A Paradoxical Growth of Large Myofibers in Growth‐Arrested Rectus Femoris Muscles of Muscular Dystrophic Mice

Abstract

Abstract The hindleg rectus femoris muscles of normal (?/+) and dystrophic (dy/dy) mice of the strain CS7BL/6J‐dy were examined morphometrically on their cross sectional microscopic images. The diameter of dystrophic muscles was similar to that of normal muscles at 11 and 14 days of postnatal age, and increased with age up to 26 days at a slower rate than that of normal muscles, and then stopped increasing. After 90 days of age, it tended to decrease slightly. The number of myofibers per muscle was always much smaller in dystrophic than in normal muscles during a period from 11 to 32 days, and remained almost unchanged in dystrophic muscles during and after the period up to 120 days of age. The mean diameter of the largest fifty myofibers in dystrophic muscles steadily increased, and the diameter‐frequency histogram of these myofibers shifted to the larger diameter, with age up to 70 days even after the diameter of muscles had stopped increasing. This paradoxical growth of a small population of the large or “hypertrophied” myofibers in growth‐arrested muscles of dystrophic mice was discussed in relation not to an abnormal “hypertrophy” preceding degeneration but to a variation in growth capabilities of myofibers greatly shifted to incompetence.