemius (GAS), soleus (SOL), and tibialis anterior (TA) muscles at 6 and 9 weeks of age. Cross-sectional areas of slow type 1, and fast type 2b fibers were moderately increased in all high growth muscles measured. Muscle fiber number was increased by approx 30% in the high growth SOL, GAS and TA muscles. We conclude that rapid growth in the high growth mouse is accompanied by an increase in muscle mass, due to fiber hyperplasia and hypertrophy, and that the hg locus may act during myogenesis to alter the proliferation rate and/or fusion kinetics of high growth mouse muscle cells.