ENDOCYTOTIC ACTIVITY OF MOUSE SKELETAL-MUSCLE FIBERS AFTER LONG-TERM DENERVATION

The endocytotic activity of skeletal muscle fibres and its relation to the denervated endplate region has been studied using horseradish per oxidase (HRP) as marker for endocytosis. In muscles denervated for a s hort time period (10-20 days) HRP-uptake occurred in small segments of the muscle fibres near the centre of the muscle (endplate region). Af ter long-term denervation (6-12 months) similar segments with high end ocytotic activity were seen preferentially in more peripheral parts of the muscle fibres. Ultrastructural characteristics of segments with h igh endocytotic activity from long-term denervated muscle fibres inclu de a proliferating transverse tubular system, HRP-containing bodies of different sizes with some very large vacuoles extending over several sarcomeres. These characteristics are similar to those described previ ously for HRP-uptake in the endplate region of short-term denervated m uscle (Tagerud et al., J. Neurol. Sci., 75 (1986) 141) except that no recognizable endplate structures were observed in the present study. T he results are discussed in relation to the fate of the denervated end plate and the receptive capacity for synapse formation in long-term de nervated muscle.