Neurogenesis in the insect brain: cellular identification and molecular characterization of brain neuroblasts in the grasshopper embryo

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Abstract
Brain neuroblasts in the embryonic grasshopper were studied by toluidine blue staining, BrdU incorporation, and immunocytochemistry in whole-mounts as well as by reconstruction of stained serial sections. Large dividing neuroblasts are observed by the 25% stage. During early neurogenesis these neuroblasts generate their progeny through mechanisms similar to those that occur in the segmental ganglia; each neuroblast divides asymmetrically to produce a chain of ganglion mother cells, and each ganglion mother cell divides symmetrically to produce a pair of neurons. Approximately 130 mitotically active, large neuroblasts are found in each brain hemisphere at the 30–45% stages. Through morphogenetic movements that occur between the 30–35% stages these neuroblasts become located in positions which are predictive of the major brain regions that they give rise to. Many of the brain neuroblasts can be identified as individuals based on their stereotyped position in the neurogenic array. Immunocytochemical experiments with antibodies against, engrailed, fasciclin I and TERM-1 show that brain neuroblasts can also be characterized by their expression of cell-specific molecular labels. These studies indicate that many features of the complex mature insect brain derive from a surprisingly simple and stereotyped set of neuronal precursor cells. Thus, many of the concepts and methods that have been used to study neurogenesis in the simpler segmental ganglia may also be applicable to the insect brain.