Differentiation of motor nerve terminals formed in the absence of muscle fibres.

During reinnervation of frog skeletal muscle, axons form functional nerve terminals at original synaptic sites on denervated myofibres. When muscle is damaged as well as denervated, myofibres decompose but their sheaths of basal lamina (BL) survive. Despite the absence of myofibres, axons regenerate to contact BL and there acquire clusters of synaptic vesicles and membrane-associated dense patches that resemble active zones; BL regulates this differentiation. We show here that these BL-associated axonal segments appear smaller and contain fewer active zones than terminals on intact myofibres in the same preparation. However, terminals formed on BL sheaths are capable of activity-dependent recycling of synaptic vesicles (demonstrated by tracer uptake), and bear an antigen normally present in terminals but not preterminal axons (demonstrated by immunofluorescence). Thus, axons can acquire functional and biochemical, as well as morphological, characteristics of normal motor nerve terminals in the absence of a postsynaptic cell.