MORPHOGENESIS OF THE BRANCHING PATTERN OF A GROUP OF SPIKING LOCAL INTERNEURONS IN RELATION TO THE ORGANIZATION OF EMBRYONIC SENSORY NEUROPILS IN LOCUST

The embryonic development of the principal tracts, commissures and neu ropils in the thoracic ganglia of the locust Schistocerca gregaria are described. We show that the major tracts and commissures are generate d during the earliest stages of axon outgrowth. Some longitudinal trac ts can be identified as early as 42% of embryonic development and by 5 5% all tracts except the dorsal median tract (DMT) and median dorsal t ract (MDT) can be recognized. DMT and MDT cannot be reliably identifie d until 65%. The major neuropilar regions, in contrast, are identifiab le relatively late in embryogenesis. They are first evident at 65-70%, but do not become fully distinct until 70-75%. This coincides with th e developmental timing of synaptogenesis. Onto this developmental grou ndplan we have mapped the growth of an identified group of local inter neurons. The early growth of these interneurons (50-65%) is characteri zed by slow and directed axon outgrowth which assembles the basic skel etal structure of the interneurons without aberrant growth. This is fo llowed by a period of extensive growth (65-80%) during which the basic scaffold is elaborated. Finally there is a maturation phase during wh ich branches are pruned away to produce the mature interneuron structu re. We show that, despite initial extensive overgrowth of branches, th ere is no branching into inappropriate neuropil regions in the embryo. The development of arborizations within specific neuropils appears to be tightly controlled. By using this information on interneuron growt h and neuropil development it is now possible to begin to understand t he developmental mechanisms that shape the neuronal architecture of th e locust central nervous system.