Major Axon Tracts Research Articles

A Diffusion Tensor Imaging White Matter Atlas of the Domestic Canine Brain

Abstract There is increasing reliance on magnetic resonance imaging (MRI) techniques in both research and clinical settings. However, few standardized methods exist to permit comparative studies of brain pathology and function. To help facilitate these studies, we have created a detailed, MRI-based white matter atlas of the canine brain using diffusion tensor imaging. This technique, which relies on the movement properties of water, permits the creation of a three-dimensional diffusivity map of white matter brain regions that can be used to predict major axonal tracts. To generate an atlas of white matter tracts, thirty neurologically and clinically normal dogs underwent MRI imaging under anesthesia. High-resolution, three-dimensional T1-weighted sequences were collected and averaged to create a population average template. Diffusion weighted imaging sequences were collected and used to generate diffusivity maps, which were then registered to the T1-weighted template. Using these diffusivity maps, individual white matter tracts—including association, projection, commissural, brainstem, olfactory and cerebellar tracts—were identified with reference to previous canine brain atlas sources. To enable the use of this atlas, we created downloadable overlay files for each white matter tract identified using manual segmentation software. In addition, using diffusion tensor imaging tractography, we created tract files to delineate major projection pathways. This comprehensive white matter atlas serves as a standard reference to aid in the interpretation of quantitative changes in brain structure and function in clinical and research settings.

Fetal development of the human amygdala.

The intricate development of the human amygdala involves a complex interplay of diverse processes, varying in speed and duration. In humans, transient cytoarchitectural structures deliquesce, leading to the formation of functionally distinct nuclei as a result of multiple interdependent developmental events. This study compares the amygdala's cytoarchitectural development in conjunction with specific antibody reactivity for neuronal, glial, neuropil, and radial glial fibers, synaptic, extracellular matrix, and myelin components in 39 fetal human brains. We recognized that the early fetal period, as a continuation of the embryonic period,is still dominated by relatively uniform histogenetic processes. The typical appearance of ovoid cell clusters in the lateral nucleus during midfetal period is most likely associated with the cell migration and axonal growth processes in the developing human brain. Notably, synaptic markers are firstly detected in the corticomedial group of nuclei, while immunoreactivity for the panaxonal neurofilament marker SMI 312 is found dorsally. The late fetal period is characterized by a protracted migration process evidenced by the presence of doublecortin and SOX-2 immunoreactivity ventrally, in the prospective paralaminar nucleus, reinforced by vimentin immunoreactivity in the last remaining radial glial fibers. Nearing the term period, SMI 99 immunoreactivity indicates that perinatal myelination becomes prominent primarily along major axonal pathways, laying the foundation for more pronounced functional maturation. This study comprehensively elucidates the rate and sequence of maturational events in the amygdala, highlighting the key role of prenatal development in its behavioral, autonomic, and endocrine regulation, with subsequent implications for both normal functioning and psychiatric disorders.

Early construction of the thalamocortical axon pathway requires c-Jun N-terminal kinase signaling within the ventral forebrain.

Thalamocortical connectivity is essential for normal brain function. This important pathway is established during development, when thalamic axons extend a long distance through the forebrain before reaching the cerebral cortex. In this study, we identify a novel role for the c-Jun N-terminal kinase (JNK) signaling pathway in guiding thalamocortical axons through intermediate target territories. Complete genetic removal of JNK signaling from the Distal-less 5/6 (Dlx5/6) domain in mice prevents thalamocortical axons from crossing the diencephalon-telencephalon boundary (DTB) and the internal capsule fails to form. Ventral telencephalic cells critical for thalamocortical axon extensions including corridor and guidepost neurons are also disrupted. In addition, corticothalamic, striatonigral, and nigrostriatal axons fail to cross the DTB. Analyses of different JNK mutants demonstrate that thalamocortical axon pathfinding has a non-autonomous requirement for JNK signaling. We conclude that JNK signaling within the Dlx5/6 territory enables the construction of major axonal pathways in the developing forebrain. Further exploration of this intermediate axon guidance territory is needed to uncover mechanisms of axonal pathfinding during normal brain development and to elucidate how this vital process may be compromised in neurodevelopmental disorders.

New insights into the anatomo-functional architecture of the right sagittal stratum and its surrounding pathways: an axonal electrostimulation mapping study.

The sagittal stratum (SS) is a large sheet-like structure where major axonal fiber tracts cross, though its anatomical delineations are still debated. Here we investigated the poorly studied anatomo-functional organization of the right SS using direct electrical stimulation (DES) in patients undergoing wide-awake surgery for a cerebral glioma. Seventeen patients were included. There were six males, the mean age was 38years old. One patient underwent surgery twice. Fourteen patients were right-handed and one was ambidextrous. Behavior tasks were used to monitor online the patients' functions during DES, including visual and somesthetic processes, semantics, language, spatial and social cognition.Beyond the cortical DES, the mapping of axonal pathways evoked various functional responses. At the level of the core of the right SS, there were visual disturbances, visual hemi-agnosia, semantic paraphasia, left spatial neglect, confusion and comprehension difficulties, anomia, and mentalizing disturbances. At the level of the surrounding axonal pathways, there were left spatial neglect, anomia, vertigo, dysesthesia, and hearing disturbances. Our functionally defined three-dimensional map indicates that this complex region has a multilayered functional architecture, and supports an organization founded on two anatomical systems: a core system formed by the optic radiations, inferior longitudinal fasciculus, and inferior fronto-occipital fasciculus, and a peripheral one composed of surrounding or intersecting white matter tracts, including the superior longitudinal fasciculus/arcuate fasciculus, thalamocortical radiations, auditory radiations, and parieto-insular vestibular system. These results should prompt neurosurgeons to achieve awake DES mapping within the right SS because of the likelihood of causing multiple and irreversible structural disconnections.

Region-specific differences and areal interactions underlying transitions in epileptiform activity.

Key points Local neocortical and hippocampal territories show different and sterotypical patterns of acutely evolving, epileptiform activity.Neocortical and entorhinal networks show tonic–clonic‐like events, but the main hippocampal territories do not, unless it is relayed from the other areas.Transitions in the pattern of locally recorded epileptiform activity can be indicative of a shift in the source of pathological activity, and may spread through both synaptic and non‐synaptic means.Hippocampal epileptiform activity is promoted by 4‐aminopyridine and inhibited by GABAB receptor agonists, and appears far more sensitive to these drugs than neocortical activity.These signature features of local epileptiform activity can provide useful insight into the primary source of ictal activity, aiding both experimental and clinical investigation. Understanding the nature of epileptic state transitions remains a major goal for epilepsy research. Simple in vitro models offer unique experimental opportunities that we exploit to show that such transitions can arise from shifts in the ictal source of the activity. These transitions reflect the fact that cortical territories differ both in the type of epileptiform activity they can sustain and in their susceptibility to drug manipulation. In the zero‐Mg2+ model, the earliest epileptiform activity is restricted to neocortical and entorhinal networks. Hippocampal bursting only starts much later, and triggers a marked transition in neo‐/entorhinal cortical activity. Thereafter, the hippocampal activity acts as a pacemaker, entraining the other territories to their discharge pattern. This entrainment persists following transection of the major axonal pathways between hippocampus and cortex, indicating that it can be mediated through a non‐synaptic route. Neuronal discharges are associated with large rises in extracellular [K+], but we show that these are very localized, and therefore are not the means of entraining distant cortical areas. We conclude instead that the entrainment occurs through weak field effects distant from the pacemaker, but which are highly effective at recruiting other brain territories that are already hyperexcitable. The hippocampal epileptiform activity appears unusually susceptible to drugs that impact on K+ conductances. These findings demonstrate that the local circuitry gives rise to stereotypical epileptic activity patterns, but these are also influenced by both synaptic and non‐synaptic long‐range effects. Our results have important implications for our understanding of epileptic propagation and anti‐epileptic drug action.

Cerebral organoids: a promising model in cellular technologies

The development of the human brain is a complex multi-stage process including the formation of various types of neural cells and their interactions. Many fundamental mechanisms of neurogenesis have been established due to the studying of model animals. However, significant differences in the brain structure compared to other animals do not allow considering all aspects of the human brain formation, which could play the main role in the development of unique cognitive abilities for human. Four years ago, Lancaster’s group elaborated human pluripotent stem cell-derived three-dimensional cerebral organoid technology, which opened a unique opportunity for researchers to model early stages of human neurogenesis in vitro. Cerebral organoids closely remodel many endogenous brain regions with specific cell composition like ventricular zone with radial glia, choroid plexus, and cortical plate with upper and deeper-layer neurons. Moreover, human brain development includes interactions between different brain regions. Generation of hybrid three-dimensional cerebral organoids with different brain region identity allows remodeling some of them, including long-distance neuronal migration or formation of major axonal tracts. In this review, we consider the technology of obtaining human pluripotent stem cell-derived three-dimensional cerebral organoids with different modifications and with different brain region identity. In addition, we discuss successful implementation of this technology in fundamental and applied research like modeling of different neurodevelopmental disorders and drug screening. Finally, we regard existing problems and prospects for development of human pluripotent stem cell-derived threedimensional cerebral organoid technology.

Fused cerebral organoids model interactions between brain regions.

Human brain development involves complex interactions between different areas, including long distance neuronal migration or formation of major axonal tracts. 3D cerebral organoids allow the growth of diverse brain regions in vitro, but the random arrangement of regional identities limits the reliable analysis of complex phenotypes. Here, we describe a co-culture method combining various brain regions of choice within one organoid tissue. By fusing organoids specified toward dorsal and ventral forebrain, we generate a dorsal-ventral axis. Using fluorescent reporters, we demonstrate robust directional GABAergic interneuron migration from ventral into dorsal forebrain. We describe methodology for time-lapse imaging of human interneuron migration that is inhibited by the CXCR4 antagonist AMD3100. Our results demonstrate that cerebral organoid fusion cultures can model complex interactions between different brain regions. Combined with reprogramming technology, fusions offer the possibility to analyze complex neurodevelopmental defects using cells from neurological disease patients, and to test potential therapeutic compounds.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans.

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

Astroglial-Mediated Remodeling of the Interhemispheric Midline Is Required for the Formation of the Corpus Callosum

The corpus callosum is the major axon tract that connects and integrates neural activity between the two cerebral hemispheres. Although ∼1:4,000 children are born with developmental absence of the corpus callosum, the primary etiology of this condition remains unknown. Here, we demonstrate that midline crossing of callosal axons is dependent upon the prior remodeling and degradation of the intervening interhemispheric fissure. This remodeling event is initiated by astroglia on either side of the interhemispheric fissure, which intercalate with one another and degrade the intervening leptomeninges. Callosal axons then preferentially extend over these specialized astroglial cells to cross the midline. A key regulatory step in interhemispheric remodeling is the differentiation of these astroglia from radial glia, which is initiated by Fgf8 signaling to downstream Nfi transcription factors. Crucially, our findings from human neuroimaging studies reveal that developmental defects in interhemispheric remodeling are likely to be a primary etiology underlying human callosal agenesis.

Highways of the emotional intellect: white matter microstructural correlates of an ability-based measure of emotional intelligence

ABSTRACTIndividuals differ in their ability to understand emotional information and apply that understanding to make decisions and solve problems effectively – a construct known as Emotional Intelligence (EI). While considerable evidence supports the importance of EI in social and occupational functioning, the neural underpinnings of this capacity are relatively unexplored. We used Tract-Based Spatial Statistics (TBSS) to determine the white matter correlates of EI as measured by the ability-based Mayer–Salovey–Caruso Emotional Intelligence Test (MSCEIT). Participants included 32 healthy adults (16 men; 16 women), aged 18–45 years. White matter integrity in key tracts was positively correlated with the Strategic Area branches of the MSCEIT (Understanding Emotions and Managing Emotions), but not the Experiential branches (Perceiving and Facilitating Emotions). Specifically, the Understanding Emotions branch was associated with greater fractional anisotropy (FA) within somatosensory and sensory-motor fiber bundles, particularly those of the left superior longitudinal fasciculus and corticospinal tract. Managing Emotions was associated with greater FA within frontal-affective association tracts including the anterior forceps and right uncinate fasciculus, along with frontal-parietal cingulum and interhemispheric corpus callosum tracts. These findings suggest that specific components of EI are directly related to the structural microarchitecture of major axonal pathways.

The Yin and Yang of Wnt/Ryk axon guidance in development and regeneration

In the developing embryo, nascent axons navigate towards their specific targets to establish the intricate network of axonal connections linking neurons within the mature nervous system. Molecular navigational systems comprising repulsive and attractive guidance cues form chemotactic gradients along the pathway of the exploring growth cone. Axon-bound receptors detect these gradients and determine the trajectory of the migrating growth cone. In contrast to their benevolent role in the developing nervous system, repulsive guidance receptors are detrimental to the axon's ability to regenerate after injury in the adult. In this review we explore the essential and beneficial role played by the chemorepulsive Wnt receptor, Ryk/Derailed in axon navigation in the embryonic nervous system (the Yin function). Specifically, we focus on the role of Wnt5a/Rykmediated guidance in the establishment of two major axon tracts in the mammalian central nervous system, the corticospinal tract and the corpus callosum. Recent studies have also identified Ryk as a major suppressor of axonal regeneration after spinal cord injury. Thus, we also discuss this opposing aspect of Ryk function in axonal regeneration where its activity is a major impediment to axon regrowth (the Yang function).

Spatiotemporal Expression of Repulsive Guidance Molecules (RGMs) and Their Receptor Neogenin in the Mouse Brain

Neogenin has been implicated in a variety of developmental processes such as neurogenesis, neuronal differentiation, apoptosis, migration and axon guidance. Binding of repulsive guidance molecules (RGMs) to Neogenin inhibits axon outgrowth of different neuronal populations. This effect requires Neogenin to interact with co-receptors of the uncoordinated locomotion-5 (Unc5) family to activate downstream Rho signaling. Although previous studies have reported RGM, Neogenin, and/or Unc5 expression, a systematic comparison of RGM and Neogenin expression in the developing nervous system is lacking, especially at later developmental stages. Furthermore, information on RGM and Neogenin expression at the protein level is limited. To fill this void and to gain further insight into the role of RGM-Neogenin signaling during mouse neural development, we studied the expression of RGMa, RGMb, Neogenin and Unc5A-D using in situ hybridization, immunohistochemistry and RGMa section binding. Expression patterns in the primary olfactory system, cortex, hippocampus, habenula, and cerebellum were studied in more detail. Characteristic cell layer-specific expression patterns were detected for RGMa, RGMb, Neogenin and Unc5A-D. Furthermore, strong expression of RGMa, RGMb and Neogenin protein was found on several major axon tracts such as the primary olfactory projections, anterior commissure and fasciculus retroflexus. These data not only hint at a role for RGM-Neogenin signaling during the development of different neuronal systems, but also suggest that Neogenin partners with different Unc5 family members in different systems. Overall, the results presented here will serve as a framework for further dissection of the role of RGM-Neogenin signaling during neural development.

Neuroblast lineage-specific origin of the neurons of the Drosophila larval olfactory system

The complete neuronal repertoire of the central brain of Drosophila originates from only approximately 100 pairs of neural stem cells, or neuroblasts. Each neuroblast produces a highly stereotyped lineage of neurons which innervate specific compartments of the brain. Neuroblasts undergo two rounds of mitotic activity: embryonic divisions produce lineages of primary neurons that build the larval nervous system; after a brief quiescence, the neuroblasts go through a second round of divisions in larval stage to produce secondary neurons which are integrated into the adult nervous system. Here we investigate the lineages that are associated with the larval antennal lobe, one of the most widely studied neuronal systems in fly. We find that the same five neuroblasts responsible for the adult antennal lobe also produce the antennal lobe of the larval brain. However, there are notable differences in the composition of larval (primary) lineages and their adult (secondary) counterparts. Significantly, in the adult, two lineages (lNB/BAlc and adNB/BAmv3) produce uniglomerular projection neurons connecting the antennal lobe with the mushroom body and lateral horn; another lineage, vNB/BAla1, generates multiglomerular neurons reaching the lateral horn directly. lNB/BAlc, as well as a fourth lineage, vlNB/BAla2, generate a diversity of local interneurons. We describe a fifth, previously unknown lineage, BAlp4, which connects the posterior part of the antennal lobe and the neighboring tritocerebrum (gustatory center) with a higher brain center located adjacent to the mushroom body. In the larva, only one of these lineages, adNB/BAmv3, generates all uniglomerular projection neurons. Also as in the adult, lNB/BAlc and vlNB/BAla2 produce local interneurons which, in terms of diversity in architecture and transmitter expression, resemble their adult counterparts. In addition, lineages lNB/BAlc and vNB/BAla1, as well as the newly described BAlp4, form numerous types of projection neurons which along the same major axon pathways (antennal tracts) used by the antennal projection neurons, but which form connections that include regions outside the “classical” olfactory circuit triad antennal lobe-mushroom body-lateral horn. Our work will benefit functional studies of the larval olfactory circuit, and shed light on the relationship between larval and adult neurons.

Imaging Axon Pathfinding in Xenopus In Vivo

Axon pathfinding in the developing animal involves a highly dynamic process in which the axonal growth cone makes continuous decisions as it navigates toward its target. Changes occurring in the growth cone with respect to retracting from or extending into complex new territories can occur in minutes. Thus, the advent of strategies to visualize axon path-finding in vivo in a live intact animal is crucial for a better understanding of how the growth cone makes such rapid decisions in response to multiple cues. Combining these strategies with loss-of-function and/or gain-of-function techniques allows one to gain some insight as to which molecules are crucial to particular growth cone behaviors at specific choice points during navigation. The main advantage of using Xenopus lies in the accessibility of major axon tracts for live microscopy, as their embryonic development occurs ex utero. Furthermore, the robust embryos remain healthy during immobilization and allow for good imaging for long periods. This protocol describes the methods for stabilizing and preparing live Xenopus embryos for imaging labeled axonal tracts at high spatial and temporal resolution for up to 72 h. This approach can been used to investigate how the knockdown of certain gene functions can affect the speed of navigation through the well-studied Xenopus retinotectal pathway. It can be adapted to visualize other axon tracts of interest.

Imaging Axon Pathfinding in Zebrafish In Vivo: Figure 1.

Axon pathfinding in the developing animal involves a highly dynamic process in which the axonal growth cone makes continuous decisions as it navigates toward its target. Changes occurring in the growth cone with respect to retracting from or extending into complex new territories can occur in minutes. Thus, the advent of strategies to visualize axon path-finding in vivo in a live intact animal is crucial for a better understanding of how the growth cone makes such rapid decisions in response to multiple cues. Combining these strategies with loss-of-function and/or gain-of-function techniques, one can gain some insight as to which molecules are crucial to particular growth cone behaviors at specific choice points during navigation. The major advantage of using zebrafish lies in the accessibility of major axon tracts for live microscopy, as their embryonic development occurs ex utero. Furthermore, the robust embryos remain healthy during immobilization and allow for good imaging for long periods. This protocol describes the method for stabilizing and preparing live zebrafish embryos for imaging labeled axonal tracts at high spatial and temporal resolution for up to 72 h. It has been used for retinotectal axon pathfinding, but can be adapted to visualize other axon tracts of interest.

The E3 Ubiquitin Ligase Protein Associated with Myc (Pam) Regulates Mammalian/Mechanistic Target of Rapamycin Complex 1 (mTORC1) Signaling in Vivo through N- and C-terminal Domains

Pam and its homologs (the PHR protein family) are large E3 ubiquitin ligases that function to regulate synapse formation and growth in mammals, zebrafish, Drosophila, and Caenorhabditis elegans. Phr1-deficient mouse models (Phr1(Δ8,9) and Phr1(Magellan), with deletions in the N-terminal putative guanine exchange factor region and the C-terminal ubiquitin ligase region, respectively) exhibit axon guidance/outgrowth defects and striking defects of major axon tracts in the CNS. Our earlier studies identified Pam to be associated with tuberous sclerosis complex (TSC) proteins, ubiquitinating TSC2 and regulating mammalian/mechanistic target of rapamycin (mTOR) signaling. Here, we examine the potential involvement of the TSC/mTOR complex 1(mTORC1) signaling pathway in Phr1-deficient mouse models. We observed attenuation of mTORC1 signaling in the brains of both Phr1(Δ8,9) and Phr1(Magellan) mouse models. Our results establish that Pam regulates TSC/mTOR signaling in vitro and in vivo through two distinct domains. To further address whether Pam regulates mTORC1 through two functionally independent domains, we undertook heterozygous mutant crossing between Phr1(Δ8,9) and Phr1(Magellan) mice to generate a compound heterozygous model to determine whether these two domains can complement each other. mTORC1 signaling was not attenuated in the brains of double mutants (Phr1(Δ8,9/Mag)), confirming that Pam displays dual regulation of the mTORC1 pathway through two functional domains. Our results also suggest that although dysregulation of mTORC1 signaling may be responsible for the corpus callosum defects, other neurodevelopmental defects observed with Phr1 deficiency are independent of mTORC1 signaling. The ubiquitin ligase complex containing Pam-Fbxo45 likely targets additional synaptic and axonal proteins, which may explain the overlapping neurodevelopmental defects observed in Phr1 and Fbxo45 deficiency.

Serum Response Factor Is Required for Cortical Axon Growth But Is Dispensable for Neurogenesis and Neocortical Lamination

Previous studies have shown that neuron-specific deletion of serum response factor (SRF) results in deficits in tangential cell migration, guidance-dependent circuit assembly, activity-dependent gene expression, and synaptic plasticity in the hippocampus. Furthermore, SRF deletion in mouse embryonic stem cells causes cell death in vitro. However, the requirement of SRF for early neuronal development including neural stem cell homeostasis, neurogenesis, and axonal innervations remains unknown. Here, we report that SRF is critical for development of major axonal tracts in the forebrain. Conditional mutant mice lacking SRF in neural progenitor cells (Srf-Nestin-cKO) exhibit striking deficits in cortical axonal projections including corticostriatal, corticospinal, and corticothalamic tracts, and they show a variable loss of the corpus callosum. Neurogenesis and interneuron specification occur normally in the absence of SRF and the deficits in axonal projections were not due to a decrease or loss in cell numbers. Radial migration of neurons and neocortical lamination were also not affected. No aberrant cell death was observed during development, whereas there was an increase in the number of proliferative cells in the ventricular zone from embryonic day 14 to day 18. Similar axonal tract deficits were also observed in mutant mice lacking SRF in the developing excitatory neurons of neocortex and hippocampus (Srf-NEX-cKO). Together, these findings suggest distinct roles for SRF during neuronal development; SRF is specifically required in a cell-autonomous manner for axonal tract development but is dispensable for cell survival, neurogenesis, neocortical lamination, and neuronal differentiation.

Appropriate Bmp7 levels are required for the differentiation of midline guidepost cells involved in corpus callosum formation

Guidepost cells are essential structures for the establishment of major axonal tracts. How these structures are specified and acquire their axon guidance properties is still poorly understood. Here, we show that in mouse embryos appropriate levels of Bone Morphogenetic Protein 7 (Bmp7), a member of the TGF-β superfamily of secreted proteins, are required for the correct development of the glial wedge, the indusium griseum, and the subcallosal sling, three groups of cells that act as guidepost cells for growing callosal axons. Bmp7 is expressed in the region occupied by these structures and its genetic inactivation in mouse embryos caused a marked reduction and disorganization of these cell populations. On the contrary, infusion of recombinant Bmp7 in the developing forebrain induced their premature differentiation. In both cases, changes were associated with the disruption of callosal axon growth and, in most animals fibers did not cross the midline forming typical Probst bundles. Addition of Bmp7 to cortical explants did not modify the extent of their outgrowth nor their directionality, when explants were exposed to a focalized source of the protein. Together, these results indicate that Bmp7 is indirectly required for corpus callosum formation by controlling the timely differentiation of its guidepost cells.

A Combinatorial Semaphorin Code Instructs the Initial Steps of Sensory Circuit Assembly in the Drosophila CNS

Longitudinal axon fascicles within the Drosophila embryonic CNS provide connections between body segments and are required for coordinated neural signaling along the anterior-posterior axis. We show here that establishment of select CNS longitudinal tracts and formation of precise mechanosensory afferent innervation to the same CNS region are coordinately regulated by the secreted semaphorins Sema-2a and Sema-2b. Both Sema-2a and Sema-2b utilize the same neuronal receptor, plexin B (PlexB), but serve distinct guidance functions. Localized Sema-2b attraction promotes the initial assembly of a subset of CNS longitudinal projections and subsequent targeting of chordotonal sensory afferent axons to these same longitudinal connectives, whereas broader Sema-2a repulsion serves to prevent aberrant innervation. In the absence of Sema-2b or PlexB, chordotonal afferent connectivity within the CNS is severely disrupted, resulting in specific larval behavioral deficits. These results reveal that distinct semaphorin-mediated guidance functions converge at PlexB and are critical for functional neural circuit assembly.

Axogenesis in the central and peripheral nervous system of the amphipod crustacean Orchestia cavimana

We describe the formation of the major axon pathways in the embryonic central and peripheral nervous systems of the amphipod crustacean Orchestia cavimana Heller, 1865 by means of antibody staining against acetylated alpha-tubulin. The data add to a long list of previous studies of various other aspects of development in Orchestia and provide a basis for future studies of neurogenesis on a deeper cellular and molecular level. Orchestia exhibits a tripartite dorsal brain, which is a characteristic feature of euarthropods. Its anlagen are the first detectable structures in the developing nervous system and can be traced back to distinct neuronal cell clusters in the early embryo. The development of the ventral nervous system proceeds with an anteroposterior gradient of development. In each trunk segment, the longitudinal connectives and the anterior commissure form first, followed by the intersegmental nerve, the posterior commissure and segmental nerves, respectively. A single commissure of a vestigial seventh pleonal segment is found. In the peripheral nervous system we observe a spatial and temporal pattern of leg innervation, which is strikingly similar in both limb types, the uniramous pereopods and the biramous pleopods. A proximal leg nerve splitting distally into two separated nerves probably reflects a general feature of crustaceans.