The influence of the olfactory placode on the development of the telencephalon in Xenopus laevis

Why Does the Face Predict the Brain? Neural Crest Induction, Craniofacial Morphogenesis, and Neural Circuit Development

Mesenchephalic and rhombencephalic neural crest cells generate the craniofacial skeleton, special sensory organs, and subsets of cranial sensory receptor neurons. They do so while preserving the anterior-posterior (A-P) identity of their neural tube origins. This organizational principle is paralleled by central nervous system circuits that receive and process information from facial structures whose A-P identity is in register with that in the brain. Prior to morphogenesis of the face and its circuits, however, neural crest cells act as "inductive ambassadors" from distinct regions of the neural tube to induce differentiation of target craniofacial domains and establish an initial interface between the brain and face. At every site of bilateral, non-axial secondary induction, neural crest constitutes all or some of the mesenchymal compartment for non-axial mesenchymal/epithelial (M/E) interactions. Thus, for epithelial domains in the craniofacial primordia, aortic arches, limbs, the spinal cord, and the forebrain (Fb), neural crest-derived mesenchymal cells establish local sources of inductive signaling molecules that drive morphogenesis and cellular differentiation. This common mechanism for building brains, faces, limbs, and hearts, A-P axis specified, neural crest-mediated M/E induction, coordinates differentiation of distal structures, peripheral neurons that provide their sensory or autonomic innervation in some cases, and central neural circuits that regulate their behavioral functions. The essential role of this neural crest-mediated mechanism identifies it as a prime target for pathogenesis in a broad range of neurodevelopmental disorders. Thus, the face and the brain "predict" one another, and this mutual developmental relationship provides a key target for disruption by developmental pathology.

The role of non-neuronal cells in hypogonadotropic hypogonadism

The hypothalamic-pituitary-gonadal axis is controlled by gonadotropin-releasing hormone (GnRH) released by the hypothalamus. Disruption of this system leads to impaired reproductive maturation and function, a condition known as hypogonadotropic hypogonadism (HH). Most studies to date have focused on genetic causes of HH that impact neuronal development and function. However, variants may also impact the functioning of non-neuronal cells known as glia. Glial cells make up 50% of brain cells of humans, primates, and rodents. They include radial glial cells, microglia, astrocytes, tanycytes, oligodendrocytes, and oligodendrocyte precursor cells. Many of these cells influence the hypothalamic neuroendocrine system controlling fertility. Indeed, glia regulate GnRH neuronal activity and secretion, acting both at their cell bodies and their nerve endings. Recent work has also made clear that these interactions are an essential aspect of how the HPG axis integrates endocrine, metabolic, and environmental signals to control fertility. Recognition of the clinical importance of interactions between glia and the GnRH network may pave the way for the development of new treatment strategies for dysfunctions of puberty and adult fertility.

The African clawed frog Xenopus laevis: A model organism to study regeneration of the central nervous system

While an injury to the central nervous system (CNS) in humans and mammals is irreversible, amphibians and teleost fish have the capacity to fully regenerate after severe injury to the CNS. Xenopus laevis has a high potential to regenerate the brain and spinal cord during larval stages (47-54), and loses this capacity during metamorphosis. The optic nerve has the capacity to regenerate throughout the frog's lifespan. Here, we review CNS regeneration in frogs, with a focus in X. laevis, but also provide some information about X. tropicalis and other frogs. We start with an overview of the anatomy of the Xenopus CNS, including the main supraspinal tracts that emerge from the brain stem, which play a key role in motor control and are highly conserved across vertebrates. We follow with the advantages of using Xenopus, a classical laboratory model organism, with increasing availability of genetic tools like transgenesis and genome editing, and genomic sequences for both X. laevis and X. tropicalis. Most importantly, Xenopus provides the possibility to perform intra-species comparative experiments between regenerative and non-regenerative stages that allow the identification of which factors are permissive for neural regeneration, and/or which are inhibitory. We aim to provide sufficient evidence supporting how useful Xenopus can be to obtain insights into our understanding of CNS regeneration, which, complemented with studies in mammalian vertebrate model systems, can provide a collaborative road towards finding novel therapeutic approaches for injuries to the CNS.

Role of Rb during Neurogenesis and Axonal Guidance in the Developing Olfactory System

The Retinoblastoma protein, Rb, was shown to regulate distinct aspects of neurogenesis in the embryonic and adult brain besides its primary role in cell cycle control. It is still unknown, however, whether Rb is required for tissue morphogenesis and the establishment of synaptic connections between adjacent tissues during development. We have investigated here the role of Rb during development of the olfactory system (OS), which heavily relies on reciprocal interactions between the olfactory epithelium (OE) and the olfactory bulb (OB). We show that mice carrying a telencephalic-specific deletion of Rb display several neurogenic defects in the OS during late development. In the OE, loss of Rb leads to ectopic proliferation of late-born progenitors (Tuj-1+), abnormal radial migration and terminal maturation of olfactory sensory neurons (OSNs). In the OB, deletion of Rb causes severe lamination defects with loss of clear boundaries between distinct layers. Importantly, starting around E15.5 when OB glomerulogenesis is initiated, many OSNs axons that project along the olfactory nerve layer (ONL) fail to properly innervate the nascent bulb, thus resulting in partial loss of connectivity between OE-OB and gradual neuronal degeneration in both tissues peaking at birth. This deficiency correlates with deregulated expressions of two key chemo-repellant molecules, Robo2/Slit1 and Nrp2/Sema3F that control the formation of dorsal-ventral topographic map of OSNs connections with OB glomeruli. This study highlights a critical requirement for Rb during neurogenesis and the establishment of proper synaptic connections inside the OS during development.

GnRH, anosmia and hypogonadotropic hypogonadism--where are we?

Gonadotropin releasing hormone (GnRH) neurons originate the nasal placode and migrate into the brain during prenatal development. Once within the brain, these cells become integral components of the hypothalamic-pituitary-gonadal axis, essential for reproductive function. Disruption of this system causes hypogonadotropic hypogonadism (HH). HH associated with anosmia is clinically defined as Kallman syndrome (KS). Recent work examining the developing nasal region has shed new light on cellular composition, cell interactions and molecular cues responsible for the development of this system in different species. This review discusses some developmental aspects, animal models and current advancements in our understanding of pathologies affecting GnRH. In addition we discuss how development of neural crest derivatives such as the glia of the olfactory system and craniofacial structures control GnRH development and reproductive function.

Neural crest and olfactory system: new prospective

Sensory neurons in vertebrates are derived from two embryonic transient cell sources: neural crest (NC) and ectodermal placodes. The placodes are thickenings of ectodermal tissue that are responsible for the formation of cranial ganglia as well as complex sensory organs that include the lens, inner ear, and olfactory epithelium. The NC cells have been indicated to arise at the edges of the neural plate/dorsal neural tube, from both the neural plate and the epidermis in response to reciprocal interactions Moury and Jacobson (Dev Biol 141:243-253, 1990). NC cells migrate throughout the organism and give rise to a multitude of cell types that include melanocytes, cartilage and connective tissue of the head, components of the cranial nerves, the dorsal root ganglia, and Schwann cells. The embryonic definition of these two transient populations and their relative contribution to the formation of sensory organs has been investigated and debated for several decades (Basch and Bronner-Fraser, Adv Exp Med Biol 589:24-31, 2006; Basch et al., Nature 441:218-222, 2006) review (Baker and Bronner-Fraser, Dev Biol 232:1-61, 2001). Historically, all placodes have been described as exclusively derived from non-neural ectodermal progenitors. Recent genetic fate-mapping studies suggested a NC contribution to the olfactory placodes (OP) as well as the otic (auditory) placodes in rodents (Murdoch and Roskams, J Neurosci Off J Soc Neurosci 28:4271-4282, 2008; Murdoch et al., J Neurosci 30:9523-9532, 2010; Forni et al., J Neurosci Off J Soc Neurosci 31:6915-6927, 2011b; Freyer et al., Development 138:5403-5414, 2011; Katoh et al., Mol Brain 4:34, 2011). This review analyzes and discusses some recent developmental studies on the OP, placodal derivatives, and olfactory system.

From the periphery to the brain: Wiring the olfactory system

The olfactory system represents a perfect model to study the interactions between the central and peripheral nervous systems in order to establish a neural circuit during early embryonic development. In addition, another important feature of this system is the capability to integrate new cells generated in two neurogenic zones: the olfactory epithelium in the periphery and the wall of the lateral ventricles in the CNS, both during development and adulthood. In all these processes the combination and sequence of specific molecular signals plays a critical role in the wiring of the olfactory axons, as well as the precise location of the incoming cell populations to the olfactory bulb. The purpose of this review is to summarize recent insights into the cellular and molecular events that dictate cell settling position and axonal trajectories from their origin in the olfactory placode to the formation of synapses in the olfactory bulb to ensure rapid and reliable transmission of olfactory information from the nose to the brain.

Disorganized olfactory bulb lamination in mice deficient for transcription factor AP-2epsilon

Within the olfactory bulb, neurons and their axonal connections are organized into distinct layers corresponding to different functionalities. Here we demonstrate that transcription factor AP-2epsilon is required for olfactory bulb development, specifically the establishment of appropriate neuronal lamination. During normal mouse embryogenesis, AP-2epsilon transcripts are one of the earliest markers of olfactory bulb formation, and expression eventually becomes refined to the projection neurons, the mitral and tufted cells. To assess the function of AP-2epsilon in olfaction, we generated a null allele (the "AK" allele) by inserting a Cre recombinase transgene into the endogenous AP-2epsilon genomic locus. AP-2epsilon-null mice exhibited defective olfactory bulb architecture. The mitral cell layer was disorganized, typified by misoriented and aberrantly positioned projection neurons, and the adjacent internal plexiform layer was absent. Despite the significant disruption of olfactory bulb organization, AP-2epsilon null mice were viable and could discriminate a variety of odors. AP-2epsilon-null mice thus provide compelling evidence for the robust nature of the mouse olfactory system, and serve as a model system to probe both the regulation of neuronal lamination and the functional circuitry of the olfactory bulb. We also show that Cre recombinase expression directed by the AP-2epsilon locus can specifically target floxed genes within the olfactory bulb, extending the utility of this AK allele.

Formation and patterning of the forebrain and olfactory system by zinc-finger genes Fezf1 and Fezf2

The zinc finger genes Fezf1 (Fez) and Fezf2 (Fez-like, Fezl, Zfp312) were initially identified as anterior neuroectoderm-specific genes in Xenopus and zebrafish. They encode transcriptional regulators containing an Engrailed homology 1 (Eh1) repressor motif, which is known to interact with Groucho/TLE (Transducin-Like Enhancer of Split)-type transcriptional co-repressors. Both Fezf1 and Fezf2 are expressed in the prospective forebrain region during early embryogenesis, and they subsequently show both overlapping and distinct expression domains in the olfactory epithelium and forebrain. Loss-of-function studies in mouse and zebrafish revealed roles for Fezf1 and Fezf2 in the development of the olfactory system and forebrain. In mice, Fezf1, expressed in olfactory sensory neurons, is required for the axonal projection of olfactory sensory neurons, and controls the layer formation of the olfactory bulb in a non-cell autonomous manner. Fezf2 is involved in the differentiation of subplate neurons and the formation of the fimbria and fornix. Fezf2 is also essential for specification of the subcerebral projection neurons in the neocortex. Fezf1 and Fezf2 control the rostro-caudal patterning of the diencephalon by repressing the caudal diencephalon fate in the rostral diencephalon in mice and zebrafish. In zebrafish, fezf2 is also required for the development of monoaminergic (dopaminergic and serotonergic) neurons in the basal forebrain.

Influence of olfactory epithelium on mitral/tufted cell dendritic outgrowth

Stereotypical connections between olfactory sensory neuron axons and mitral cell dendrites in the olfactory bulb establish the first synaptic relay for olfactory perception. While mechanisms of olfactory sensory axon targeting are reported, molecular regulation of mitral cell dendritic growth and refinement are unclear. During embryonic development, mitral cell dendritic distribution overlaps with olfactory sensory axon terminals in the olfactory bulb. In this study, we investigate whether olfactory sensory neurons in the olfactory epithelium influence mitral cell dendritic outgrowth in vitro. We report a soluble trophic activity in the olfactory epithelium conditioned medium which promotes mitral/tufted cell neurite outgrowth. While the trophic activity is present in both embryonic and postnatal olfactory epithelia, only embryonic but not postnatal mitral/tufted cells respond to this activity. We show that BMP2, 5 and 7 promote mitral/tufted cells neurite outgrowth. However, the BMP antagonist, Noggin, fails to neutralize the olfactory epithelium derived neurite growth promoting activity. We provide evidence that olfactory epithelium derived activity is a protein factor with molecular weight between 50–100 kD. We also observed that Follistatin can effectively neutralize the olfactory epithelium derived activity, suggesting that TGF-beta family proteins are involved to promote mitral/tufted dendritic elaboration.

Sall1 regulates mitral cell development and olfactory nerve extension in the developing olfactory bulb

Sall1 is a zinc finger containing transcription factor that is highly expressed during mammalian embryogenesis. In humans, the developmental disorder Townes Brocks Syndrome is associated with mutations in the SALL1 gene. Sall1-deficient animals die at birth due to kidney deficits; however, its function in the nervous system has not been characterized. We examined the role of Sall1 in the developing olfactory system. We demonstrate that Sall1 is expressed by cells in the olfactory epithelium and olfactory bulb (OB). Sall1-deficient OBs are reduced in size and exhibit alterations in neurogenesis and mitral cell production. In addition, the olfactory nerve failed to extend past the ventral-medial region of the OB in Sall1-deficient animals. We observed intrinsic patterns of neurogenesis during olfactory development in control animals. In Sall1-mutant animals, these patterns of neurogenesis were disrupted. These findings suggest a role for Sall1 in regulating neuronal differentiation and maturation in developing neural structures.

Role of a transcription factor Pax6 in the developing vertebrate olfactory system

The olfactory system is responsible for capturing and processing odorant information, which significantly influences a variety of behaviors in animals. The vertebrate olfactory system consists of several neuronal components including the olfactory epithelium, olfactory bulb and olfactory cortex, which originate from distinct embryonic tissues. The transcription factor Pax6 is strongly expressed in the embryonic and postnatal olfactory systems, and regulates neuronal specification, migration and differentiation. Here we review classical and recent studies focusing on the role of Pax6 in the developing olfactory system, and highlight the cellular and molecular mechanisms underlying the highly coordinated developmental processes of the vertebrate olfactory system.

Brain regeneration in anuran amphibians

Urodele amphibians are highly regenerative animals. After partial removal of the brain in urodeles, ependymal cells around the wound surface proliferate, differentiate into neurons and glias and finally regenerate the lost tissue. In contrast to urodeles, this type of brain regeneration is restricted only to the larval stages in anuran amphibians (frogs). In adult frogs, whereas ependymal cells proliferate in response to brain injury, they cannot migrate and close the wound surface, resulting in the failure of regeneration. Therefore frogs, in particular Xenopus, provide us with at least two modes to study brain regeneration. One is to study normal regeneration by using regenerative larvae. In this type of study, the requirement of reconnection between a regenerating brain and sensory neurons was demonstrated. Functional restoration of a regenerated telencephalon was also easily evaluated because Xenopus shows simple responses to the stimulus of a food odor. The other mode is to compare regenerative larvae and non-regenerative adults. By using this mode, it is suggested that there are regeneration-competent cells even in the non-regenerative adult brain, and that immobility of those cells might cause the failure of regeneration. Here we review studies that have led to these conclusions.

Mice without transcription factor KLF7 provide new insight into olfactory bulb development

Recent genetic studies have excluded that peripheral innervation plays a substantial role in the initial outgrowth of the olfactory bulb. Mice without Kruppel-like factor 7 activity die at birth and display hypoplastic olfactory bulbs which lack peripheral innervation. Here, we report that incomplete penetrance of the mutation is responsible for partial bulb innervation in a small fraction of Klf7 null mice. Analysis of the partially innervated bulbs of mutant embryos, newborns and adult mice revealed an obligatory correlation with local restoration of laminar architecture, neuronal cell differentiation and neuronal activity. The degree of normal OB maturation in Klf7-/- OBs was proportional to the degree of peripheral innervation. These findings therefore indicate that peripheral innervation contributes to bulb maturation late in development by promoting cell morphogenesis and differentiation.

Functional regeneration of the olfactory bulb requires reconnection to the olfactory nerve in Xenopus larvae

Larvae of the South African clawed frog (Xenopus laevis) can regenerate the telencephalon, which consists of the olfactory bulb and the cerebrum, after it has been partially removed. Some authors have argued that the telencephalon, once removed, must be reconnected to the olfactory nerve in order to regenerate. However, considerable regeneration has been observed before reconnection. Therefore, we have conducted several experiments to learn whether or not reconnection is a prerequisite for regeneration. We found that the olfactory bulb did not regenerate without reconnection, while the cerebrum regenerated by itself. On the other hand, when the brain was reconnected by the olfactory nerve, both the cerebrum and the olfactory bulb regenerated. Morphological and histological investigation showed that the regenerated telencephalon was identical to the intact one in morphology, types and distributions of cells, and connections between neurons. Froglets with a regenerated telencephalon also recovered olfaction, the primary function of the frog telencephalon. These results suggest that the Xenopus larva requires reconnection of the regenerating brain to the olfactory nerve in order to regenerate the olfactory bulb, and thus the regenerated brain functions, in order to process olfactory information.

Zinc-finger gene Fez in the olfactory sensory neurons regulates development of the olfactory bulb non-cell-autonomously

Fez is a zinc-finger gene encoding a transcriptional repressor that is expressed in the olfactory epithelium, hypothalamus, ventrolateral pallium and prethalamus at mid-gestation. To reveal its function, we generated Fez-deficient mice. The Fez-deficient mice showed several abnormalities in the olfactory system: (1) impaired axonal projection of the olfactory sensory neurons; (2) reduced size of the olfactory bulb; (3) abnormal layer formation in the olfactory bulb; and (4) aberrant rostral migration of the interneuron progenitors. Fez was not expressed in the projection neurons, interneurons or interneuron progenitors. Transgene-mediated expression of Fez in olfactory sensory neurons significantly rescued the abnormalities in olfactory axon projection and in the morphogenesis of the olfactory bulb in Fez-knockout mice. Thus, Fez is cell-autonomously required for the axon termination of olfactory sensory neurons, and Fez non-cell-autonomously controls layer formation and interneuron development in the olfactory bulb. These findings suggest that signals from olfactory sensory neurons contribute to the proper formation of the olfactory bulb.

Noses and neurons: induction, morphogenesis, and neuronal differentiation in the peripheral olfactory pathway

Non-axial mesenchymal/epithelial (M/E) induction guides peripheral olfactory pathway differentiation using cellular and molecular mechanisms similar to those in the developing limbs, aortic arches, and branchial arches. At each of these bilaterally symmetric sites off the midline axis, a thickened ectodermal epithelium is apposed to a specialized mesenchyme derived largely, but not exclusively, from the neural crest. The capacity of M/E interaction in the olfactory primordia (the combined olfactory placodal epithelium and adjacent mesenchyme) to induce a distinct class of sensory receptor neurons-olfactory receptor neurons-suggests that this mechanism has been modified to accommodate neurogenesis, neurite outgrowth, and axon guidance, in addition to musculoskeletal differentiation, chondrogenesis, and vasculogenesis. Accordingly, although the olfactory primordia share signaling molecules and transcriptional regulators with other bilaterally symmetric, non-axial sites such as limb buds, their activity may be adapted to mediate distinct aspects of cellular differentiation and process outgrowth during the initial assembly of a sensory pathway-the primary olfactory pathway-during early forebrain development.

Olfactory epithelium influences the orientation of mitral cell dendrites during development

We have established previously that, although the olfactory epithelium is absent in the homozygous Pax-6 mutant mouse, an olfactory bulb-like structure (OBLS) does develop. Moreover, this OBLS contains cells that correspond to mitral cells, the primary projection neurons in the olfactory bulb. The current study aimed to address whether the dendrites of mitral cells in the olfactory bulb or in the OBLS mitral-like cells, exhibit a change in orientation in the presence of the olfactory epithelium. The underlying hypothesis is that the olfactory epithelium imparts a trophic signal on mitral and mitral-like cell that influences the growth of their primary dendrites, orientating them toward the surface of the olfactory bulb. Hence, we cultured hemibrains from wild-type and Pax 6 mutant mice from two different embryonic stages (embryonic days 14 and 15) either alone or in coculture with normal olfactory epithelial explants or control tissue (cerebellum). Our results indicate that the final dendritic orientation of mitral and mitral-like cells is directly influenced both by age and indeed by the presence of the olfactory epithelium.

Arx homeobox gene is essential for development of mouse olfactory system

The olfactory system provides an excellent model in which to study cell proliferation, migration, differentiation, axon guidance, dendritic morphogenesis, and synapse formation. We report here crucial roles of the Arx homeobox gene in the developing olfactory system by analyzing its mutant phenotypes. Arx protein was expressed strongly in the interneurons and weakly in the radial glia of the olfactory bulb, but in neither the olfactory sensory neurons nor bulbar projection neurons. Arx-deficient mice showed severe anatomical abnormalities in the developing olfactory system: (1) size reduction of the olfactory bulb, (2) reduced proliferation and impaired entry into the olfactory bulb of interneuron progenitors, (3) loss of tyrosine hydroxylase-positive periglomerular cells, (4) disorganization of the layer structure of the olfactory bulb, and (5) abnormal axonal termination of olfactory sensory neurons in an unusual axon-tangled structure, the fibrocellular mass. Thus, Arx is required for not only the proper developmental processes of Arx-expressing interneurons, but also the establishment of functional olfactory neural circuitry by affecting Arx-non-expressing sensory neurons and projection neurons. These findings suggest a likely role of Arx in regulating the expression of putative instructive signals produced in the olfactory bulb for the proper innervation of olfactory sensory axons.

Dock and Pak regulate olfactory axon pathfinding in Drosophila

The convergence of olfactory axons expressing particular odorant receptor (Or) genes on spatially invariant glomeruli in the brain is one of the most dramatic examples of precise axon targeting in developmental neurobiology. The cellular and molecular mechanisms by which olfactory axons pathfind to their targets are poorly understood. We report here that the SH2/SH3 adapter Dock and the serine/threonine kinase Pak are necessary for the precise guidance of olfactory axons. Using antibody localization, mosaic analyses and cell-type specific rescue, we observed that Dock and Pak are expressed in olfactory axons and function autonomously in olfactory neurons to regulate the precise wiring of the olfactory map. Detailed analyses of the mutant phenotypes in whole mutants and in small multicellular clones indicate that Dock and Pak do not control olfactory neuron (ON) differentiation, but specifically regulate multiple aspects of axon trajectories to guide them to their cognate glomeruli. Structure/function studies show that Dock and Pak form a signaling pathway that mediates the response of olfactory axons to guidance cues in the developing antennal lobe (AL). Our findings therefore identify a central signaling module that is used by ONs to project to their cognate glomeruli.

Pax6 regulates regional development and neuronal migration in the cerebral cortex

Mutations in the Pax6 gene disrupt telencephalic development, resulting in a thin cortical plate, expansion of proliferative layers, and the absence of the olfactory bulb. The primary defect in the neuronal cell population of the developing cerebral cortex was analysed by using mouse chimeras containing a mixture of wild-type and Pax6-deficient cells. The chimeric analysis shows that Pax6 influences cellular activity throughout corticogenesis. At early stages, Pax6-deficient and wildtype cells segregate into exclusive patches, indicating an inability of different cell genotypes to interact. At later stages, cells are sorted further based on telencephalic domains. Pax6-deficient cells are specifically reduced in the mediocaudal domain of the dorsal telencephalon, indicating a role in regionalization. In addition, Pax6 regulates the process of radial migration of neuronal precursors. Loss of Pax6 particularly affects movement of neuronal precursors at the subventricular zone/intermediate zone boundary at a transitional migratory phase essential for entry into the intermediate zone. We suggest that the primary role of Pax6 is the continual regulation of cell surface properties responsible for both cellular identity and radial migration, defects of which cause regional cell sorting and abnormalities of migration in chimeras.

DLX5 regulates development of peripheral and central components of the olfactory system

Induction, neurogenesis, and synaptogenesis of the olfactory bulb are thought to require interactions with the olfactory epithelium. The Dlx family of homeobox genes is expressed in both the olfactory bulb and olfactory epithelium. In particular, Dlx5 is expressed in the olfactory placode, olfactory epithelium, and local circuit neurons of the olfactory bulb. Here we analyzed mice lacking DLX5 function. The Dlx5-/- mutation reduces the size of the olfactory epithelium. Although some olfactory neurons are formed, they fail to generate olfactory axons that innervate the olfactory bulb. Despite the lack of innervation, the olfactory bulb forms, and neurogenesis of projection and local circuit neurons proceeds. However, the mutation has a cell-autonomous effect on the ability of neural progenitors to produce olfactory bulb local circuit neurons, with granule cells more severely affected than periglomerular cells. In addition, the mutation has a noncell-autonomous effect on the morphogenesis of mitral cells.

Apoptosis following peripheral sensory deafferentation in the olfactory bulb of adult zebrafish

Removal of the olfactory organ in adult zebrafish results in a significant decrease in volume of the ipsilateral olfactory bulb. The current study investigated the potential role of apoptosis in this phenomenon. It was hypothesized that cells in the adult olfactory bulb normally undergo minimal apoptosis and that apoptosis increases upon removal of sensory stimulation. By using both the terminal transferase-mediated deoxyuridine nick-end labeling method and bis-benzimide labeling, the current study showed that, in the normal adult olfactory bulb, cells exhibiting apoptotic profiles were scarce and were localized to the outer layers of the bulb. However, in deafferented animals, there was a significant increase in the number of apoptotic cells. The apoptotic response occurred in two phases and was confined to the rostral half of the bulb. The first phase of cell death peaked at 1 hour postsurgery. These apoptotic profiles appeared to be primarily nonneuronal in nature, in that they exhibited no immunohistochemical labeling to the neuron-specific protein Hu. The second phase of cell death peaked at 24 hours and declined to normal levels by 1 week. At the 24 hour time point, only a fraction of the apoptotic cells was neuronal in nature. Thus, apoptosis of nonneuronal and neuronal elements accounts for at least part of the deafferentation-induced volume decrease in the zebrafish olfactory bulb. This model of anterograde transneuronal degeneration will be useful in elucidating the afferent signals involved in survival and maintenance of mature brain neurons.

The olfactory bulb as an independent developmental domain

The olfactory system is a good model to study the mechanisms underlying guidance of growing axons to their appropriate targets. The formation of the olfactory bulb involves differentiation of several populations of cells and the initiation of the central projections, all under the temporal and spatial patterns of gene expression. Moreover, the nature of interactions between the olfactory epithelium, olfactory bulb and olfactory cortex at early developmental stages is currently of great interest. To explore these questions more fully, the present review aims to correlate recent data from different developmental studies, to gain insight into the mechanisms involved in the specification and development of the olfactory system. From our studies in the pax6 mutant mice (Sey(Neu)/Sey(Neu)), it was concluded that the initial establishment of the olfactory bulb central projections is able to proceed independently of the olfactory sensory axons from the olfactory epithelium. The challenge that now remains is to consider the validity of the olfactory bulb as an independent development domain. In the course of evaluating these ideas, we will review the orchestra of molecular cues involved in the formation of the projection from the OB to the olfactory cortex.

Coordinate regulation and synergistic actions of BMP4, SHH and FGF8 in the rostral prosencephalon regulate morphogenesis of the telencephalic and optic vesicles

We investigated the roles of bare morphogenetic protein (BMP), sonic hedgehog (SHH) and fibroblast growth factor (FGF)-expressing signaling centers in regulating the patterned outgrowth of the telencephalic and optic vesicles. Implantation of BMP4 beads in the anterior neuropore of stage 10 chicken embryos repressed FGF8 and SHH expression. Similarly, loss of SHH expression in Shh mutant mice leads to increased BMP signaling and loss of Fgf8 expression in the prosencephalon. Increased BMP signaling and loss of FGF and SHH expression was correlated with decreased proliferation, increased cell death, and hypoplasia of the telencephalic and optic vesicles. However, decreased BMP signaling, through ectopic expression of Noggin, a BMP-binding protein, also caused decreased proliferation and hypoplasia of the telencephalic and optic vesicles, but with maintenance of Fgf8 and Shh expression, and no detectable increase in cell death. These results suggest that optimal growth requires a balance of BMP, FGF8 and SHH signaling. We suggest that the juxtaposition of Fgf8, Bmp4 and Shh expression domains generate patterning centers that coordinate the growth of the telencephalic and optic vesicles, similar to how Fgf8, Bmp4 and Shh regulate growth of the limb bud. Furthermore, these patterning centers regulate regional specification within the forebrain and eye, as exemplified by the regulation of Emx2 expression by different levels of BMP signaling. In summary, we present evidence that there is cross-regulation between BMP-, FGF- and SHH-expressing signaling centers in the prosencephalon which regulate morphogenesis of, and regional specification within, the telencephalic and optic vesicles.

Development of the facial midline

"Intellectual excellence lies in having faith in the observation of apparently nontranscendental and unimportant facts. To observe an anatomic element calmly, with an open, analytical spirit, and with spiritual freedom, can lead to an explosive vortex of new knowledge."-Miguel Orticochea, M.D.(1) Traditional descriptive embryology based upon the interaction of frontonasal, lateral nasal, and medial nasal prominences is incapable of explaining the three-dimensional development of the facial midline. The internal structure of the nose and that of the oronasal midline can best be explained by the presence of paired A fields originating from the prechordal mesendoderm, associated with the nasal and optic placodes, supplied by the internal carotid artery, and sharing a common genetic coding with the prosomeres of the forebrain. Mesial drift of these fields leads to fusion of their medial walls; this in turn provides bilateral functional matrics within which form the orbits ethmoids, lacrimals, turbinates, premaxillae, vomerine bones, and the cartilages of the nose. This two-part paper reports six lines of evidence supporting the field theory model of facial development: (1) An apparent watershed exists in the midline of the base between the territories of the internal and external carotid systems. Isolation of the ICA in injected fetal specimens confirmed that the demarcation was distinct and restricted to the embryonic nasal capsule. (2) Field theory explains the developmental anatomy of the contents of the nasal capsule. (3) The neuromeric model of CNS development provides a genetic basis for the anatomy and behavior of fields. (4) Mutants for the Dlx5 gene demonstrate A field deletion patterns. These experiments relate the nasal placode to the structures of the A fields. (5) Separate regions of the original nasal placodes give rise to neurons, which are dedicated to separate sensory and endocrine systems. The A fields constitute the pathways by which these neurons reach the brain. (6) Non-cleft lip-related cleft palate, holoprosencephaly, and the Kallmann syndrome are clinical models that demonstrate the effects of anatomic disturbances within the A fields.

Cellular interactions in the development of the olfactory system: an ablation and homotypic transplantation analysis

In the current study, we addressed two questions: First, is the olfactory placode necessary for the development of the olfactory bulb and the entire telencephalon? Second, does the olfactory placode contribute cells to the olfactory bulb? We addressed these questions by unilaterally ablating the olfactory placode in chick embryos before an olfactory nerve was produced and, in a second series of experiments, by replacing the ablated chick olfactory placode with a quail olfactory placode. Our results indicate that the olfactory placode is critical for olfactory bulb development, but is not necessary for the development of the rest of the telencephalon. Further, our results support the hypothesis that LHRH neurons and olfactory nerve glia originate in the olfactory placode, but do not support an olfactory placodal origin for other cell types within the olfactory bulb.

Vertebrate cranial placodes I. Embryonic induction

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

Focal denervation alters cellular phenotypes and survival in the rat olfactory bulb: a developmental analysis

Our previous work (Couper Leo et al. [2000] J. Comp. Neurol. 417:325-336) introduced a technique for focally denervating the olfactory bulb soon after birth and described the pattern of changes incurred by this procedure by postnatal day (P) 30. The current study extends these findings with a developmental analysis of the effects of focal denervation in P10 and P20 rats. The results suggest that denervation begins to affect bulb architecture and cell survival soon after the procedure is performed, but that alterations within the bulb occur over an extended time period. For example, at P10, bulb and laminar sizes and mitral/tufted cell profile number had begun their decline, and nearly all measurements were significantly reduced by P20. Furthermore, a superficial-to-deep gradient of alterations in bulb architecture and a temporal separation of the effects on mitral/tufted cell dendrites vs. somata were observed. Immunohistochemical analyses of olfactory marker protein (OMP)-, calretinin- calbindin-, parvalbumin-, tyrosine hydroxylase-, and glutamic acid decarboxylase-stained sections indicated that: 1) denervation alters the interaction between olfactory axons and their targets in a developmentally significant manner; 2) the fine structure of denervated cells is altered; 3) cell phenotypes are differentially affected by loss of afferent contact, perhaps due to the age-dependent expression of their defining antigens; and 4) specific cell populations may be lost as a result of denervation.

Ace/Fgf8 is required for forebrain commissure formation and patterning of the telencephalon

Fibroblast growth factors (Fgfs) form a large family of secreted signalling proteins that have a wide variety of roles during embryonic development. Within the central nervous system (CNS) Fgf8 is implicated in patterning neural tissue adjacent to the midbrain-hindbrain boundary. However, the roles of Fgfs in CNS tissue rostral to the midbrain are less clear. Here we examine the patterning of the forebrain in zebrafish embryos that lack functional Fgf8/Ace. We find that Ace is required for the development of midline structures in the forebrain. In the absence of Ace activity, midline cells fail to adopt their normal morphology and exhibit altered patterns of gene expression. This disruption to midline tissue leads to severe commissural axon pathway defects, including misprojections from the eye to ectopic ipsilateral and contralateral targets. Ace is also required for the differentiation of the basal telencephalon and several populations of putative telencephalic neurons but not for overall regional patterning of forebrain derivatives. Finally, we show that ace expression co-localises with anterior neural plate cells that have previously been shown to have forebrain patterning activity. Removal of these cells leads to a failure in induction of ace expression indicating that loss of Ace activity may contribute to the phenotypes observed when anterior neural plate cells are ablated. However, as ace mutant neural plate cells still retain at least some inductive activity, then other signals must also be produced by the anterior margin of the neural plate.

Deafferentation-induced changes in the olfactory bulb of adult zebrafish

The influence of the olfactory organ on maintenance of olfactory bulb structure was examined in zebrafish, using peripheral deafferentation. This fish provides a model in which the olfactory organ is easily accessible for removal, the animals easily survive the surgery, and the olfactory bulbs are small enough to allow rigorous analysis of the resulting effects. Unilateral olfactory organ ablations were performed on anesthetized adult zebrafish using a small-vessel cautery iron. Fish were allowed to survive for 1, 3, or 6 weeks following the procedure. Analysis of deafferented animals revealed that most, if not all, of the olfactory organ was missing on the ablated side, and the structure did not regenerate. The morphology of the olfactory bulb was affected notably by the removal of its primary afferent innervation. The olfactory nerve layer was diminished at 1 week and absent by 3 weeks post-deafferentation. At all of the survival times the deafferented bulb appeared significantly smaller at the gross level, and there was a statistically significant effect on bulb size and cell number after 6 weeks. Tyrosine hydroxylase expression, as revealed by immunohistochemistry, was decreased noticeably on the ablated side. In conclusion, the olfactory organ is important in the preservation of normal olfactory bulb anatomy and neurochemistry in adult zebrafish. Thus, the influence of the periphery does not end with the formation of the mature olfactory bulb.

Focal denervation alters cellular phenotypes and survival in the developing rat olfactory bulb

Several studies have demonstrated that contact between the olfactory nerve and the forebrain is critical for normal olfactory bulb development. Removal of the embryonic olfactory placode results in a failure of the olfactory bulb to form, as well as causing other forebrain malformations. The current study introduces a technique that permits removal of contact between specific regions of the olfactory nerve and the bulb early in development, without causing damage to other brain regions, and without removing the peripheral olfactory organ. The manipulation, which involves insertion of a small Teflon chip between the cribriform plate and the bulb, prohibits growth of new axons into the "shadow" region behind the implant. Focal denervation of the olfactory bulb causes a decrease in bulb and layer sizes, a reduction in mitral cell number, and changes to bulb architecture. Using a battery of antibodies (OMP, MAP2, TuJ1, calretinin, calbindin, parvalbumin, TH, and GAD), we further demonstrated that 1) focal denervation alters the relationship between the olfactory nerve and the bulb, 2) the fine structure of cells in denervated regions is disrupted, and 3) cellular phenotypes change in response to loss of afferent contact. These results suggest that contact between the olfactory nerve and the bulb is important for maintaining bulb architecture and cell survival, structure, and phenotype. They also point to focal denervation as a useful technique for examining the role of neural contact in olfactory development and maintenance of the central nervous system.

Axonal signals in the assembly of neural circuitry

Recent work in Drosophila and rodents has revealed that proteins transported along axons and delivered to pathway and target cell populations play important roles in the construction of neural circuitry. Interestingly, the parallels between these systems may extend to the identities of some of the molecules involved.

The role of the brain in metamorphosis of the olfactory epithelium in the frog, Xenopus laevis

Retrograde signaling from the brain to the olfactory sensory epithelium is important for neuronal survival, but the importance of the olfactory bulb in retrograde signaling during the naturally-induced, neuronal plasticity occurring during metamorphosis is unclear. The olfactory system of the African clawed frog (Xenopus laevis) undergoes dramatic rearrangements during metamorphosis, making this an ideal system in which to examine interactions between the brain and the olfactory sensory epithelium. The main olfactory epithelium of larvae, located in the principal cavity (PC), changes at metamorphosis in function, receptor neuron morphology, biochemistry, and axon termination sites. A new, "middle", cavity forms during metamorphosis that assumes all the characteristics of the larval PC. Using a combination of bulbectomy and olfactory transplantation, we investigated changes in expression of a marker protein (E7) and in apical ultrastructure in olfactory receptor neurons either (1) connected to the olfactory bulb, (2) connected to non-olfactory brain regions, or (3) with no apparent central nervous system (CNS) connections. We find that neurons in the middle cavity (MC) lacking connections with the CNS appear mature but neurons in the PC do not. Supporting cells in the PC undergo the changes normally observed during metamorphosis. Neurons connected to non-olfactory brain regions, either after bulbectomy or transplantation, appeared normal with regard to the changes normally expected after metamorphosis. These results suggest that influence from the brain is necessary for metamorphic changes in the X. laevis olfactory epithelium, but that these signals are not confined to the olfactory bulb; non-olfactory brain regions can also support these metamorphic changes.

Development of the chick olfactory nerve

Gonadotropin releasing hormone (GnRH) is produced and secreted by neurons dispersed throughout the septal-preoptic and anterior hypothalamic areas in adult birds and mammals. These neurons, essential for a functional brain-pituitary-gonadal axis, differentiate in the olfactory placode, the superior aspect of which forms the olfactory epithelium. To reach their final placement within the brain, GnRH neurons migrate out of the epithelium and along the olfactory nerve to the CNS. This nerve is essential for the entrance of GnRH neurons into the CNS. Due to the importance of the nerve for the proper migration of these neurons, we have used immunocytochemistry, DiI labeling and 1 microm serial plastic-embedded sections to characterize the nerve's earliest development in the embryonic chick (stages 17-21). Initially (stage 17) the zone between the placode and prosencephalon is a cellular mass contiguous with the placode. This cluster, known as epithelioid cells, is positive for some but not all neuronal markers studied. The epithelium itself is negative for all neuronal and glial markers at this early stage. By stage 18, the first neurites emerge from the epithelium; this was confirmed at stage 19 by examination of serial 1 microm plastic sections. There is sequential acquisition of immunoreactivity to neuronal markers from stage 18 to 21. The glial component of the nerve appears at stage 21. Axons originating from epithelium, extend to the border of the CNS as confirmed by DiI labeling at stage 21. Small fascicles have entered the CNS at this stage. As previously reported, GnRH neurons begin their migration between stages 20-21 and have also arrived at the border of the brain at stage 21. Despite the penetration of neurites from the olfactory nerve into the CNS, GnRH neurons pause at the nerve-brain junction until stage 29 (2 1/2 days later) before entering the brain. Subsequent studies will examine the nature of the impediment to continued GnRH neuronal migration.

Axons of olfactory receptor cells of transsexually grafted antennae induce development of sexually dimorphic glomeruli in Manduca sexta

The influence of olfactory receptor cell (ORC) axons from transsexually grafted antennae on the development of glomeruli in the antennal lobes (ALs), the primary olfactory centers, was studied in the moth Manduca sexta. Normally during metamorphic adult development, the pheromone-specific macroglomerular complex (MGC) forms only in the ALs of males, whereas two lateral female-specific glomeruli (LFGs) develop exclusively in females. A female AL innervated by ORC axons from a grafted male antenna developed an MGC with three glomeruli, like the MGC of a normal male AL. Conversely, a male AL innervated by ORC axons from a grafted female antenna lacked the MGC but exhibited LFGs. ORC axons from grafted male antenna terminated in the MGC-specific target area, even in cases when the antennal nerve (AN) entered the AL via an abnormal route. Within ectopic neuromas formed by ANs that had become misrouted and failed to enter the brain, male-specific axons were not organized and formed terminal branches in many areas. The results suggest the presence of guidance cues within the AL for male-specific ORC axons. Depending on the sex of the antennal innervation, glial borders formed in a pattern characteristic of the MGC or LFGs. The sex-specific number of projection neurons (PNs) in the medial group of AL neurons remained unaffected by the antennal graft, but significant changes occurred in the organization of PN arborizations. In gynandromorphic females, LFG-specific PNs extended processes into the induced MGC, whereas in gynandromorphic males, PNs became restricted to the LFGs. The results indicate that male-and female-specific ORC axons play important roles in determining the position, anatomical features, and innervation of sexually dimorphic glomeruli.

Influence of olfactory innervation on neurogenesis in the developing olfactory bulb of the frog, Xenopus laevis

Previous research on development of the Xenopus olfactory bulb from our laboratory has shown that mitral cells begin to differentiate after olfactory axons make contact with the olfactory bulb, and the number of olfactory axons is correlated with the number of mitral cells throughout development. In embryos, removal of all afferent innervation before the mitral cells begin to differentiate results in a failure of the bulb to form; removal of half the olfactory axons, results in development of half the normal number of mitral cells. At larval stages, transection of the olfactory nerve results in a decrease in the number of neurons in the olfactory bulb. Thus, the olfactory axons play a major role in stimulating or maintaining development of the olfactory bulb neurons. Since we have found that neurogenesis in the bulb continues through metamorphosis, the goal of the current study was to determine whether olfactory axons influence cell genesis and/or neuronal maturation in the larval olfactory bulb. To study cell genesis, we cut the olfactory nerves, and 6 days later, injected the animals with [3H]thymidine. After 6 hr, the animals were killed and the tissue was processed for autoradiography. The number of labeled cells in the ventricular zone of the olfactory bulb was determined in every fifth section through the bulb in control and experimental animals. There was no statistical difference (Mann-Whitney rank sum test) in the number of labeled ventricular cells between controls and experimentals. Thus, olfactory axon innervation does not appear to play a role in precursor cell division during larval development. To study the influence of olfactory axon innervation on the ability of newly generated neurons to mature, we followed the same procedures. However, the animals were killed 21 days after the [3H]thymidine injection. The results from this experiment showed that there are many fewer labeled mitral cells in the experimental animals at 21 days. Together these results suggest that sensory deafferentation influences mitral cell differentiation or survival even during late stages of larval development.

A retinal axon fascicle uses spitz, an EGF receptor ligand, to construct a synaptic cartridge in the brain of Drosophila

Photoreceptor axons arriving in the Drosophila brain organize their postsynaptic target field into a precise array of five neuron "cartridge" ensembles. Here we show that Hedgehog, an initial inductive signal transported along retinal axons from the developing eye, induces postsynaptic precursor cells to express the Drosophila homolog of the epidermal growth factor receptor (EGFR). The EGFR ligand Spitz, a signal for ommatidial assembly in the compound eye, is transported to retinal axon termini in the brain where it acts as a local cue for the recruitment of five cells into a cartridge ensemble. Hedgehog and Spitz thus bring about the concerted assembly of ommatidial and synaptic cartridge units, imposing the "neurocrystalline" order of the compound eye on the postsynaptic target field.

Regionalization of the prosencephalic neural plate

Recent embryological studies are beginning to establish that the underlying organization of the forebrain may be reduced to relatively simple elements that are common to all vertebrates. We begin this chapter by reviewing studies that describe the similarities in prospective fate and molecular organization of the developing neural plate in fish, frogs, chickens, and mice. The chapter next addresses mechanisms that regulate regional specification in the anterior central nervous system. There is now evidence that the axial mesendoderm anterior to the notochord (the prechordal plate) has a central role in induction of the floor and basal plate primordia (hypothalamus) of the forebrain. Patterning of the anterolateral neural plate (telencephalon) may be regulated by FGF8 produced in the anterior neural ridge. Thus, the synthesis of information from fate mapping and experimental embryological and genetic studies is illuminating the mechanisms that generate the different components of the forebrain.

An Fgf8 mutant allelic series generated by Cre- and Flp-mediated recombination

We describe a strategy for generating an allelic series of mutations at a given locus that requires the production of only one targetted mouse line. The 'allelogenic' mouse line we produced carries a hypomorphic allele of Fgf8, which can be converted to a null allele by mating to cre transgenic animals. The hypomorphic allele can also be reverted to wild-type by mating the allelogenic mice to flp transgenic animals, thereby generating a mouse line suitable for Cre-induced tissue-specific knockout experiments. Analysis of embryos carrying different combinations of these alleles revealed requirements for Fgf8 gene function during gastrulation, as well as cardiac, craniofacial, forebrain, midbrain and cerebellar development.

Inductive interactions direct early regionalization of the mouse forebrain

The cellular and molecular mechanisms that regulate regional specification of the forebrain are largely unknown. We studied the expression of transcription factors in neural plate explants to identify tissues, and the molecules produced by these tissues, that regulate medial-lateral and local patterning of the prosencephalic neural plate. Molecular properties of the medial neural plate are regulated by the prechordal plate perhaps through the action of Sonic Hedgehog. By contrast, gene expression in the lateral neural plate is regulated by non-neural ectoderm and bone morphogenetic proteins. This suggests that the forebrain employs the same medial-lateral (ventral-dorsal) patterning mechanisms present in the rest of the central nervous system. We have also found that the anterior neural ridge regulates patterning of the anterior neural plate, perhaps through a mechanism that is distinct from those that regulate general medial-lateral patterning. The anterior neural ridge is essential for expression of BF1, a gene encoding a transcription factor required for regionalization and growth of the telencephalic and optic vesicles. In addition, the anterior neural ridge expresses Fgf8, and recombinant FGF8 protein is capable of inducing BF1, suggesting that FGF8 regulates the development of anterolateral neural plate derivatives. Furthermore, we provide evidence that the neural plate is subdivided into distinct anterior-posterior domains that have different responses to the inductive signals from the prechordal plate, Sonic Hedgehog, the anterior neural ridge and FGF8. In sum, these results suggest that regionalization of the forebrain primordia is established by several distinct patterning mechanisms: (1) anterior-posterior patterning creates transverse zones with differential competence within the neural plate, (2) patterning along the medial-lateral axis generates longitudinally aligned domains and (3) local inductive interactions, such as a signal(s) from the anterior neural ridge, further define the regional organization.

Neurogenesis in the olfactory bulb of the frog Xenopus laevis shows unique patterns during embryonic development and metamorphosis

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog, Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.

Activity blockade does not prevent the construction of olfactory glomeruli in the moth Manduca sexta

During metamorphic development, the arrival at the olfactory (antennal) lobe of olfactory receptor axons initiates the process of glomerulus formation. The glomeruli are discrete spheroidal regions of neuropil that are the sites of synaptic interactions among receptor neurons and their target antennal-lobe neurons. The process of glomerulus formation begins as groups of receptor axons form protoglomeruli. These dense clusters of terminal branches mostly are discrete entities from the time they can be recognized, although a few branches from neighboring protoglomeruli overlap laterally. A previous study by Schweitzer et al. [Schweitzer E. S., Sanes J. R. and Hildebrand J. G. (1976) Ontogeny of electroantennogram responses in the moth, Manduca sexta. J. Insect Physiol. 22, 955-960] has shown that odor-induced activity in the receptor neurons can be detected first in recordings from the axons in the antennal nerve only in the last few days of metamorphic development and thus could not influence the process of glomerulus formation. In this study, we have tested directly the possibility that an earlier presence of spontaneous activity in either the receptor axons or the antennal-lobe neurons could affect the process. Tetrodotoxin, a Na(+)-channel blocker, was injected into the hemolymph prior to the onset of glomerulus formation to block any spontaneous Na(+)-dependent activity. Subsequent intracellular recordings from antennal-lobe neurons revealed no spike activity. Comparison with vehicle-injected control animals at stages during and after glomerulus formation revealed no differences in the localization of receptor-axon terminal branches in the glomeruli, in the border of glial cells that forms around each glomerulus, or in the morphology of the tufted glomerular arbors of one of the antennal-lobe neurons. We conclude that: (1) the process of glomerulus formation is largely independent of activity; and (2) glomeruli as modular units of the CNS more closely resemble cortical barrels than cortical columns, both in their ontogeny and in the lack of an obvious effect of activity on the morphology of the neurons arborizing within them.

Influence of receptor axons on the formation of olfactory glomeruli in a hemimetabolous insect, the cockroach Periplaneta americana

The embryonic development of the hemimetabolous insect Periplaneta americana requires approximately 31 days. Deafferentation experiments were used to investigate the role of ingrowing receptor axons during embryogenesis, specifically their influence 1) on the subdivision of the antennal lobe neuropil into glomeruli, 2) on the morphology and number of glial cells, and 3) on the arborization pattern of central neurons. The flagellum of one antenna was removed from embryos at different developmental stages starting with day 10. Subsequently, they were raised in culture until a total age of 26 days. At day 10, the deutocerebrum has received only a very small number (ca. 0.4%) of antennal receptor axons; deafferentation at this stage allowed us to deprive the deutocerebrum of approximately 99% of its normal antennal input. Deafferentation has marked effects on the organization of the antennal lobe neuropil. The deafferented lobe is reduced in volume compared to the control side; the characteristic glomeruli are missing. During normal development glomeruli are formed between day 19 and 22, first in dorsal and then in ventral antennal lobe regions. By removing the antenna before day 20, their formation is disturbed in all parts of the antennal lobe. If deafferentation is performed after stage 20, glomeruli persist in dorsal regions, but are missing in ventral regions. On day 24 or later, glomeruli in both dorsal and ventral regions are unaffected by deafferentation. Glial cells continue to extend fine processes into the neuropil in the absence of ingrowing receptor axons. The number of glial cells is reduced compared to control lobes. Multiglomerular local interneurons and other gamma-amino butyric acid-immunoreactive neurons, as well as projection neurons, fail to develop glomerular arborization patterns in antennal lobes deprived of sensory axons.

Multiple factors shape development of olfactory glomeruli: insights from an insect model system

The antennal system of the moth Manduca sexta is a useful model for studies of the development of olfactory glomeruli, the complex synaptic structures that typically underlie the initial processing of olfactory input in vertebrates and invertebrates. In this review, we summarize cellular events in the construction of glomeruli in Manduca and highlight experiments that reveal factors that influence glomerulus development. By methodically manipulating each of various cell types, both neuronal and glial, that contribute to glomerular architecture, we have found that: olfactory receptor axons lay a template for developing glomeruli, stabilization of the template by glial cells is necessary to permit subsequent steps in development of the glomeruli, and the hormone that regulates adult development causes production of adequate numbers of glial cells. Neither electrical activity nor the presence of a serotonin-containing neuron that persists throughout development is required for a glomerular pattern to develop; these factors might, however, influence the synaptic organization of individual glomeruli.