Abstract
The compartmental and clonal relationships between primary motoneurons and the myotubes they innervate have been studied in Xenopus laevis embryos by initiating clones at blastula stages (32 to 512 cells) with intracellular injections of horseradish peroxidase (HRP). Primary motoneurons and ventral myotome belong to the posterior-ventral compartment, whereas sensory neurons and dorsal myotome belong to the posterior-dorsal compartment (Jacobson, M. (1983) J. Neurosci. 3: 1019–1038). The pathways of HRP-labeled primary motor axons, which pioneer the peripheral pathway to the adjacent myotome beginning at stage 21/22, were traced. Within the spinal cord, the axons remained in the posterior-ventral compartment. Upon leaving the cord they were confronted with both dorsal and ventral myotome; they remained almost exclusively in the ventral myotome. At late embryonic stages (40/42) primary motoneurons could be retrogradely labeled by applying HRP to the dorsal myotome, indicating that their axons or a branch had crossed the compartmental boundary. The primary motor axon also displayed a highly significant preferred association with clonally related myotubes within the ventral myotome. Axon growth only in the compartmentally related myotome and preferred association with clonally related myotubes suggest that the guidance of pioneer axons in the periphery may be based upon factors derived from common ancestry and lineage.