Evidence that the primary brain commissure is pioneered by neurons with a peripheral-like ontogeny in the grasshopper Schistocerca gregaria

The commissures represent a major neuroarchitectural feature of the central nervous system of insects and vertebrates alike. The adult brain of the grasshopper comprises 72 such commissures, the first of which is established in the protocerebral midbrain by three sets of pioneer cells at around 30% of embryogenesis. These pioneers have been individually identified via cellular, molecular and intracellular dye injection techniques. Their ontogenies, however, remain unclear. The progenitor cells of the protocerebral midbrain are shown via Annulin immunocytochemistry to be compartmentalized, belonging either to the protocerebral hemispheres or the so-called median domain. Serial reconstructions based on bromodeoxyuridine incorporation confirm that their lineages do not intermingle. Dye injection into progenitor cells and progeny confirms this compartmentalization, and reveals that none of the pioneers are associated with a lineage of cells deriving from a protocerebral neuroblast or midline precursor. Immunocytochemical data as well as dye injection into identified pioneers over several developmental stages indicate that they differentiate directly from epithelial cells, but not from classical progenitor cells. That the commissural pioneers of the protocerebrum represent modified epithelial cells involves a different ontogeny to that described for pioneers in the ventral nerve cord, but parallels that of pioneer neurons of the peripheral nervous system.