Specificity of neuromuscular connections during early development and following regeneration of motor axons in the bullfrog.

The specificity of neuromuscular connectivity was examined in unoperated bullfrog (Rana catesbeiana) tadpoles and in tadpoles that had undergone transection of the three ventral roots that normally innervate the hindlimb. The specificity of motoneuron projections was assessed by applying small amounts of horseradish peroxidase to circumcribed hindlimb regions and mapping the locations of retrogradely labeled motoneurons within the lumbar lateral motor column (LMC). In unoperated tadpoles, the locations of retrogradely labeled motoneurons in the LMC were as circumscribed at early stages of development as in tadpoles examined after motoneuron number in the LMC had stabilized. Six to eight weeks after ventral root transection in young tadpoles, localization of retrogradely labeled motoneurons was almost as circumscribed as found in unoperated tadpoles. However, localization following regeneration became less precise in more advanced tadpoles. If the ventral roots of adult frogs were crushed rather than transected, motor axon regeneration was considerably more precise, confirming previous reports (Westerfield and Powell, 1983). The implications of these results for hypotheses of neuromuscular specificity are discussed.