Lazarillo expression reveals a subset of neurons contributing to the primary axon scaffold of the embryonic brain of the grasshopper Schistocerca gregaria

The authors studied the contribution of seven clusters of Lazarillo-express ing cells to the primary axon scaffold of the brain in the grasshopper Schi stocerca gregaria from 26% to 43% of embryogenesis. Each cluster, which was numbered according to when Lazarillo expression first appeared, was unique ly identifiable on the basis of its stereotypic position in the brain and t he number of Lazarillo-expressing cells it contained. At no time during emb ryogenesis was Lazarillo expression found in brain neuroblasts: It was foun d only in progeny. For ease of analysis, axogenesis was followed in a cell cluster that contained only a single Lazarillo-expressing cell (the lateral cell) in the dorsal median domain of the brain midline. Bromodeoxyuridine incorporation revealed the presence of only a single midline precursor cell in this region during embryogenesis. Intracellular injection of Lucifer ye llow into the lateral cell at various ages showed that there was no dye cou pling to the midline precursor or to the nearby term-1-expressing primary c ommissure pioneers; The lateral cell is not related lineally to these cells and most likely differentiates directly from the neuroectoderm of the brai n midline. Lazarillo expression appears at the onset of axogenesis as the l ateral cell projects an axon laterally toward the next Lazarillo-expressing cell cluster. The cells of this target; cluster direct axons into separate brain regions, thereby establishing an orthogonally organized scaffold tha t the lateral cell axon follows as it navigates away from the brain midline . The primary axon scaffold of the brain results from a stepwise interlinki ng of discrete brain regions, as exemplified by axons from neighboring Laza rillo-expressing cell. clusters. J. Comp. Neurol. 419:394-405, 2000. (C) 20 00 Wiley-Liss, Inc.