Effects of selection for body size on the weight and cellular structure of seven mouse muscles

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Abstract
SUMMARY: The effects of selection for increased and decreased body weight in mice at 5 and at 10 weeks of age on the weight and mean fibre diameter and number of seven different muscles were studied by dissecting and histologically examining animals from large, small and unselected control lines. The purpose was to learn how selection for body size affects muscle weight and its components. Selection for increased body weight significantly increased the weight of m. biceps brachii, m. tibialis anterior, m. pectoralis major, m. brachio-radialis, m. soleus, m. rectus femoris and m. psoas major. These increases in muscle weight reflected increases in both fibre diameter and number although there were a few exceptions. Selection for low body weight produced decreases in the weight of all muscles. These reflected significant reductions in mean fibre diameter and, in most cases, fibre number. Significant estimates of heterosis in mean fibre diameter were obtained in m. biceps brachii and m. tibialis anterior from crosses between the Large and Small lines selected at 5 weeks and also between the lines selected at 10 weeks of age, respectively.