MUSCULAR WEAKNESS IN THE MDX MOUSE

mdx mice are believed to be virtually free from neuromuscular symptoms , despite the presence of a degenerative/regenerative process that inv olves all skeletal muscles. We analyzed both the spontaneous motility and treadmill motor activity of mdx mice aged 15 days to 6 months. Our results indicate that there is an early period, between the end of th e second and up to the fifth week of life, when mdx mice experience ex treme weakness. After this critical period, both spontaneous motility and endurance of mdx mice, although lower than those of controls, do n ot show statistically significant differences up to 6 months of age. W e also carried out a detailed histological analysis of proximal and di stal muscle groups in mdx mice during this early critical motility per iod. The occurrence of extensive necrosis followed by regeneration and involving proximal muscles before distal ones was documented in mice as young as 16-17 days of age and reached a peak at day 18. We conclud e that dystrophin deficiency induces muscle degeneration and significa nt weakness in mdx mice, but only in an early period. Later on, during development, mdx mice adapt to the lack of this protein and do not sh ow detectable in vivo functional muscle impairment up to 6 months of a ge.