Ontogeny and development of the tritocerebral commissure giant (TCG): an identified neuron in the brain of the grasshopper Schistocerca gregaria
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The tritocerebral commissure giant (TCG) of the grasshopper Schistocerca gregaria is one of the best anatomically and physiologically described arthropod brain neurons. A member of the so-called Ventral Giant cluster of cells, it integrates sensory information from visual, antennal and hair receptors, and synapses with thoracic motor neurons in order to initiate and regulate flight behavior. Its ontogeny, however, remains unclear. In this study, we use bromodeoxyuridine incorporation and cyclin labeling to reveal proliferative neuroblasts in the region of the embryonic brain where the ventral giant cluster is located. Engrailed labeling confirms the deutocerebral identity of this cluster. Comparison of soma locations and initial neurite projections into tracts of the striate deutocerebrum help identify the cells of the ventral cluster in both the embryonic and adult brain. Reconstructions of embryonic cell lineages suggest deutocerebral NB1 as being the putative neuroblast of origin. Intracellular dye injection coupled with immunolabeling against neuron-specific horseradish peroxidase is used to identify the VG1 (TCG) and VG3 neurons from the ventral cluster in embryonic brain slices. Dye injection and backfilling are used to document axogenesis and the progressive expansion of the dendritic arbor of the TCG from mid-embryogenesis up to hatching. Comparative maps of embryonic neuroblasts from several orthopteroid insects suggest equivalent deutocerebral neuroblasts from which the homologous TCG neurons already identified in the adult brain could originate. Our data offer the prospect of identifying further lineage-related neurons from the cluster and so understand a brain connectome from both a developmental and evolutionary perspective.
KeywordsInsect Brain Development Neuroblast Lineage Identified neuron
George Boyan and Leslie Williams thank Dr. E. Ball for the gift of the engrailed antibody, Dr. E. Ehrhardt for assistance with the confocal microscopy, and Karin Fischer for general technical assistance. George Boyan and Leslie Williams received support from the Deutsche Forschungsgemeinschaft (BO 1434/3-5) and the Graduate School of Systemic Neuroscience, LMU. Tobias Müller acknowledges the personal support of Prof. Dr. W. Kutsch, Universität Konstanz, and received financial support from the Deutsche Akademische Austauschdienst (DAAD) and from ARC grant 313-ARC-VII 93/50 to Jonathan Bacon.
Compliance with ethical standards
All experiments were performed in accordance with the guidelines for animal welfare as laid down by the Deutsche Forschungsgemeinschaft.
Conflict of interest
The authors declare that they have no competing interests.
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