Building the antennal lobe: engrailed expression reveals a contribution from protocerebral neuroblasts in the grasshopper Schistocerca gregaria

Developmental expression of cell recognition molecules in the mushroom body and antennal lobe of the locust Locusta migratoria

We examined the development of olfactory neuropils in the hemimetabolous insect Locusta migratoria with an emphasis on the mushroom bodies, protocerebral integration centers implicated in memory formation. Using a marker of the cyclic adenosine monophosphate (cAMP) signaling cascade and lipophilic dye labeling, we obtained new insights into mushroom body organization by resolving previously unrecognized accessory lobelets arising from Class III Kenyon cells. We utilized antibodies against axonal guidance cues, such as the cell surface glycoproteins Semaphorin 1a (Sema 1a) and Fasciclin I (Fas I), as embryonic markers to compile a comprehensive atlas of mushroom body development. During embryogenesis, all neuropils of the olfactory pathway transiently expressed Sema 1a. The immunoreactivity was particularly strong in developing mushroom bodies. During late embryonic stages, Sema 1a expression in the mushroom bodies became restricted to a subset of Kenyon cells in the core region of the peduncle. Sema 1a was differentially sorted to the Kenyon cell axons and absent in the dendrites. In contrast to Drosophila, locust mushroom bodies and antennal lobes expressed Fas I, but not Fas II. While Fas I immunoreactivity was widely distributed in the midbrain during embryogenesis, labeling persisted into adulthood only in the mushroom bodies and antennal lobes. Kenyon cells proliferated throughout the larval stages. Their neurites retained the embryonic expression pattern of Sema 1a and Fas I, suggesting a role for these molecules in developmental mushroom body plasticity. Our study serves as an initial step toward functional analyses of Sema 1a and Fas I expression during locust mushroom body formation.

Tracking down the "head blob": comparative analysis of wingless expression in the developing insect procephalon reveals progressive reduction of embryonic visual system patterning in higher insects

The evolution of larval head morphology in holometabolous insects is characterized by reduction of antennal appendages and the visual system components. Little insight has been gained into molecular developmental changes underlying this morphological diversification. Here we compare the expression of the segment polarity gene wingless (wg) in the pregnathal head of fruit fly, flour beetle and grasshopper embryos. We provide evidence that wg activity contributes to segment border formation, and, subsequently, the separation of the visual system and protocerebrum anlagen in the anterior procephalon. In directly developing insects like grasshopper, seven expression domains are formed during this process. The activation of four of these, which correspond to polar expression pairs in the optic lobe anlagen and the protocerebral ectoderm, has shifted to postembryonic stages in flour beetle and Drosophila. The remaining three domains map to the protocerebral neuroectoderm, and form by disintegration of a large precursor domain in flour beetle and grasshopper. In Drosophila, the precursor domain remains intact, constituting the previously described "head blob". These data document major changes in the expression of an early patterning gene correlated with the dramatic evolution of embryonic visual system development in the Holometabola.

The organization and evolutionary implications of neuropils and their neurons in the brain of the onychophoran Euperipatoides rowelli

This account describes the organization of the brain of the adult Euperipatoides rowelli, a member of the Onychophora or "velvet worms." The present account identifies three cerebral divisions, the first of which contains primary olfactory neuropils, visual neuropils, and brain regions that correspond anatomically to the mushroom bodies of annelids, chelicerates, myriapods, and insects. In common with the brains of many chelicerates, the onychophoran brain is supplied by many thousands of uniformly small basophilic perikarya. Other chelicerate-like features include mushroom body lobes that extend across the brain's midline, an unpaired arch-shaped midline neuropil, and visual pathways that supply midline neuropil and that of the mushroom bodies. These and other similarities with chelicerate brains are discussed in the context of arthropod evolution and with reference to recent molecular phylogenies.

F-actin at identified synapses in the mushroom body neuropil of the insect brain

The distribution of f-actin stained by fluorescent phalloidin was investigated in the brain of several insect species, with a special focus on the mushroom body. For localizing f-actin in identified neurons and at synapses, additional staining with fluorescent dextrans and anti-synapsin I immunostaining was employed. Intense f-actin staining was consistently found in synaptic complexes of the mushroom body calyces (calycal microglomeruli [MG]). These MG contain a central core of presynaptic boutons, predominantly belonging to deutocerebral cholinergic excitatory projection neurons, which are surrounded by a shell of numerous Kenyon cell (KC) dendritic tips. In the cricket Gryllus bimaculatus, high-resolution confocal laser scanning imaging revealed colocalization of f-actin with KC dendritic spine parts within MG. Although presynaptic boutons appear to be mainly devoid of f-actin-phalloidin fluorescence, there appears to be an accumulation of f-actin in KC dendritic spines synaptically contacting the boutons. Electron microscopy of boutons and dextran-stained KC dendrites revealed their pre- and postsynaptic sites, with KCs being strictly postsynaptic elements. Their subsynaptic membrane appositions are considered to be associated with f-actin. Focal accumulation of f-actin in the dendritic tips of KCs was found to be a general feature of MG, with either spheroidal or indented boutons of different sizes, as encountered in the mushroom bodies of the cricket, honey bee, ant, and fruit fly. The structural similarities of calycal MG and f-actin accumulation in KC dendrites with cerebellar microglomeruli are considered comparatively. The accumulation of f-actin in KC dendrites is discussed in view of mushroom body plasticity and its potential role in learning and memory formation.

Embryonic development of the sensory innervation of the clypeo-labral complex: further support for serially homologous appendages in the locust

The clypeo-labrum, or upper lip, of insects is intimately involved in feeding behavior and is accordingly endowed with a rich sensory apparatus. In the present study we map the temporal appearance of all major clusters of sensory cells on this structure in the locust during the first half of embryogenesis. The identities of these sensory cell clusters were defined according to the origin of the branching point of their axons from the labral sensory nerve as seen at mid-embryogenesis. The first sensory cells to differentiate from the labral epithelium do so at stereotypic sites beginning at around 32% of embryogenesis. Bilaterally symmetrical clusters of differentiated neurons rapidly appear and pioneering of the labral sensory nerve on each side is performed by a specific cell from each cluster. This cell directs its axon anteriorly towards a bilaterally symmetrical pair of cells, the frontal commissure pioneers, on either side of the developing frontal ganglion. The final trajectory of the sensory nerve within the labrum closely matches the pattern of Repo-expressing glial cells. The majority of the sensory cell clusters differentiate during embryogenesis, but the number of sensory cells in some clusters are modified significantly during postembryonic development. Comparing the innervation pattern of the clypeo-labrum with that of other mouthparts and the leg at mid-embryogenesis, we find a striking similarity in organization which we interpret as support for the homologous appendage hypothesis.

Early steps in building the insect brain: neuroblast formation and segmental patterning in the developing brain of different insect species

In insects, morphological, molecular and genetic studies have provided a detailed insight into the ontogenetic processes that shape the ventral nerve cord. On the other hand, owing to its complexity and less obvious segmental composition, the knowledge about the development of the brain is still fragmentary. A promising approach towards gaining insight into fundamental processes underlying brain development is the comparison of embryonic brain development among different insect species. However, so far such comparative analyses are scarce. In this review, we summarize and compare data on the early steps in brain formation in different hemi- and holometabolous insects. We show that basic aspects of the spatial and temporal development of the embryonic brain neuroblast pattern are conserved among insects. Furthermore, we compare the number and proliferation patterns of neuroblasts related to major neuropil structures such as mushroom bodies, central complex, and antennal lobe. Finally, comparing the expression patterns of engrailed in different species, and considering new data from Drosophila melanogaster, we discuss the segmental organization of the insect brain.

Commissure formation in the embryonic insect brain

The primary axon scaffold of the insect brain is established early in embryogenesis and comprises a preoral protocerebral commissure, a postoral tritocerebral commissure and longitudinal fiber pathways linking the two. In both grasshopper and fly its form is approximately orthogonal and is centered around the stomodeum. We show how pioneer fibers from the protocerebrum and tritocerebrum cross the brain midline directly via their respective commissures. The deutocerebrum, however, lacks its own commissure and we describe how deutocerebral pioneers circumnavigate the gut to cross the midline either via the protocerebral commissure or the tritocerebral commissure. In contrast to all other commissures of the central nervous system, the protocerebral commissure persists, albeit in reduced form, in the commissureless mutation in the fly. Besides the com gene, a further, as yet unidentified, mechanism must regulate this commissure. The formation of the tritocerebral commissure involves labial, a member of the Hox gene group. Genetic rescue experiments in labial mutants reveal that the formation of this commissure can be rescued by all other Hox genes except Abdominal-B. However, only in the labial and Deformed null mutants are the commissures associated with the respective expression domains (tritocerebral, mandibular, respectively) absent. This suggests that the molecular mechanisms regulating postoral brain commissure formation are distinct from those in the neuromeres of the ventral nerve cord.

Segment polarity and DV patterning gene expression reveals segmental organization of the Drosophila brain

The insect brain is traditionally subdivided into the trito-, deuto- and protocerebrum. However, both the neuromeric status and the course of the borders between these regions are unclear. The Drosophila embryonic brain develops from the procephalic neurogenic region of the ectoderm, which gives rise to a bilaterally symmetrical array of about 100 neuronal precursor cells, called neuroblasts. Based on a detailed description of the spatiotemporal development of the entire population of embryonic brain neuroblasts, we carried out a comprehensive analysis of the expression of segment polarity genes (engrailed, wingless, hedgehog, gooseberry distal, mirror) and DV patterning genes (muscle segment homeobox, intermediate neuroblast defective, ventral nervous system defective) in the procephalic neuroectoderm and the neuroblast layer (until stage 11, when all neuroblasts are formed). The data provide new insight into the segmental organization of the procephalic neuroectodem and evolving brain. The expression patterns allow the drawing of clear demarcations between trito-, deuto- and protocerebrum at the level of identified neuroblasts. Furthermore, we provide evidence indicating that the protocerebrum (most anterior part of the brain) is composed of two neuromeres that belong to the ocular and labral segment, respectively. These protocerebral neuromeres are much more derived compared with the trito- and deutocerebrum. The labral neuromere is confined to the posterior segmental compartment. Finally, similarities in the expression of DV patterning genes between the Drosophila and vertebrate brains are discussed.

Spatial and temporal pattern of neuroblasts, proliferation, and Engrailed expression during early brain development in Tenebrio molitor L. (Coleoptera)

In insects, the knowledge of embryonic brain development is still fragmentary, and comparative data are scarce. In this study, we explored aspects of embryonic brain development in the coleopteran Tenebrio molitor. A detailed description is provided of the spatial and temporal pattern of the embryonic brain neuroblasts during 18-60% of embryonic development. Approximately 125 brain NBs have been identified in each hemisphere of the brain at about 40% of embryonic development. A subset of five neuroblasts, among them the two progenitors of the mushroom bodies and two progenitors of the larval antennal lobe, are morphologically identifiable by their larger size. As revealed by incorporation of BrdU, their mitotic behaviour is distinct from that of all other brain NBs, exhibiting an extended period of proliferation into postembryonic stages, and a significantly higher rate of division. To gain insight into the segmental organization of the T. molitor brain, Engrailed expression was examined in the head ectoderm and the deriving components of the CNS (including neuroblasts and their progeny) at different stages of embryonic development.

Development of the nervous system in the "head" of Limulus polyphemus (Chelicerata: Xiphosura): morphological evidence for a correspondence between the segments of the chelicerae and of the (first) antennae of Mandibulata

We investigated brain development in the horseshoe crab Limulus polyphemus and several other arthropods via immunocytochemical methods, i.e. antibody stainings against acetylated alpha-tubulin and synapsin. According to the traditional view, the first appendage-bearing segment in chelicerates (the chelicerae) is not homologous to the first appendage-bearing segment of mandibulates (first antenna, deutocerebrum) but to the segment of the second antenna (tritocerebrum) or the intercalary segment in hexapods and myriapods. Accordingly, the segment of the deutocerebrum in chelicerates would be completely reduced. The main arguments for this view are: (1) the postoral origin of the cheliceral ganglion, (2) a poststomodaeal commissure, and (3) a connection of the cheliceral ganglion to the stomatogastric system. Our data show that these arguments are not convincing. During the development of horseshoe crabs there is no evidence for a former additional segment in front of the chelicerae. Instead, comparison of the brain structure (neuropil ring) between chelicerates, crustaceans and insects shows remarkable similarities. Furthermore, the cheliceral commissure in horseshoe crabs runs mainly praestomodaeal, which would be unique for a tritocerebral commissure. An unbiased view of the developing nervous system in the "head" of chelicerates, crustaceans and insects leads to a homologisation of the cheliceral segment and that of the (first) antenna (= deutocerebrum) of mandibulates that is also congruous to the interpretation of the Hox gene expression patterns. Thus, our data provide morphological evidence for the existence of a chelicerate deutocerebrum.

Morphological and molecular data argue for the labrum being non-apical, articulated, and the appendage of the intercalary segment in the locust

Our analysis of head segmentation in the locust embryo reveals that the labrum is not apical as often interpreted but constitutes the topologically fused appendicular pair of appendages of the third head metamere. Using molecular, immunocytochemical and retrograde axonal staining methods we show that this metamere, the intercalary segment, is innervated by the third brain neuromere-the tritocerebrum. Evidence for the appendicular nature of the labrum is firstly, the presence of an engrailed stripe within its posterior epithelium as is typical of all appendages in the early embryo. Secondly, the labrum is innervated by a segmental nerve originating from the third brain neuromere (the tritocerebrum). Immunocytochemical staining with Lazarillo and horseradish peroxidase antibodies reveal that sensory neurons on the labrum contribute to the segmental (tritocerebral) nerve via the labral nerve in the same way as for the appendages immediately anterior (antenna) and posterior (mandible) on the head. All but one of the adult and embryonic motoneurons innervating the muscles of the labrum have their cell bodies and dendrites located completely within the tritocerebral neuromere and putatively derive from engrailed expressing tritocerebral neuroblasts. Molecular evidence (repo) suggests the labrum is not only appendicular but also articulated, comprising two jointed elements homologous to the coxa and trochanter of the leg.

A single cell analysis of engrailed expression in the early embryonic brain of the grasshopper Schistocerca gregaria: ontogeny and identity of the secondary headspot cells

The expression pattern of the segment polarity gene engrailed was studied at the single cell level in the protocerebrum of the early embryonic brain of the grasshopper Schistocerca gregaria, the neuromere containing the secondary headspot cells. The engrailed protein is first expressed in the protocerebrum at about 22% of embryogenesis by a group of identified neuroblasts bordering the antennal lobe. The number of immunoreactive neuroblasts increases up to 26% of embryogenesis and then rapidly declines so that by 30% only the three most posterior remain immunoreactive. These three neuroblasts become incorporated into the developing antennal lobe of the deutocerebrum. Subsequently, there is a progressive re-expression of the engrailed protein in the protocerebrum by the so-called six secondary headspot cells. These are the first born sibling progeny of three identified protocerebral neuroblasts which themselves expressed the engrailed protein prior to generating their lineages, and so represents a reacquisition of engrailed expression within identified clones. The secondary headspot cells are neurons which direct axonal processes into the developing optic tract and so contribute to the primary axon scaffold of the brain. From our analysis of their ontogeny, we conclude that the secondary headspot cells do not represent a segmental border in the brain.