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The anterior midline of the brain, also known as the pars intercerebralis, contains the largest collection of neurosecretory cells in the central nervous system of the grasshopper. In this study, we use immunocytochemical, intracellular staining, and histological methods to establish the ontogenies of the various cell types in the brain midline, and show how these cells contribute to the pars intercerebralis of the adult brain. We show that the adult pars intercerebralis develops from three distinct embryonic cell groups: (1) the median neurosecretory cells, which derive from a subset of neuroblasts in the protocerebral hemispheres, and which project axons to the corpora cardiaca; (2) the paired primary commissure pioneers, which derive directly from the mesectoderm of the dorsal median domain and whose axons project to the ventral nerve cord via the midline tract; and (3) the six progeny of the median precursor in the dorsal median domain, which share a common axonal projection with the primary commissure pioneers. Since the adult pars intercerebralis is a fusion product of these independent cellular components, it can only be understood in terms of its origins in the embryonic brain. When the expression pattern of the TERM-1 antigen is compared in subsets of median neurosecretory cells in a wide range of insect orders, the results suggests a common organizational Bauplan for the pars intercerebralis. This hypothesis is supported by the identification of putative homologs of the grasshopper primary commissure pioneers in all these insects.

The authors studied the contribution of seven clusters of Lazarillo-expressing cells to the primary axon scaffold of the brain in the grasshopper Schistocerca gregaria from 26% to 43% of embryogenesis. Each cluster, which was numbered according to when Lazarillo expression first appeared, was uniquely identifiable on the basis of its stereotypic position in the brain and the number of Lazarillo-expressing cells it contained. At no time during embryogenesis was Lazarillo expression found in brain neuroblasts: It was found 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 yellow into the lateral cell at various ages showed that there was no dye coupling to the midline precursor or to the nearby term-1-expressing primary commissure pioneers;The lateral cell is not related lineally to these cells and most likely differentiates directly from the neuroectoderm of the brain midline. Lazarillo expression appears at the onset of axogenesis as the lateral 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 that the lateral cell axon follows as it navigates away from the brain midline. The primary axon scaffold of the brain results from a stepwise interlinking of discrete brain regions, as exemplified by axons from neighboring Lazarillo-expressing cell. clusters. J. Comp. Neurol. 419:394-405, 2000. (C) 2000 Wiley-Liss, Inc.

The bilaterally paired primary commissure pioneer neurons in the median domain of the grasshopper brain are large, descending interneurons that uniquely express the TERM-1 antigen, even in the adult. After pioneering the primary interhemispheric brain commissure, these neurons extend TERM-1-immunoreactive collaterals into most parts of the brain except the mushroom bodies. In this report, the authors show that the TERM-1 antigen is located in the cell body cytoplasm of these neurons and not on the membranes. Screening with antisera to insect neuropeptides reveals that an antiserum recognizing peptides of the leucokinin family labels the cell body cytoplasm of the primary commissure neurons. Leucokinin-related peptides are known to modulate motility of visceral muscle, play a role in diuresis, and are likely to be neuromodulators in the insect nervous system. The primary commissure neurons differ ultrastructurally from median neurosecretory cells in that their cell body cytoplasm is more extensive, contains high numbers of mitochondria and extensive endoplasmic reticulum, but does not contain neurosecretory granules. In the adult, the cell somata are enveloped by multiple glia membranes and associated trophospongia. According to these ultrastructural characteristics, the primary commissure pioneers are not classical neurosecretory cells.

Embryonic development in the median domain of the brain of the grasshopper Schistocerca gregaria was investigated with immunohistochemical, histological, and intracellular dye injection techniques. The early head midline is divisible into a dorsal median domain and a ventral median domain based on the orientation of cell somata in each region. At 25% of embryogenesis, a single large midline precursor differentiates in the dorsal median domain and produces a lineage of six neuronal progeny before degenerating. No further precursors arise. In addition, the primary commissure pioneers and a pair of lateral neurons differentiate directly from the ectoderm in this region. Lucifer yellow dye injected into the midline precursor stains only this cell and its progeny. Similarly, there is no dye coupling from the primary commissure pioneers to the midline lineage or to neuroblasts of the brain hemispheres. Neurogenesis in the dorsal median domain therefore proceeds separately within each subset of cells, and is not related to development in the brain hemispheres. Beginning at 42% of embryogenesis, the primary commissure pioneers undergo a morphological transformation and concomittantly express the Term-1 antigen. Expression continues throughout embryogenesis and into the adult, where the midline primary commissure pioneer cells are the only ones labeled by Term-1 in the entire brain. The cellular organization of the dorsal median domain therefore remains remarkably conserved throughout embryogenesis, even as the brain undergoes extensive morphological transformation.

We have studied the morphogenetic reorganization that occurs in the grasshopper brain during embryogenesis. We find that morphogenetic movements occur at three organizational levels during brain development. First, the entire developing brain changes its orientation with respect to the segmental chain of ventral ganglia. A 90 degrees shift in the attitude of the brain neuraxis occurs during embryogenesis due to a gradual upward movement of the cerebral structures in the head. Second, the clusters of proliferating neuroblasts and progeny that generate the neuroarchitecture of the mature brain move relative to one another and to nonneural structures such as the stomodeum. This is especially pronounced for the pars intercerebralis and for the tritocerebrum, as shown by annulin and engrailed immunoreactivity. Third, individual neuroblasts within a given proliferative cluster undergo positional reorganization during embryogenesis. Identified neuroblasts of the tritocerebrum and the pars intercerebralis are displaced within the brain. We conclude that the transformation of the simple sheet-like structure of the early embryonic brain into the highly differentiated structure of the mature brain involves a series of morphogenetic movements that occur in virtually all parts of the brain.

The brain (supraoesophageal ganglion) is the most complex of the segmental ganglia composing the nerve cord of the locusts Schistocerca gregaria and Locusta migratoria. In this paper, we describe the ground plan of the commissures crossing the midline of the brain and propose a nomenclature with the aim of making a complex neuropil more understandable at the level of individual neurons. For developmental and comparative reasons the neuroarchitecture of the brain is related to the neural axis, not to the body axis. We have identified 73 commissural fiber bundles belonging to the adult brain, and these are named according to their location (ventral, dorsal, anterior, posterior, medial) with respect to the central complex as reference point. Reconstructions of identified neurons from intracellular stainings, cobalt backfills, or immunohistochemical studies demonstrate the various configurations in which fibers cross the brain.

The aim of the present study was to examine the pattern of synaptic interactions among a set of identified homologous interneurons in the segmental nervous system of the locust. This paper presents two main findings: first, serially homologous interneurons that are the progeny of neuroblast 7-4 in the mesothoracic, metathoracic, and first abdominal neuromeres of the locust central nervous system all receive synaptic drive from one and the same presynaptic interneuron. This interneuron has its entire arborization located in these three neuromeres of the central nervous system. It synapses with cells that are siblings, bilateral homologs, and serial homologs, and is itself connected monosynaptically with auditory afferents. The neuronal network that results comprises postsynaptic cells with the same developmental lineage. The second finding is that there is an additional set of synaptic connections among the homologous neurons themselves. All these connections are excitatory, and the pattern of information flow within the network is highly directional. This may relate to the morphologies of the neurons involved and will influence the contribution of homologs from different segments to behavior.

We describe the neuroarchitecture of the noctuid thoracic nerve cord and use this framework to interpret the organization of the auditory pathway responsible for escape behaviour in noctuid moths. Noctuid moths possess only two auditory receptors (A1, A2), in each ear. The axon of the A1 cell projects initially to a glomerulus located ventrally and medially in the metathoracic ganglion, where it bifurcates. One branch ascends in the ventral intermediate tract to the brain, the other descends in the ventral intermediate tract into abdominal neuromeres of the metathoracic ganglion. Both axons arborize in the median ventral and ring tracts in each neuromere. The central projections of the A2 cell remain largely within the metathoracic ganglion. The axon bifurcates at the midline and directs arborizations dorsally to the dorsal intermediate and median dorsal tracts, and ventrally into the ring tract where the arborizations overlap those of the A1 afferent. The afferent projections remain ipsilateral to the ear of origin. We describe a posterior auditory association area in the metathoracic ganglion in which the major arborizations of several identified interneurones overlap those of the A1 afferent and make monosynaptic connections with it. These interneurones all respond tonically to sound stimuli. We have also identified the projections of the A1 afferent, interneurones, and motor neurones in the segmentally equivalent anterior auditory association area of the mesothoracic ganglion. An interneurone with major arborizations in the same tracts as the A1 afferent, and receiving monosynaptic input from it, is described. The arborizations of higher order interneurones lie mainly in dorsal tracts along with those of flight motor neurones. All the interneurones in this anterior centre respond phasically or phasic/tonically to sound stimuli. The relevance of this anatomical organization for predator avoidance behaviour is considered and the organization of auditory pathways in tympanate insects compared.