Abstract
Following intramuscular injection of horseradish peroxidase into mouse skeletal muscle adjacent to areas of innervation, rapid uptake of the label was observed by coated but not synaptic vesicles. The tracer was also found in Schwann cells, external mesaxons and within peripheral myelin lamellae, but never in axoplasm of intramuscular nerves proximal to the injection site. The data suggest that the tracer is taken up by the coated vesicles and may be rapidly discharged into the synaptic clefts from which it is cleared by a combination of phagocytic activity and absorption via pinocytotic vesicles and intercellular clefts of adjacent capillaries or, more probably, enzyme activity is lost within the coated vesicles. The significance of these observations as related to neuromuscular function is discussed.