Longitudinal growth of skeletal muscle fibres in lines of mice selected for high and low body weight.

The effect of selection for increased and decreased body weight on the weight, mean fibre length, mean estimated resting fibre length, mean sarcomere length and mean sarcomere number per fibre of the biceps brachii muscle was studied in 10 week old mice. The expected alterations in muscle weight were present and were due in part to changes in fibre length which was increased in the high line and decreased in the low line. The changes in fibre length were due to differences in sarcomere number per fibre which was increased in the high line and decreased in the low line. The mean sarcomere length of mice in the high and low lines did not differ significantly from control indicating that this parameter is unaffected by selection for altered body weight. It is suggested that genetic factors influence longitudinal muscle fibre growth in two ways. Species differ in the length of their myofilaments whereas differences between individuals and lines within a species are due to the formation of additional sarcomeres.