Reorganization of intrinsic components in the distal motor axon during outgrowth

We have studied the changes in the distribution of three intrinsic axonal components during the growth and maturation of sprouts in vivo. Neurofilaments, tubulin and synaptophysin, a synaptic vesicle protein, were visualized in motor axons and their sprouts using immunocytochemical staining of frozen longitudinal sections of muscle. We examined changes in these elements in sprouts regenerating after axonal crush injury and in those evoked from intact axons by denervation changes in muscle. Our results show that intrinsic axonal components move into newly formed motor axon sprouts in different temporal patterns. Based on the patterns of reorganization of staining of intrinsic axonal components, two types of outgrowth can be distinguished. One type, synaptic elaboration, is manifest by short, broad axonal processes that produce enlargement of the synaptic zone (synaptophysin staining) with little change in the distribution of intrinsic cytoskeletal elements. A second type of outgrowth, axonal elongation, occurs during axonal regeneration and ultraterminal sprouting and is longitudinal in form. In these sprouts there is a sequential appearance of neurofilament and then, several days later, tubulin immunostaining. Synaptophysin only accumulates in these sprouts after two weeks at points of synaptic contact with a muscle fibre.