AXOGENESIS IN THE EMBRYONIC BRAIN OF THE GRASSHOPPER SCHISTOCERCA-GREGARIA - AN IDENTIFIED CELL ANALYSIS OF EARLY BRAIN-DEVELOPMENT

Axogenesis in the embryonic brain was studied at the single cell level in the grasshopper Schistocerca gregaria. A small set of individually identifiable pioneer neurons establishes a primary axon scaffold duri ng early embryogenesis. At the beginning of scaffold formation, pionee ring axons navigate along and between glial borders that surround clus ters of proliferating neuroblasts, In each brain hemisphere, an axonal outgrowth cascade involving a series of pioneer neurons establishes a path way from the optic ganglia to the brain midline, At the midline the primary preoral commissural interconnection in the embryonic brain is pioneered by a pair of midline-derived pioneer neurons. A second p reoral commissural connection is pioneered by two pairs of pars interc erebralis pioneer neurons. Descending tracts are pioneered by the prog eny of identified neuroblasts in the pars intercerebralis, deutocerebr um and tritecerebrum; the postoral tritocerebral commissure is pioneer ed by a pair of tritocerebral neurons. All of the pioneering brain neu rons express the cell adhesion molecule fasciclin I during initial axo n outgrowth and fasciculation. Once established, the primary axon scaf fold of the brain is used for fasciculation by subsequently differenti ating neurons and, by the 40% stage of embryogenesis, axonal projectio ns that characterize the mature brain become evident. The single cell analysis of grasshopper brain development presented here sets the stag e for manipulative cell biological experiments and provides the basis for comparative molecular genetic studies of embryonic brain developme nt in Drosophila.