Primary Motor Axons Research Articles

Anterior-posterior subdivision of the somite in embryonic zebrafish: implications for motor axon guidance.

The ventral branches of the segmental peripheral nerves in the zebrafish embryo are pioneered by the caudal primary (CaP) motor axons, which extend midsegmentally at the interface of the somite and the notochord. The signals that define the CaP pathway are not well understood. To gain insight into the nature of the guidance cues, we have examined the environment of the CaP motor axons by using electron microscopy and histochemistry. Specifically, we have mapped the distribution of the transcription factor engrailed, of a chondrotin sulfate epitope, and of the recognition molecules zebrafish semaphorin z1b and zebrafish tenascin C. Ultrastructural examination of dye-labeled CaP motor axons revealed a close association with the medial surface of the somite but not with the notochord. The CaP axons were always accompanied by cells that appeared to migrate at the interface of somite and notochord. These cells were confined to the posterior half of the somite. Some of the cells may be neural crest derived, many others are probably of sclerotomal origin. The putative migratory cells expressed a chondroitin sulfate epitope that is a marker of sclerotome in the chick. The pathway of the CaP axon and the distribution of the putative neural crest and sclerotome cells correlated with a subdivision of the myotome into an anterior and posterior components, which were evident at the histological level and by the expression of the markers engrailed, semaphorin z1b, chondroitin sulfate, and tenascin C. We suggest that both the pathway choice of the CaP axon and the route of migratory cells reflect this anterior-posterior bipartition of the myotome.

Sclerotome development and peripheral nervous system segmentation in embryonic zebrafish.

Vertebrate embryos display segmental patterns in many trunk structures, including somites and peripheral nervous system elements. Previous work in avian embryos suggests a role for somite-derived sclerotome in segmental patterning of the peripheral nervous system. We investigated sclerotome development and tested its role in patterning motor axons and dorsal root ganglia in embryonic zebrafish. Individual somite cells labeled with vital fluorescent dye revealed that some cells of a ventromedial cell cluster within each somite produced mesenchymal cells that migrated to positions expected for sclerotome. Individual somites showed anterior/posterior distinctions in several aspects of development: (1) anterior ventromedial cluster cells produced only sclerotome, (2) individual posterior ventromedial cluster cells produced both sclerotome and muscle, and (3) anterior sclerotome migrated earlier and along a more restricted path than posterior sclerotome. Vital labeling showed that anterior sclerotome colocalized with extending identified motor axons and migrating neural crest cells. To investigate sclerotome involvement in peripheral nervous system patterning, we ablated the ventromedial cell cluster and observed subsequent development of peripheral nervous system elements. Primary motor axons were essentially unaffected by sclerotome ablation, although in some cases outgrowth was delayed. Removal of sclerotome did not disrupt segmental pattern or development of dorsal root ganglia or peripheral nerves to axial muscle. We propose that peripheral nervous system segmentation is established through interactions with adjacent paraxial mesoderm which develops as sclerotome in some vertebrate species and myotome in others.

Pathfinding by zebrafish motoneurons in the absence of normal pioneer axons.

Individually identified primary motoneurons of the zebrafish embryo pioneer cell-specific peripheral motor nerves. Later, the growth cones of secondary motoneurons extend along pathways pioneered by primary motor axons. To learn whether primary motor axons are required for pathway navigation by secondary motoneurons, we ablated primary motoneurons and examined subsequent pathfinding by the growth cones of secondary motoneurons. We found that ablation of the primary motoneuron that pioneers the ventral nerve delayed ventral nerve formation, but a normal-appearing nerve eventually formed. Therefore, the secondary motoneurons that extend axons in the ventral nerve were able to pioneer that pathway in the absence of the pathway-specific primary motoneuron. In contrast, in the absence of the primary motoneuron that normally pioneers the dorsal nerve, secondary motoneurons did not pioneer a nerve in the normal location, instead they formed dorsal nerves in an atypical position. This difference in the ability of these two groups of motoneurons to pioneer their normal pathways suggests that the guidance rules followed by their growth cones may be very different. Furthermore, the observation that the atypical dorsal nerves formed in a consistent incorrect location suggests that the growth cones of the secondary motoneurons that extend dorsally make hierarchical pathway choices.

Compartmental relationships between anuran primary spinal motoneurons and somitic muscle fibers that they first innervate

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.