Wide-ranging migration and destination of early olfactory placode-derived neurons in chick embryos.

Cell migration from the olfactory placode (OP) is a well-known phenomenon wherein various cell types, such as gonadotropin-releasing hormone (GnRH)-producing neurons, migrate toward the telencephalon (TEL) during early embryonic development. However, the spatial relationship between early migratory cells and the forebrain is unclear. We examined the early development of whole-mount chick embryos to observe the three-dimensional spatial relationship among OP-derived migratory neurons, olfactory nerve (ON), and TEL. Migratory neurons that express highly polysialylated neural cell adhesion molecule (PSA-NCAM) emerge from the OP and spread over a relatively wide TEL area at the Hamburger and Hamilton (HH) stage 17. Most migratory neurons form a cellular cord between the olfactory pit and rostral TEL within HH18-20. The earliest axons from the olfactory epithelium (OE) were detected along this neuronal cord using DiI-labeling at HH21. Furthermore, a few PSA-NCAM-positive neurons were dispersed around the cellular cord and over the lateral TEL at HH18. A long cellular cord branch extending to the lateral TEL was transiently observed within HH18-24. These results suggest a novel migratory route of OP-derived neurons during the early developmental stages. Following GFP vector introduction into the OP of HH13-15 embryos, labeled neurons were detected around and within the lateral TEL at HH23 and HH27. At HH36, labeled cells were observed in the rostral-lateral TEL, including the olfactory bulb (OB) region. GFP-labeled and calretinin-positive neurons were detected in the OB, suggesting that early OP-derived neurons enter the forebrain and function as interneurons in the OB.

Olig2 defines a subset of neural stem cells that produce specific olfactory bulb interneuron subtypes in the subventricular zone of adult mice.

Distinct neural stem cells (NSCs) reside in different regions of the subventricular zone (SVZ) and generate multiple olfactory bulb (OB) interneuron subtypes in the adult brain. However, the molecular mechanisms underlying such NSC heterogeneity remain largely unknown. Here, we show that the basic helix-loop-helix transcription factor Olig2 defines a subset of NSCs in the early postnatal and adult SVZ. Olig2-expressing NSCs exist broadly but are most enriched in the ventral SVZ along the dorsoventral axis complementary to dorsally enriched Gsx2-expressing NSCs. Comparisons of Olig2-expressing NSCs from early embryonic to adult stages using single cell transcriptomics reveal stepwise developmental changes in their cell cycle and metabolic properties. Genetic studies further show that cross-repression contributes to the mutually exclusive expression of Olig2 and Gsx2 in NSCs/progenitors during embryogenesis, but that their expression is regulated independently from each other in adult NSCs. Finally, lineage-tracing and conditional inactivation studies demonstrate that Olig2 plays an important role in the specification of OB interneuron subtypes. Altogether, our study demonstrates that Olig2 defines a unique subset of adult NSCs enriched in the ventral aspect of the adult SVZ.

Estrogens regulate early embryonic development of the olfactory sensory system via estrogen-responsive glia.

Estrogens are well-known to regulate development of sexual dimorphism of the brain; however, their role in embryonic brain development prior to sex-differentiation is unclear. Using estrogen biosensor zebrafish models, we found that estrogen activity in the embryonic brain occurs from early neurogenesis specifically in a type of glia in the olfactory bulb (OB), which we name estrogen-responsive olfactory bulb (EROB) cells. In response to estrogen, EROB cells overlay the outermost layer of the OB and interact tightly with olfactory sensory neurons at the olfactory glomeruli. Inhibiting estrogen activity using an estrogen receptor antagonist, ICI182,780 (ICI), and/or EROB cell ablation impedes olfactory glomerular development, including the topological organisation of olfactory glomeruli and inhibitory synaptogenesis in the OB. Furthermore, activation of estrogen signalling inhibits both intrinsic and olfaction-dependent neuronal activity in the OB, whereas ICI or EROB cell ablation results in the opposite effect on neuronal excitability. Altering the estrogen signalling disrupts olfaction-mediated behaviour in later larval stage. We propose that estrogens act on glia to regulate development of OB circuits, thereby modulating the local excitability in the OB and olfaction-mediated behaviour.

Dissecting the neural divide: a continuous neurectoderm gives rise to the olfactory placode and bulb.

The olfactory epithelia arise from morphologically identifiable structures called olfactory placodes. Sensory placodes are generally described as being induced from the ectoderm suggesting that their development is separate from the coordinated cell movements generating the central nervous system. Previously, we have shown that the olfactory placodes arise from a large field of cells bordering the telencephalic precursors in the neural plate, and that cell movements, not cell division, underlie olfactory placode morphogenesis. Subsequently by image analysis, cells were tracked as they moved in the continuous sheet of neurectoderm giving rise to the peripheral (olfactory organs) and central (olfactory bulbs) nervous system (Torres-Paz and Whitlock, 2014). These studies lead to a model whereby the olfactory epithelia develop from within the border of the neural late and are a neural tube derivative, similar to the retina of the eye (Torres-Paz and Whitlock, 2014; Whitlock, 2008). Here we show that randomly generated clones of cells extend across the morphologically differentiated olfactory placodes/olfactory bulbs, and test the hypothesis that these structures are patterned by a different level of distal-less (dlx) gene expression subdividing the anterior neurectoderm into OP precursors (high Dlx expression) and OB precursors (lower Dlx expression). Manipulation of DLX protein and RNA levels resulted in morphological changes in the size of the olfactory epithelia and olfactory bulb. Thus, the olfactory epithelia and bulbs arise from a common neurectodermal region and develop in concert through coordinated morphological movements.

The evolutionary origins of the vertebrate olfactory system.

Vertebrates develop an olfactory system that detects odorants and pheromones through their interaction with specialized cell surface receptors on olfactory sensory neurons. During development, the olfactory system forms from the olfactory placodes, specialized areas of the anterior ectoderm that share cellular and molecular properties with placodes involved in the development of other cranial senses. The early-diverging chordate lineages amphioxus, tunicates, lampreys and hagfishes give insight into how this system evolved. Here, we review olfactory system development and cell types in these lineages alongside chemosensory receptor gene evolution, integrating these data into a description of how the vertebrate olfactory system evolved. Some olfactory system cell types predate the vertebrates, as do some of the mechanisms specifying placodes, and it is likely these two were already connected in the common ancestor of vertebrates and tunicates. In stem vertebrates, this evolved into an organ system integrating additional tissues and morphogenetic processes defining distinct olfactory and adenohypophyseal components, followed by splitting of the ancestral placode to produce the characteristic paired olfactory organs of most modern vertebrates.

Comparative growth in the olfactory system of the developing chick with considerations for evolutionary studies.

Despite the long-held assumption that olfaction plays a relatively minor role in the behavioral ecology of birds, crown-group avians exhibit marked phylogenetic variation in the size and form of the olfactory apparatus. As part of a larger effort to better understand the role of olfaction and olfactory tissues in the evolution and development of the avian skull, we present the first quantitative analysis of ontogenetic scaling between olfactory features [olfactory bulbs (OBs) and olfactory turbinates] and neighboring structures (cerebrum, total brain, respiratory turbinates) based on the model organism Gallus gallus. The OB develops under the predictions of a concerted evolutionary model with rapid early growth that is quickly overcome by the longer, sustained growth of the larger cerebrum. A similar pattern is found in the nasal cavity where the morphologically simple (non-scrolled) olfactory turbinates appear and mature early, with extended growth characterizing the larger and scrolled respiratory turbinates. Pairwise regressions largely recover allometric relationships among the examined structures, with a notable exception being the isometric trajectory of the OB and olfactory turbinate. Their parallel growth suggests a unique regulatory pathway that is likely driven by the morphogenesis of the olfactory nerve, which serves as a structural bridge between the two features. Still, isometry was not necessarily expected given that the olfactory epithelium covers more than just the turbinate. These data illuminate a number of evolutionary hypotheses that, moving forward, should inform tradeoffs and constraints between the olfactory and neighboring systems in the avian head.

Multifaceted actions of Zeb2 in postnatal neurogenesis from the ventricular-subventricular zone to the olfactory bulb.

The transcription factor Zeb2 controls fate specification and subsequent differentiation and maturation of multiple cell types in various embryonic tissues. It binds many protein partners, including activated Smad proteins and the NuRD co-repressor complex. How Zeb2 subdomains support cell differentiation in various contexts has remained elusive. Here, we studied the role of Zeb2 and its domains in neurogenesis and neural differentiation in the young postnatal ventricular-subventricular zone (V-SVZ), in which neural stem cells generate olfactory bulb-destined interneurons. Conditional Zeb2 knockouts and separate acute loss- and gain-of-function approaches indicated that Zeb2 is essential for controlling apoptosis and neuronal differentiation of V-SVZ progenitors before and after birth, and we identified Sox6 as a potential downstream target gene of Zeb2. Zeb2 genetic inactivation impaired the differentiation potential of the V-SVZ niche in a cell-autonomous fashion. We also provide evidence that its normal function in the V-SVZ also involves non-autonomous mechanisms. Additionally, we demonstrate distinct roles for Zeb2 protein-binding domains, suggesting that Zeb2 partners co-determine neuronal output from the mouse V-SVZ in both quantitative and qualitative ways in early postnatal life.

Regional differences in mitral cell development in mouse olfactory bulb.

Olfactory sensory neurons (OSNs) located in the dorsomedial and ventromedial regions of the olfactory epithelium (OE) are distinguished from one another based on their molecular expression patterns. This difference is reflected in the separation of the glomerular layer of the olfactory bulb (OB) into dorsomedial and ventrolateral regions. However, it is unclear whether a complementary separation is also evident in the projection neurons that innervate the OB glomeruli. In this study, we compared the development of the OB between different regions by focusing on the transcription factor, Tbx21, which is expressed by mitral and tufted cells in the mature OB. Examining the OB at different developmental ages, we found that Tbx21 expression commenced in the anteromedial region called the tongue-shaped area, followed by the dorsomedial and then ventrolateral areas. We also showed that the tongue-shaped area was innervated by the OSNs located in the most dorsomedial part of the ventrolateral OE, the V-zone:DM. Interestingly, the generation of OSNs occurred first in the dorsomedial zone including the V-zone:DM, suggesting a correlation between the time course of OSN generation in the OE and Tbx21 expression in their target region of the OB. In contrast, expression of vGluT1, which is also found in all mitral cells in the mature OB, was first detected in the ventrolateral region during development. Our findings demonstrate that the development of projection neurons occurs in a compartmentalized manner in the OB; tongue-shaped, dorsomedial, and ventrolateral areas, and that not all projection neurons follow the same developmental pathway.

Dlx1/2 are Central and Essential Components in the Transcriptional Code for Generating Olfactory Bulb Interneurons.

Generation of olfactory bulb (OB) interneurons requires neural stem/progenitor cell specification, proliferation, differentiation, and young interneuron migration and maturation. Here, we show that the homeobox transcription factors Dlx1/2 are central and essential components in the transcriptional code for generating OB interneurons. In Dlx1/2 constitutive null mutants, the differentiation of GSX2+ and ASCL1+ neural stem/progenitor cells in the dorsal lateral ganglionic eminence is blocked, resulting in a failure of OB interneuron generation. In Dlx1/2 conditional mutants (hGFAP-Cre; Dlx1/2F/- mice), GSX2+ and ASCL1+ neural stem/progenitor cells in the postnatal subventricular zone also fail to differentiate into OB interneurons. In contrast, overexpression of Dlx1&2 in embryonic mouse cortex led to ectopic production of OB-like interneurons that expressed Gad1, Sp8, Sp9, Arx, Pbx3, Etv1, Tshz1, and Prokr2. Pax6 mutants generate cortical ectopia with OB-like interneurons, but do not do so in compound Pax6; Dlx1/2 mutants. We propose that DLX1/2 promote OB interneuron development mainly through activating the expression of Sp8/9, which further promote Tshz1 and Prokr2 expression. Based on this study, in combination with earlier ones, we propose a transcriptional network for the process of OB interneuron development.

Neural stem cells: origin, heterogeneity and regulation in the adult mammalian brain.

In the adult rodent brain, neural stem cells (NSCs) persist in the ventricular-subventricular zone (V-SVZ) and the subgranular zone (SGZ), which are specialized niches in which young neurons for the olfactory bulb (OB) and hippocampus, respectively, are generated. Recent studies have significantly modified earlier views on the mechanisms of NSC self-renewal and neurogenesis in the adult brain. Here, we discuss the molecular control, heterogeneity, regional specification and cell division modes of V-SVZ NSCs, and draw comparisons with NSCs in the SGZ. We highlight how V-SVZ NSCs are regulated by local signals from their immediate neighbors, as well as by neurotransmitters and factors that are secreted by distant neurons, the choroid plexus and vasculature. We also review recent advances in single cell RNA analyses that reveal the complexity of adult neurogenesis. These findings set the stage for a better understanding of adult neurogenesis, a process that one day may inspire new approaches to brain repair.

Repeated gestational exposure to diesel engine exhaust affects the fetal olfactory system and alters olfactory-based behavior in rabbit offspring.

BACKGROUND: Airborne pollution, especially from diesel exhaust (DE), is known to have a negative effect on the central nervous system in exposed human populations. However, the consequences of gestational exposure to DE on the fetal brain remain poorly explored, with various effects depending on the conditions of exposure, as well as little information on early developmental stages. We investigated the short-term effects of indirect DE exposure throughout gestation on the developing brain using a rabbit model. We analyzed fetal olfactory tissues at the end of gestation and tested behaviors relevant to pups' survival at birth. Pregnant dams were exposed by nose-only inhalation to either clean air or DE with a content of particles (DEP) adjusted to 1 mg/m3 by diluting engine exhaust, for 2 h/day, 5 days/week, from gestational day 3 (GD3) to day 27 (GD27). At GD28, fetal olfactory mucosa, olfactory bulbs and whole brains were collected for anatomical and neurochemical measurements. At postnatal day 2 (PND2), pups born from another group of exposed or control female were examined for their odor-guided behavior in response to the presentation of the rabbit mammary pheromone 2-methyl-3-butyn-2-ol (2MB2). RESULTS: At GD28, nano-sized particles were observed in cilia and cytoplasm of the olfactory sensory neurons in the olfactory mucosa and in the cytoplasm of periglomerular cells in the olfactory bulbs of exposed fetuses. Moreover, cellular and axonal hypertrophies were observed throughout olfactory tissues. Concomitantly, fetal serotoninergic and dopaminergic systems were affected in the olfactory bulbs. Moreover, the neuromodulatory homeostasis was disturbed in a sex-dependent manner in olfactory tissues. At birth, the olfactory sensitivity to 2MB2 was reduced in exposed PND2 pups. CONCLUSION: Gestational exposure to DE alters olfactory tissues and affects monoaminergic neurotransmission in fetuses' olfactory bulbs, resulting in an alteration of olfactory-based behaviors at birth. Considering the anatomical and functional continuum between the olfactory system and other brain structures, and due to the importance of monoamine neurotransmission in the plasticity of neural circuits, such alterations could participate to disturbances in higher integrative structures, with possible long-term neurobehavioral consequences.

Immunohistochemical analysis of the development of olfactory organs in two species of turtles Pelodiscus sinensis and Mauremys reevesii.

The nasal cavity of turtles is composed of the upper and lower chambers, lined by the upper and lower chamber epithelia, respectively. In many turtles including the Reeve's turtle Mauremys reevesii, the upper chamber epithelium contains ciliated olfactory receptor neurons (ORNs) and the lower chamber epithelium contains microvillous ORNs. However, in the olfactory organ of the Chinese soft-shelled turtle Pelodiscus sinensis, both the upper and lower chamber epithelia contain ciliated ORNs. In the present study, we immunohistochemically examined the developmental process of olfactory organs in soft-shelled turtle and the Reeve's turtle to clarify the developmental origins of the lower chamber epithelium in these turtles. Obtained data indicate that olfactory organs of these turtles have identical origin and follow similar process of development, suggesting that, in the lower chamber epithelium of the nasal cavity, ciliated ORNs differentiate in soft-shelled turtle whereas microvillous ORNs differentiate in the Reeve's turtle.

Olfactory ensheathing cells abutting the embryonic olfactory bulb express Frzb, whose deletion disrupts olfactory axon targeting.

We and others previously showed that in mouse embryos lacking the transcription factor Sox10, olfactory ensheathing cell (OEC) differentiation is disrupted, resulting in defective olfactory axon targeting and fewer gonadotropin-releasing hormone (GnRH) neurons entering the embryonic forebrain. The underlying mechanisms are unclear. Here, we report that OECs in the olfactory nerve layer express Frzb-encoding a secreted Wnt inhibitor with roles in axon targeting and basement membrane breakdown-from embryonic day (E)12.5, when GnRH neurons first enter the forebrain, until E16.5, the latest stage examined. The highest levels of Frzb expression are seen in OECs in the inner olfactory nerve layer, abutting the embryonic olfactory bulb. We find that Sox10 is required for Frzb expression in OECs, suggesting that loss of Frzb could explain the olfactory axon targeting and/or GnRH neuron migration defects seen in Sox10-null mice. At E16.5, Frzb-null embryos show significant reductions in both the volume of the olfactory nerve layer expressing the maturation marker Omp and the number of Omp-positive olfactory receptor neurons in the olfactory epithelium. As Omp upregulation correlates with synapse formation, this suggests that Frzb deletion indeed disrupts olfactory axon targeting. In contrast, GnRH neuron entry into the forebrain is not significantly affected. Hence, loss of Frzb may contribute to the olfactory axon targeting phenotype, but not the GnRH neuron phenotype, of Sox10-null mice. Overall, our results suggest that Frzb secreted from OECs in the olfactory nerve layer is important for olfactory axon targeting.

Morphological and Cellular Characterization of the Fetal Canine (Canis lupus familiaris) Subventricular Zone, Rostral Migratory Stream, and Olfactory Bulb.

The existence of neurogenesis in the adult brain is a widely recognized phenomenon, occurring in the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone of the dentate gyrus in several vertebrate species. Neural precursors originated in the SVZ migrate to the main olfactory bulb (MOB), originating the rostral migratory stream (RMS) in the process. To better understand the formation of the adult neurogenic niches in dogs, we investigated the cellular composition and morphological organization of these areas in 57 days-old dog fetuses. Using multiple immunohistochemical markers, we demonstrated that the SVZ in the canine fetus is remarkably similar to the adult SVZ, with glial GFAP-immunoreactive (-ir) cells, DCX-ir neuroblasts and SOX2-ir neuronal progenitors tangentially organized along the dorsal lateral ventricle. The fetal RMS has all the features of its adult counterpart and closely resembles the RMS of other mammalian species. The late-development canine MOB has most of the neurochemical features of the adult MOB, including an early-developed TH-ir population and maturing CALR-ir interneurons, but CALB-ir neurons in the granule cell layer will only appear in the post-partum period. Taken together, our results suggest that the canine fetal development of adult neurogenic niches closely resembles those of primates, and dogs may be suitable models of human adult neurogenesis. Anat Rec, 301:1570-1584, 2018. © 2018 Wiley Periodicals, Inc.

Embryonic and postnatal neurogenesis produce functionally distinct subclasses of dopaminergic neuron.

Most neurogenesis in the mammalian brain is completed embryonically, but in certain areas the production of neurons continues throughout postnatal life. The functional properties of mature postnatally generated neurons often match those of their embryonically produced counterparts. However, we show here that in the olfactory bulb (OB), embryonic and postnatal neurogenesis produce functionally distinct subpopulations of dopaminergic (DA) neurons. We define two subclasses of OB DA neuron by the presence or absence of a key subcellular specialisation: the axon initial segment (AIS). Large AIS-positive axon-bearing DA neurons are exclusively produced during early embryonic stages, leaving small anaxonic AIS-negative cells as the only DA subtype generated via adult neurogenesis. These populations are functionally distinct: large DA cells are more excitable, yet display weaker and - for certain long-latency or inhibitory events - more broadly tuned responses to odorant stimuli. Embryonic and postnatal neurogenesis can therefore generate distinct neuronal subclasses, placing important constraints on the functional roles of adult-born neurons in sensory processing.

Regulation of continuous but complex expression pattern of Six1 during early sensory development.

BACKGROUND: In vertebrates, cranial sensory placodes give rise to neurosensory and endocrine structures, such as the olfactory epithelium, inner ear, and anterior pituitary. We report here the establishment of a transgenic mouse line that expresses Cre recombinase under the control of Six1-21, a major placodal enhancer of the homeobox gene Six1. RESULTS: In the new Cre-expressing line, mSix1-21-NLSCre, the earliest Cre-mediated recombination was induced at embryonic day 8.5 in the region overlapping with the otic-epibranchial progenitor domain (OEPD), a transient, common precursor domain for the otic and epibranchial placodes. Recombination was later observed in the OEPD-derived structures (the entire inner ear and the VIIth-Xth cranial sensory ganglia), olfactory epithelium, anterior pituitary, pharyngeal ectoderm and pouches. Other Six1-positive structures, such as salivary/lacrimal glands and limb buds, were also positive for recombination. Moreover, comparison with another mouse line expressing Cre under the control of the sensory neuron enhancer, Six1-8, indicated that the continuous and complex expression pattern of Six1 during sensory organ formation is pieced together by separate enhancers. CONCLUSIONS: mSix1-21-NLSCre has several unique characteristics to make it suitable for analysis of cell lineage and gene function in sensory placodes as well as nonplacodal Six1-positive structures. Developmental Dynamics 247:250-261, 2018. © 2017 Wiley Periodicals, Inc.

Organizing activity of Fgf8 on the anterior telencephalon.

The anterior part of the embryonic telencephalon gives rise to several brain regions that are important for animal behavior, including the frontal cortex (FC) and the olfactory bulb. The FC plays an important role in decision-making behaviors, such as social and cognitive behavior, and the olfactory bulb is involved in olfaction. Here, we show the organizing activity of fibroblast growth factor 8 (Fgf8) in the regionalization of the anterior telencephalon, specifically the FC and the olfactory bulb. Misexpression of Fgf8 in the most anterior part of the mouse telencephalon at embryonic day 11.5 (E11.5) by ex utero electroporation resulted in a lateral shift of dorsal FC subdivision markers and a lateral expansion of the dorsomedial part of the FC, the future anterior cingulate and prelimbic cortex. Fgf8-transfected brains had lacked ventral FC, including the future orbital cortex, which was replaced by the expanded olfactory bulb. The olfactory region occupied a larger area of the FC when transfection efficiency of Fgf8 was higher. These results suggest that Fgf8 regulates the proportions of the FC and olfactory bulb in the anterior telencephalon and has a medializing effect on the formation of FC subdivisions.

The temporal expression profile of a Nos3-related natural antisense RNA in the brain suggests a possible role in neurogenesis.

Experimental work over the past several years has revealed an unexpected abundance of long natural antisense transcripts (NATs) in eukaryotic species. In light of the proposed role of such RNA molecules in the regulation of gene expression in the brain, attention is now focused on specific examples of neuronal NATs. Of particular interest are NATs that are complementary to mRNAs encoding nitric oxide synthase (NOS), the enzyme responsible for production of the important gaseous neurotransmitter nitric oxide (NO). Here we study the temporal expression profile of murine Nos3as NAT in the brain. Notably, Nos3as NAT is known to act as a negative regulator of Nos3 gene expression. The results of our quantitative analysis reveal differential expression of Nos3as NAT during embryonic and post-embryonic stages of development of the brain. Also, they show that the low levels of Nos3as NAT coincides with active neurogenesis. In addition we report on an inverse correlation between the relative expression level of Nos3as NAT and the level of Nos3 protein. Thus our data raise the hypothesis that the Nos3as NAT regulates neurogenesis through suppression of Nos3 gene activity. This idea is further supported by experiments conducted on the olfactory bulbs and cultured neuroblastoma cells.

Olfactory sensory axons target specific protoglomeruli in the olfactory bulb of zebrafish.

BACKGROUND: The axons of Olfactory Sensory Neurons (OSNs) project to reproducible target locations within the Olfactory Bulb (OB), converting odorant experience into a spatial map of neural activity. We characterized the initial targeting of OSN axons in the zebrafish, a model system suitable for studying axonal targeting early in development. In this system the initial targets of OSN axons are a small number of distinct, individually identifiable neuropilar regions called protoglomeruli. Previously, Olfactory Marker Protein-expressing and TRPC2-expressing classes of OSNs were shown to project to specific, non-overlapping sets of protoglomeruli, indicating that particular subsets of OSNs project to specific protoglomerular targets. We set out to map the relationship between the classical Odorant Receptor (OR) an OSN chooses to express and the protoglomerulus its axon targets. METHODS: A panel of BACs were recombineered so that the axons of OSNs choosing to express modified ORs were fluorescently labeled. Axon projections were followed into the olfactory bulb to determine the protoglomeruli in which they terminated. RESULTS: RNA-seq demonstrates that OSNs express a surprisingly wide variety of ORs and Trace Amine Associated Receptors (TAARs) very early when sensory axons are arriving in the bulb. Only a single OR is expressed in any given OSN even at these early developmental times. We used a BAC expression technique to map the trajectories of OSNs expressing specific odorant receptors. ORs can be divided into three clades based upon their sequence similarities. OSNs expressing ORs from two of these clades project to the CZ protoglomerulus, while OSNs expressing ORs from the third clade project to the DZ protoglomerulus. In contrast, OSNs expressing a particular TAAR project to multiple protoglomeruli. Neither OR choice nor axonal targeting are related to the position an OSN occupies within the olfactory pit. CONCLUSIONS: Our results demonstrate that it is not the choice of a particular OR, but of one from a category of ORs, that is related to initial OSN target location within the olfactory bulb. These choices are not related to OSN position within the olfactory epithelium.

Extrinsic mechanical forces mediate retrograde axon extension in a developing neuronal circuit.

To form functional neural circuits, neurons migrate to their final destination and extend axons towards their targets. Whether and how these two processes are coordinated in vivo remains elusive. We use the zebrafish olfactory placode as a system to address the underlying mechanisms. Quantitative live imaging uncovers a choreography of directed cell movements that shapes the placode neuronal cluster: convergence of cells towards the centre of the placodal domain and lateral cell movements away from the brain. Axon formation is concomitant with lateral movements and occurs through an unexpected, retrograde mode of extension, where cell bodies move away from axon tips attached to the brain surface. Convergence movements are active, whereas cell body lateral displacements are of mainly passive nature, likely triggered by compression forces from converging neighbouring cells. These findings unravel a previously unknown mechanism of neuronal circuit formation, whereby extrinsic mechanical forces drive the retrograde extension of axons.How neuronal migration and axon growth coordinate during development is only partially understood. Here the authors use quantitative imaging to characterise the morphogenesis of the zebrafish olfactory placode and report an unexpected phenomenon, whereby axons extend through the passive movement of neuron cell bodies away from tethered axon tips.