The early development of motor axon pathways in the locust embryo: the establishment of the segmental nerves in the thoracic ganglia

Summary This study has identified the first five motor neurones to send axons out of the segmental nerves in the thoracic ganglia of the locust and has traced the pathways followed by these axons up to their divergence into the ganglionic nerve roots. These motor neurones send out axons in a stereotyped sequence over a short period, corresponding to 2 % of embryonic development. Motor axons initially grow dorsally to contact the dorsal basal lamina and then posteriolaterally in a parallel array just beneath this membrane. At the edge of the CNS the axons diverge into either of two pathways: an anterior pathway, corresponding to nerve root 3 which is pioneered by the first motor axon to leave the CNS; and a posterior pathway, corresponding to nerve root 5, which is pioneered by the second motor axon. The first motor axon appears to grow circumferentially around the segmental border between the body wall and the base of the coxa, while the second is closely associated with the filopodia or axons of the afferent peripheral pioneer neurones. The later motor axons reliably follow the pathways pioneered by these first two axons. A small number of molecular markers would be sufficient to generate the observed patterns of axon growth by these early motor neurones and some of these same cues may be used to guide afferent axons into the CNS.