Fig. 1 Progressive muscular dystrophy causes age-dependent histomorphological changes and muscle weakness in SJL mice. a, Quadriceps femoris muscle of a 24-day-old SJL mouse discloses only mild myopathic lesions in terms of small-calibre muscle fibres with internal myonuclei (arrowhead). b, Dystrophic and inflammatory changes in the quadriceps femoris muscle of a 279-day-old SJL mouse. Note the abnormally shaped muscle fibres, increased variability of fibre diameters, degenerating fibres (arrowheads), small, regenerating ‘basophilic’fibres and foci of inflammatory cells (arrows). c, In an aged SJL mouse (450 days), the proximal limb muscles (for example, quadriceps muscle) display changes compatible with an advanced muscular dystrophy: numerous rounded muscle fibres with internal myonuclei are separated by fibro-connective tissue infiltrated by foci of mononucleated cells and scattered fat cells (asterisks). Different behaviour of a normal (BALB/c, 4 weeks) mouse (d) and a four-week-old SJL mouse (e) is seen when they are suspended by the tail. d, The first reflex of control mice is to spread their limbs and digits and the trunk is held in an extended position. e, SJL mice, in contrast, keep their limbs in an adducted and flexed position and tend to curl in on themselves, grasping at their own fur. They often fail in attempts to extend their trunk and they are not able to turn round and reach the suspending hand, indicating trunk muscle weakness. d a b e c quadriceps femoris