Axonal projections from transplanted ectopic legs in an insect

Axon guidance factors in invertebrate development

The contribution that studies in the invertebrates have made to our understanding of the factors responsible for directing axon growth is reviewed. Cellular mechanisms for axon guidance are considered, particularly the question of the accuracy of initial axon growth, and the implications of these observations for models of growth cone turning. The cellular substrates followed by growing axons during embryogenesis are identified, together with the experimental evidence that each is essential for reliable axon navigation. The significance of these studies for investigations into the molecular nature of axon guidance factors is discussed, and the likely cellular roles of putative axon guidance molecules considered.

Central projections of axons from taste hairs on the labellum and tarsi of the blowfly,Phormia regina Meigen

Taste hairs are located on the labellum and tarsi of blowflies. These multimodal hairs consist of four functionally distinct chemoreceptors and a mechanoreceptor. By staining selected multimodal hairs, we sought to identify the central projection patterns of multiple and single axons from those hairs. On each side of the labellum there are 11 "largest" hairs (LH). The neurons associated with the anteriormost (LH-1), posteriormost (LH-11), and one lateral (LH-6) hair on the labellum were stained selectively with cobaltous sulfide. The overall projection pattern in the central nervous system (CNS) for axons from LH-1 and LH-11 is similar and differs markedly from axons from LH-6. At least three individual axon-projection patterns were determined for each labellar hair filled, indicating a partial functional organization for axons from multimodal hairs. One identified axon, the dorsalmost axon, has terminal arborizations that do not differ with the location of its associated hair. Another axon, thicker than the others, projects to a region that is distinct from the four thin axons. Within this region the arborizations of the thick axons occupy different areas depending on the location of their associated hair. Neurons from the largest hairs on the distalmost tarsomere (D5) of each leg were also stained and consisted of one thick and four thin axons. All axons except one thin axon from tarsal D5 hairs terminate in their respective leg neuromeres. The remaining thin axon projects to the suboesophageal ganglion ipsilateral to the hair filled and terminates in the same region as a branch of the labellar dorsalmost axon. These data suggest that axonal arbors from multimodal hairs have a limited functional and somatotopic organization in the blowfly CNS.

Regeneration of the projection and synaptic connections of tympanic receptor fibers of Locusta migratoria (Orthoptera) after axotomy

The tergite nerve N6 of the first abdominal segment of the locust Locusta migratoria contains receptor fibers, from the tympanic organ, and hair sensilla as well as motoric axons. The nerve was axotomized in nymphal instars or adults, and the regeneration of nerve fibers was studied. The sensory fibers regrow and regenerate their projection pattern within the central nervous system. They recognize their specific neuropile areas even after entering the ganglion through different pathways. The receptor fibers of the tympanic organ reestablish synaptic connections to auditory interneurons, even though the physiological characteristics of the interneurons are not fully restored. This regenerative capability contrasts with the lack of regeneration of peripheral structures in locusts, but supports the described plasticity in the auditory system of monaural locusts (Lakes, Kalmring, and Engelhard, 1990). The motor fibers do not regenerate nerves innervating muscles of the body wall.

Development of axon paths of motorneurons after removal of target muscles in a holometabolous insect

The fate of leg motor neurons and the outgrowth of their axons was studied in fleshflies after removal of their target muscles before the onset of axogenesis. When a mesothoracic leg imaginal disc was extirpated in the prepupa the corresponding leg nerve was missing on the operated side of the five-legged adult fly. Horseradish peroxidase and cobalt filling revealed that the target-deprived motorneurons survive and their axons grow out through other nerve roots. The choice of exit of the axons is variable: adjacent pro- or metathoracic leg nerves on the ipsilateral side or nerves supplying thoracic muscles (e.g. indirect flight muscles). In order to see whether factors in the mesothoracic leg disc can attract mesothoracic motorneurons, implantation of supernumerary leg discs was made in prepupae. Motor innervation of the implanted legs was never seen. Our data thus indicate that under the present experimental conditions leg motor neurons are not dependent on their specific targets for survival. They can track along adjacent preexisting axonal paths to innervate other targets. Under normal conditions specific markers may be present in the appropriate targets and the appropriate preexisting pathways (that we disrupt in our experiments) may lead the axons to these markers.