Formation of an olfactory glomerulus: morphological aspects of development and organization

Neurotrophin-3 is expressed in a discrete subset of olfactory receptor neurons in the mouse

In transgenic neurotrophin-3 lacZ-neo (NT-3(lacZneo)) mice, in which the coding region for NT-3 is replaced by Eschericia coli lacZ, the expression of beta-galactosidase faithfully mimics the expression of NT-3 (Vigers AJ, Baquet ZC, Jones KR [2000], J Comp Neurol 416:398-416). During embryonic and early postnatal development, beta-galactosidase is detected in the olfactory system, beginning at embryonic day 11.5 in the nasal epithelium and at embryonic day 16.5 in the olfactory bulb. Levels of beta-galactosidase rise with age, reaching a peak during the second postnatal week, when beta-galactosidase reactivity is visible in up to 50% of the glomeruli. As the animal matures, the beta-galactosidase levels decline, but staining remains present in axons and cell bodies of a specific subset of olfactory receptor neurons (ORNs) projecting to a limited subset of glomeruli. The heavily labeled ORNs do not follow the typical OR expression zones in the epithelium but appear similar to the "patch" expression pattern of mOR37 receptors. The most heavily reactive glomeruli exhibit a striking reproducible pattern in the ventral olfactory bulb (OB). Some glomeruli of the OB contain calcitonin gene-related peptide (CGRP)-immunoreactive fibers of the trigeminal nerve. However, double-label immunocytochemistry for CGRP and beta-galactosidase rendered no correlation between trigeminal innervation and the degree of innervation by NT-3-expressing ORNs. Thus, the timing and presence of beta-galactosidase in a subset of ORNs suggests that NT-3 plays a role in synaptogenesis and/or synapse function in a specific subset of ORNs within the olfactory bulb.

Expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in the developing olfactory bulb and subventricular zone rostral extension

The olfactory bulb (OB) presents a unique pattern of permanent acquisition of primary afferents and interneurons, but not much detail is known about the differentiation of its oligodendroglial cells. We studied the expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a protein related to axonal ensheathment by myelinating cells. Expression of CNPase in OB follows a general caudorostral gradient, with the exception of the glomerular layer (GL). At postnatal day 5-6 (P5-P6), the first CNPase(+) profiles appeared in the dorsal lateral olfactory tract adjacent to the accessory OB (AOB), followed by rare cell bodies and processes in AOB internal plexiform layer at P7. At P9, the main OB (MOB) granular cell layer (GrCL) already showed intensely stained CNPase(+) processes. From P5 to P12, small numbers of CNPase(+) cells were found in the subventricular zone (SVZ), throughout its rostral extension (SVZ-RE), and in the intrabulbar subependymal layer. The appearance of CNPase(+) profiles delimiting glomeruli started in the GL rostralmost region at P12, extending to all GL levels, but glomeruli remained open caudally at P15. At P18, oligodendroglial glomeruli were evident throughout OB, but the adult pattern was established only after P30. There was no age-related loss of CNPase immunoreactivity in glial cell bodies, possibly indicating de novo ensheathment of neurites. Our results show an earlier onset of oligodendroglial differentiation in OB than previously reported and a rostrocaudal gradient of formation of oligodendroglial glomeruli. They also raise the possibility that a minor fraction of OB oligodendrocytes might derive from the SVZ-RE.

Reciprocal interactions between olfactory receptor axons and olfactory nerve glia cultured from the developing moth Manduca sexta

In olfactory systems, neuron-glia interactions have been implicated in the growth and guidance of olfactory receptor axons. In the moth Manduca sexta, developing olfactory receptor axons encounter several types of glia as they grow into the brain. Antennal nerve glia are born in the periphery and enwrap bundles of olfactory receptor axons in the antennal nerve. Although their peripheral origin and relationship with axon bundles suggest that they share features with mammalian olfactory ensheathing cells, the developmental roles of antennal nerve glia remain elusive. When cocultured with antennal nerve glial cells, olfactory receptor growth cones readily advance along glial processes without displaying prolonged changes in morphology. In turn, olfactory receptor axons induce antennal nerve glial cells to form multicellular arrays through proliferation and process extension. In contrast to antennal nerve glia, centrally derived glial cells from the axon sorting zone and antennal lobe never form arrays in vitro, and growth-cone glial-cell encounters with these cells halt axon elongation and cause permanent elaborations in growth cone morphology. We propose that antennal nerve glia play roles similar to olfactory ensheathing cells in supporting axon elongation, yet differ in their capacity to influence axon guidance, sorting, and targeting, roles that could be played by central olfactory glia in Manduca.

Microenvironmental elements supporting adult hippocampal neurogenesis

Neurogenesis continues into adulthood in two distinct regions, the subventricular zone of the forebrain and the subgranular zone of the dentate gyrus. Transplantation experiments have suggested that the two neurogenic regions have a special microenvironment to support the proliferation and differentiation of stem or progenitor cells. As the candidates of the microenvironment, three elements have so far been proposed: (i) astrocytes; (ii) polysialyl neural cell adhesion molecule (PSA-NCAM)-expressing immature neurons; and (iii) blood vessels. In the early developmental process of neurogenesis, newly born cells make clusters within the neurogenic regions and the clusters are found to interact structurally with astrocytes, polysialic acid-expressing immature cells, endothelium and extravascular basal laminae of blood vessels. Furthermore, recent reports have shown that astrocytes support the proliferation and differentiation of stem cells in vitro. These results suggest that these microenvironmental elements contribute to the cell proliferation and differentiation of stem or progenitor cells. However, it remains to be determined how the microenvironmental elements support adult neurogenesis functionally and coordinate with each other.

Inverse expression of olfactory cell adhesion molecule in a subset of olfactory axons and a subset of mitral/tufted cells in the developing rat main olfactory bulb

The projection of olfactory sensory neuron (OSN) axons from the olfactory epithelium (OE) to the olfactory bulb (OB) is highly organized but topographically complex. Evidence suggests that odorant receptor expression zones in the OE map to the OB about orthogonal axes. One candidate molecule for the formation of zone-specific targeting of OSN axon synapses onto the OB is the olfactory cell adhesion molecule (OCAM). OCAM(+) OSNs are restricted to three of the four zones in the OE and project their axons to the ventral OB where they form synapses with mitral/tufted (M/T) cells. To determine when this zonal connection is established, we have examined OCAM expression in rat olfactory system, during seminal periods of glomerular formation. OCAM(+) axons sort out in the ventral olfactory nerve layer of the OB before glomerular formation. Surprisingly, OCAM was also expressed transiently by subsets of M/T cell dendrites located in the dorsal OB. The expression of OCAM by OSN axons and M/T dendrites was asymmetrical; in the dorsal OB, OCAM(-) OSN axons synapsed on OCAM(+) M/T dendrites, whereas in the ventral OB, OCAM(+) OSN axons synapsed on OCAM(-) M/T dendrites. The restricted spatial map of OCAM(+) M/T cells appeared earlier in development than the zonal segregation of OCAM(+) OSN axons. Thus, OCAM on M/T cell dendrites may act in a spatiotemporal window to specify regions of the developing rat OB, thereby establishing a foundation for mapping of the OE zonal organization onto the OB.

Distorted odor maps in the olfactory bulb of semaphorin 3A-deficient mice

Semaphorin 3A (Sema3A) repels growing olfactory axons that express neuropilin-1 (NP-1), a receptor for Sema3A. The Sema3A-mediated axon guidance seems to be essential for the formation of the glomerular sensory map in the olfactory bulb (OB). To understand whether and how Sema3A is involved in sensory map formation, we examined the glomerular map in the OB of adult Sema3A-deficient mice. In wild-type mice, NP-1-positive glomeruli form the lateral and medial bands and avoid the anteromedial and ventral regions of the OB. In the Sema3A-deficient OB, NP-1-positive glomeruli spread over the entire OB, and we consistently found the ectopic arrangement of NP-1-positive glomeruli in the anteromedial and ventral regions. In addition, a specific subset of NP-1-negative and olfactory cell adhesion molecule-positive glomeruli, especially those in the anteromedial region, disappeared from the mutant OB. These results show a critical role for Sema3A in the spatial arrangement of glomeruli in the OB. Optical imaging from the dorsal OB showed that the distorted glomerular map conserved molecular-feature domains. However, the positions of the domains were shifted, which suggests a secondary rearrangement of the glomerular map in the Sema3A-deficient OB.

Olfactory ensheathing cells of the lamina propria in vivo and in vitro

Olfactory ensheathing cells (OECs) continuously support the regeneration of olfactory receptor neurons (ORNs). In addition, OECs promote regeneration of neurons within the CNS in a number of transplantation paradigms, but details of exactly how they support regeneration remain elusive. The majority of studies using OECs to promote regeneration have thus far focused on understanding the cell biology of OECs purified from the olfactory bulb (OB). Here we show that a population of OECs similar to those obtained from the OB is present in the lamina propria (LP) beneath the olfactory epithelium (OE). These OECs are the first glial cells encountered by the axons of developing ORNs as they exit the OE and display distinct and variable expression of p75, S100beta, GFAP, and O4, characteristic markers of bulb OECs. Once purified in vitro, they display Schwann cell-like and astrocyte-like properties and expand rapidly. In addition to resembling OB-OECs, LP-OECs also express a unique combination of developmentally important proteins-CD 44, beta1 integrin, P200, Notch 3, NG2, VEGF, and PACAP and CREB binding protein (CBP/p300)-not previously reported in OB-OECs. These data suggest that LP-OECs, like OB-OECs, are a developmentally distinct class of glia that are capable of both immature and mature function, depending on environmental stimuli, within the adult nervous system.

Spatiotemporal distribution of the insulin-like growth factor receptor in the rat olfactory bulb

Insulin-like growth factor I (IGF-I) and its receptor (IGF-IR) are involved in growth of neurons. In the rat olfactory epithelium, we previously showed IGF-IR immunostaining in subsets of olfactory receptor neurons. We now report that IGF-IR staining was heaviest in the olfactory nerve layer of the rat olfactory bulb at embryonic days 18, and 19 and postnatal day 1, with labeling of protoglomeruli. In the adult, only a few glomeruli were IGF-IR-positive, some of which were unusually small and strongly labeled. Some IGF-IR-positive fibers penetrated deeper into the external plexiform layer, even in adults. In developing tissues, IGF-IR staining co-localized with that for olfactory marker protein and growth associated protein GAP-43, but to a lesser extent with synaptophysin. In the adult, IGF-IR-positive fibers were compartmentalized within glomeruli. IGF-I may play a role in glomerular synaptogenesis and/or plasticity, possibly contributing to development of coding patterns for odor detection or identification.

Anatomic mapping of neuronal odor responses in the developing rat olfactory bulb

Behavioral evidence indicates that altricial mammals possess olfactory function at early developmental ages, before the olfactory bulb has matured anatomically. We monitored the early anatomic and functional development of the olfactory bulb in rat pups stimulated with odors using in situ localization of c-fos mRNA to identify responsive postsynaptic neurons. Odor-specific spatial patterns of neuronal activation in the glomerular layer were evident from birth, were sharply defined rather than diffuse, and remained relatively unchanged in terms of their bulbar distribution during the first 3 postnatal weeks. In neonates, focal postsynaptic responses in the glomerular layer occurred in the form of clusters of activated tufted neurons. Broad zones of activated mitral cells were located beneath these cell clusters, with scattered neurons in the underlying granule cell layer also expressing c-fos. The cellular composition of these functional neuronal groups shifted from predominantly output neurons at the earliest ages, to increasing incorporation of interneurons as they developed postnatally. The characteristic distribution of activated neurons in the mature glomerular layer, in which the boundaries of individual glomeruli are precisely defined by cells expressing c-fos, emerged near the end of the first week. Broad zones of cRNA hybridization in the mitral cell layer became increasingly restricted as the size of the activated granule cell population increased postnatally, correlating with the functional maturation of inhibitory circuitry. These results provide evidence that the types and distributions of neurons collectively activated by sensory input to glomeruli change as the rat olfactory bulb matures and that distinct, functional odor maps in the glomerular layer are established from birth.

Why are olfactory systems of different animals so similar?

As we learn more about the neurobiology of olfaction, it is becoming increasingly clear that olfactory systems of animals in disparate phyla possess many striking features in common. Why? Do these features provide clues about the ways the nervous system processes olfactory information? This might be the case if these commonalities are convergent adaptations that serve similar functions, but similar features can be present in disparate animals for other reasons. For example, similar features may be present because of inheritance from a common ancestor (homology), may represent responses to similar constraints, or may be superficial or reflect strategies used by researchers studying the system. In this paper, I examine four examples of features of olfactory systems in members of different phyla: the presence of odorant binding proteins in the fluid overlying olfactory receptor neurons; the use of G protein-coupled receptors as odorant receptors; the use of a two-step pathway in the transduction of odorant signals; and the presence of glomerular neuropils in the first central target of the axons of olfactory receptor cells. I analyze data from nematodes, arthropods, molluscs, and vertebrates to investigate the phylogenetic distribution of these features, and to try to explain the presence of these features in disparate animals. Phylogenetic analyses indicate that these features are not homologous across phyla. Although these features are often interpreted as convergent adaptations, I find that alternative explanations are difficult to dismiss. In many cases, it seems that olfactory system features that are similar across phyla may reflect both responses to similar constraints and adaptations for similar tasks.

EphA receptors and ephrin-A ligands exhibit highly regulated spatial and temporal expression patterns in the developing olfactory system

The spatiotemporal expression patterns of the chemorepulsive EphA receptors, EphA4 and EphA7, and three ephrins-A2, A4 and A5, were examined in the developing rat primary olfactory system. Unlike the visual system that has simple and stable gradients of Ephs and ephrins, the olfactory system demonstrates complex spatiotemporal expression patterns of these molecules. Using immunohistochemistry, we demonstrate that expression of these molecules is dynamic and tightly regulated both within and between different cell types. We reveal restricted targeting of these proteins within subcellular compartments of some neurons. EphA4, ephrin-A2 and ephrin-A5 were expressed by primary olfactory axons during the embryonic formation of the olfactory nerve. There were no gradients in expression along the rostrocaudal or ventrodorsal axes in the nasal cavity and olfactory bulb. However, during the early neonatal period, axons expressing different levels of ephrin-A5 sorted out and terminated in a subpopulation of glomeruli that were mosaically dispersed throughout the bulb. The expression of EphA4 and ephrin-A2 was dramatically down-regulated on all axons during the early neonatal period of glomerular formation. The uniform co-expression of receptors and ligands before glomerular formation suggests they play a generic role in axon-axon interactions in the olfactory nerve and nerve fibre layer. In contrast, loss of EphA4 from axons during glomerular formation may facilitate the interaction of ephrin-A5 with Eph receptors on target cells in the bulb. While EphA4, EphA5 and EphA7 are not mosaically expressed by bulbar neurons, other Eph receptors may have expression patterns complementary to the ephrin-A5-positive subpopulation of glomeruli.

Migration of LHRH neurons into the spinal cord: evidence for axon-dependent migration from the transplanted chick olfactory placode

In the chick embryo, luteinizing hormone-releasing hormone (LHRH) neurons originate in the olfactory placode and migrate along the olfactory nerve to the forebrain. In previous studies, we demonstrated that LHRH neurons followed the trigeminal nerve when the olfactory nerve was physically interrupted. To examine whether LHRH neurons possess the capacity to migrate along the different type of axons, the olfactory placode was transplanted into the base of the forelimb. Three to five days after the transplantation, LHRH neurons were detectable in the spinal nerve, the dorsal root ganglion, the sympathetic ganglion and the spinal cord. Double or triple labelling studies for LHRH, somatostatin and/or axonin-1 showed that LHRH neurons entered the spinal nerve in contact with the olfactory axons, which are specifically immunoreactive to somatostatin. Migrating LHRH neurons continued to associate closely with the olfactory axons in the spinal nerve. However, some LHRH neurons often migrated along with the axonin-1 positive spinal sensory axons, maintaining a distance from the olfactory axons. Furthermore, a few LHRH neurons were observed in the ventral root and the ventral funiculus independent of olfactory axons. As LHRH neurons were observed in the motor component of the spinal nerve, it is probable that LHRH neurons also invaded the spinal cord using the motor axons as a guiding substrate for their migration. These results suggest that the migration mode of LHRH neurons is axon dependent in the peripheral region, however, chemical identity with regard to axonal substrate choice for migration was not specified in the present study.

Olfactory ensheathing cells: their potential use for repairing the injured spinal cord

STUDY DESIGN

The literature concerning the potential use of olfactory ensheathing cells for repairing damaged spinal cord was reviewed.

OBJECTIVE

To engender a better understanding of the role that olfactory ensheathing cells play in spinal cord regeneration.

SUMMARY OF BACKGROUND DATA

Intraspinal transplants (e.g., fetal neuronal cells, progenitor stem cells, and olfactory ensheathing cells) have been used to restore intraspinal circuitry or to serve as a "bridge" for damaged axons. Among these transplants, olfactory ensheathing cells provide a particularly favorable substrate for spinal axonal regeneration because these cells can secrete extracellular molecules and neurotrophic factors and have the ability to migrate into gliotic scar tissue, an important attribute that might be associated with high potential for axonal regeneration.

METHODS

Recent advances using centrally and peripherally derived olfactory ensheathing cells to promote spinal cord regeneration were reviewed.

RESULTS

Both centrally and peripherally derived olfactory ensheathing cells can lead to a degree of functional and anatomic recovery after spinal cord injury in adult animals.

CONCLUSION

Olfactory ensheathing cells from olfactory lamina propria in the nose are among the best transplants for "bridging" descending and ascending pathways in damaged spinal cord.

Laminar disorganisation of mitral cells in the olfactory bulb does not affect topographic targeting of primary olfactory axons

Primary olfactory neurons expressing the same odorant receptor protein typically project to topographically fixed olfactory bulb sites. While cell adhesion molecules and odorant receptors have been implicated in guidance of primary olfactory axons, the postsynaptic mitral cells may also have a role in final target selection. We have examined the effect of disorganisation of the mitral cell soma layer in mutant mice heterozygous for the beta-subunit of platelet activating factor acetylhydrolase (Lis1(-/+)) on the targeting of primary olfactory axons. Lis1(-/+) mice display abnormal lamination of neurons in the olfactory bulb. Lis1(-/+) mice were crossed with the P2-IRES-tau:LacZ line of transgenic mice that selectively expresses beta-galactosidase in primary olfactory neurons expressing the P2 odorant receptor. LacZ histochemistry revealed blue-stained P2 axons that targeted topographically fixed glomeruli in these mice in a manner similar to that observed in the parent P2-IRES-tau:LacZ line. Thus, despite the aberrant organisation of postsynaptic mitral cells in Lis1(-/+) mice, primary olfactory axons continued to converge and form glomeruli at correct sites in the olfactory bulb. Next we examined whether challenging primary olfactory axons in adult Lis(-/+) mice with regeneration would affect their ability to converge and form glomeruli. Following partial chemical ablation of the olfactory neuroepithelium with dichlobenil, primary olfactory neurons die and are replaced by newly differentiating neurons that project axons to the olfactory bulb where they converge and form glomeruli. Despite the aberrant mitral cell layer in Lis(-/+) mice, primary olfactory axons continued to converge and form glomeruli during regeneration. Together these results demonstrate that the convergence of primary olfactory axons during development and regeneration is not affected by gross perturbations to the lamination of the mitral cell layer. Thus, these results support evidence from other studies indicating that mitral cells do not play a major role in the convergence and targeting of primary olfactory axons in the olfactory bulb.

Novel microglomerular structures in the olfactory bulb of mice

The murine olfactory system consists of two primary divisions: (1) a main olfactory system, in which olfactory sensory neurons (OSNs) located in the main olfactory epithelium (MOE) send their axons to glomeruli in the main olfactory bulb (MOB); and (2) an accessory olfactory system, in which OSNs located in the vomeronasal organ send their axons to glomeruli in the accessory olfactory bulb (AOB). In labeling studies using the lectin Ulex europaeus agglutinin (UEA), we discovered a novel subset of small neuropilar structures in the MOB that are distinct from other glomeruli both in the MOB and AOB. These "microglomeruli" are morphologically similar to MOB glomeruli in many respects: they receive innervation from processes present in the olfactory nerve layer and are isolated from other glomeruli by juxtaglomerular cells; in addition, the compartmental pattern of UEA labeling suggests the presence of UEA (-) processes within their neuropil. Microglomeruli contained processes that express the olfactory marker protein, a marker common to mature OSN axons. However, unlike other glomerular structures, the microglomeruli did not contain neural cell adhesion molecule-labeled processes. Within microglomeruli, UEA(+) processes interdigitated with MAP2(+) dendrites, some of which likely originate from interneurons, as indicated by glutamic acid decarboxylase labeling. Synaptophysin labeling in microglomeruli strongly suggested that synapses occur between UEA(+) processes and dendrites. Anterograde labeling of OSNs, by injection of rhodamine-dextran into one naris, demonstrated that UEA(+) processes in microglomeruli originated in the MOE. The unique morphology, protein expression, and location of microglomeruli have led us to hypothesize that they represent a novel class of glomerular structures in the murine olfactory system.

Structure and emergence of specific olfactory glomeruli in the mouse

Olfactory sensory neurons (OSNs) expressing a given odorant receptor (OR) gene project their axons to a few specific glomeruli that reside at recognizable locations in the olfactory bulb. Connecting approximately 1000 populations of OSNs to the approximately 1800 glomeruli of the mouse bulb poses a formidable wiring problem. Additional progress in understanding the mechanisms of neuronal connectivity is dependent on knowing how these axonal pathways are organized and how they form during development. Here we have applied a genetic approach to this problem. We have constructed by gene targeting novel strains of mice in which either all OSNs or those that express a specific OR gene, M72 or M71, also produce green fluorescent protein (GFP) or a fusion of tau with GFP. We visualized OSNs and their axons in whole mounts with two-photon laser scanning microscopy. The main conclusion we draw from the three-dimensional reconstructions is the high degree of morphological variability of mature glomeruli receiving axonal input from OR-expressing OSNs and of the pathways taken by the axons to those glomeruli. We also observe that axons of OR-expressing OSNs do not innervate nearby glomeruli in mature mice. Postnatally, a tangle of axons from M72-expressing OSNs occupies a large surface area of the bulb and coalesces abruptly into a protoglomerulus at a reproducible stage of development. These results differ in several aspects from those reported for the development of glomeruli receiving input from OSNs expressing the P2 OR, suggesting the need for a more systematic examination of OR-specific glomeruli.

Brain targeting and glomerulus formation of two olfactory neuron populations expressing related receptor types

Olfactory sensory neurons expressing different members of the mOR37 odourant receptor subfamily send their axons to distinct glomeruli located in the immediate vicinity in the olfactory bulb [Strotmann, J., Conzelmann, S., Beck, A., Feinstein, P., Breer, H. & Mombaerts, P. (2000) J. Neurosci., 20, 6927-6938]. In this study, the potential of transgenic mouse lines was used to explore the onset of receptor expression, the outgrowth of axons as well as the glomerulus formation for two neuron populations expressing different mOR37 subtypes. The data indicate a synchronous time course of these features for both neuron populations. From E15 until the day of birth, the axons of the two mOR37 populations terminate in a common, small area of the presumptive olfactory bulb. During a short postnatal phase, the two axon populations segregate into distinct, protoglomerular structures; some aberrant fibers can still be observed during this period.

Immunohistochemical localization of calbindin D-28k in the migratory pathway from the rat olfactory placode

The spatiotemporal localization of calbindin D-28k (Calb), a calcium-binding protein, was examined immunohistochemically in the developing rat olfactory system with special reference to cell migration from the olfactory placode. Calb immunoreactivity was first detected at embryonic day 12 (E12) in a few cells just outside the olfactory epithelium, and at E13, Calb-immunoreactive cells were found scattered in the laminin-rich mesenchyme. By E14, Calb-immunoreactive cells had increased in number and were seen along the entire migratory route between the vomeronasal organ, a derivative of the medial olfactory pit, and the ventromedial surface of the telencephalic vesicle. Calb neurones were not seen in the olfactory epithelium, a derivative of the lateral olfactory pit. Although the distribution pattern of Calb-immunoreactive cells was similar to that of luteinizing hormone releasing hormone (LHRH)-producing neurones, which are known to originate in the vomeronasal organ and migrate into the forebrain, Calb and LHRH immunoreactivities were contained in separate neuronal populations. Calb-immunoreactive cells were localized along the vomeronasal nerves, identified by labelling the vomeronasal organ with the lipophilic dye, DiI, and strongly immunoreactive for neural cell adhesion molecule (NCAM). These data strongly suggest that, in addition to LHRH neurones, the rat vomeronasal organ generates Calb-immunoreactive neurones which migrate along the vomeronasal nerves to enter the forebrain. The final fate and functional importance of these cells remains to be determined.

Radial glia development in the mouse olfactory bulb

Radial glia are critical for cell migration and lamination of the cortex. In most developing cortical structures, radial glia, as their name suggests, extend processes from the ventricle to the pia in regular parallel arrangements. However, immunohistochemical labeling from several laboratories suggests that radial glia have a more branched morphology in the olfactory bulb. To investigate the morphology of radial glia in the mouse olfactory bulb we (1) labeled radial glia and olfactory receptor neuron axons at 24-hour intervals by immunohistochemistry; and (2) developed a novel method of generating and applying "nanocrystals" of 1,1'-dioctadecyl-3,3,3',3'- tetramethylindocarbocyanine perchlorate (DiI) to the ventricle surface such that the processes of single olfactory bulb radial glia are labeled in the embryonic olfactory bulb. We examined the structure and interactions of radial glia with ingrowing olfactory receptor neuron (ORN) axons in late embryonic olfactory bulb development. These results showed that olfactory bulb radial glia do not form straight parallel structures as do radial glia in the neocortex but rather have a convoluted trajectory from the ventricle to the bulb surface. Moreover, olfactory bulb radial glia consistently extend tangential branches at the level of the internal plexiform layer. Beginning at embryonic day 17.5, two types of radial glia can be distinguished: type I radial glia have a process that extends from the ventricle into the glomerular layer. These apical processes form highly restricted tufts, or "glial glomeruli" at the same time that ORN axons are forming "axonal glomeruli." In type II radial glia the apical process does not enter the glomerular layer but instead ramifies within the external plexiform layer. The tight spatiotemporal relationship between the glomerulization of radial glia processes and ORN axons during development suggest that radial glia processes could play a role in the formation and/or stabilization of mammalian glomeruli.

EphB2 and two of its ligands have dynamic protein expression patterns in the developing olfactory system

Primary olfactory neurons are located in the olfactory neuroepithelium lining the nasal cavity. Their axons converge and form glomeruli with the dendrites of second-order neurons in the olfactory bulb. The molecular basis of primary olfactory axon guidance, targeting and subsequent arborisation is largely unknown. In this study we examined the spatio-temporal expression of the Eph receptor EphB2 and its ligands, ephrin-B1 and ephrin-B2, during development of the rat primary olfactory system. Unlike in other regions of the nervous system where receptor and ligand expression patterns are usually non-overlapping, EphB2, ephrin-B1 and ephrin-B2 were all expressed by primary and second-order olfactory neurons. In the embryonic animal we found that these three proteins had distinct and different expression patterns. EphB2 was first expressed at E18.5 by the perikarya of primary olfactory neurons. In contrast, ephrin-B1 was expressed from E13.5 and was localised to the axons of these cells up to E18.5 but was then restricted to the perikarya. Ephrin-B2, however, was expressed by olfactory ensheathing cells. EphB2, ephrin-B1 and ephrin-B2 were also expressed in the prenatal olfactory bulb and were restricted to the perikarya of mitral cells. In the post-natal olfactory bulb there was a shift in the localisation of both EphB2 and ephrin-B1 to the dendritic arborisations of mitral cells. The dynamic and tightly regulated spatio-temporal expression patterns of EphB2, ephrin-B1 and ephrin-B2 by specific olfactory cell populations suggest that these molecules have the potential to regulate important developmental events in the olfactory system.

Olfactory ensheathing cells: bridging the gap in spinal cord injury

Spinal cord injury (SCI) continues to be an insidious and challenging problem for scientists and clinicians. Recent neuroscientific advances have changed the pessimistic notion that axons are not capable of significant extension after transection. The challenges of recovering from SCI have been broadly divided into four areas: 1) cell survival; 2) axon regeneration (growth); 3) correct targeting by growing axons; and 4) establishment of correct and functional synaptic appositions. After acute SCI, there seems to be a therapeutic window of opportunity within which the devastating consequences of the secondary injury can be ameliorated. This is supported by several observations in which apoptotic glial cells have been identified up to 1 week after acute SCI. Moreover, autopsy studies have identified anatomically preserved but unmyelinated axons that could potentially subserve normal physiological properties. These observations suggest that therapeutic strategies after SCI can be directed into two broad modalities: 1) prevention or amelioration of the secondary injury, and 2) restorative or regenerative interventions. Intraspinal transplants have been used after SCI as a means for restoring the severed neuraxis. Fetal cell transplants and, more recently, progenitor cells have been used to restore intraspinal circuitry or to serve as relay for damaged axons. In an attempt to remyelinate anatomically preserved but physiologically disrupted axons, newer therapeutic interventions have incorporated the transplantation of myelinating cells, such as Schwann cells, oligodendrocytes, and olfactory ensheathing cells. Of these cells, the olfactory ensheathing cells have become a more favorable candidate for extensive remyelination and axonal regeneration. Olfactory ensheathing cells are found along the full length of the olfactory nerve, from the basal lamina of the epithelium to the olfactory bulb, crossing the peripheral nervous system-central nervous system junction. In vitro, these cells promote robust axonal growth, in part through cell adhesion molecules and possibly by secretion of neurotrophic growth factors that support axonal elongation and extension. In animal models of SCI, transplantation of ensheathing cells supports axonal remyelination and extensive migration throughout the length of the spinal cord. Although the specific properties of these cells that govern enhanced axon regeneration remain to be elucidated, it seems certain that they will contribute to the establishment of new horizons in SCI research.

Expression of neuropeptide Y in olfactory ensheathing cells during prenatal development

Neuropeptide Y (NPY) is expressed in a special type of glial cell, the olfactory ensheathing cells, that surround the axons of olfactory sensory neurons on their way from the olfactory epithelium to the glomeruli in the olfactory bulb. The expression of NPY in ensheathing cells was examined during prenatal development of the olfactory system by using immunohistochemistry and in situ hybridization. NPY expression was compared with the expression of growth associated protein-43, olfactory marker protein, the low-affinity nerve growth factor receptor (p75) and S-100, factors expressed in the olfactory system at known stages of development. NPY-like immunoreactivity (NPY-LI) and NPY mRNA expression was first detected in the olfactory nerve layer of the olfactory bulb at embryonic day 15. From embryonic day 16 and onward, a clear segregation could be observed in the intensity of both NPY-LI and NPY mRNA expression within the olfactory nerve layer. NPY expression was most intense in the inner part of the olfactory nerve layer. In the outer olfactory nerve layer, a clear decrease in NPY expression was observed. The inner olfactory nerve layer, showing high NPY expression, did not stain for S-100 or p75. However, NPY-LI was found to coexist with S-100-LI from the outer olfactory nerve layer until the olfactory epithelium and with p75-LI in cells surrounding the olfactory nerve. These results show that NPY is expressed in ensheathing cells before olfactory sensory neurons mature and the formation of the glomerular layer starts. NPY might be involved in the guidance, growth, or both, of olfactory sensory axons toward their target glomeruli in the olfactory bulb or have a function in the maturation of the olfactory sensory neurons.

Levels of amino acid neurotransmitters during mouse olfactory bulb neurogenesis and in histotypic olfactory bulb cultures

The developmental changes in the levels of amino acid neurotransmitters were analyzed by high pressure liquid chromatography during mouse olfactory bulb neurogenesis, from embryonic day (E)13 until the young adult age, between postnatal days (P)30 and P40. During the embryonic period, high levels of glutamate, aspartate and GABA were observed, with the values of GABA about 2-fold higher than those of glutamate and aspartate. At P0, the production of these neurotransmitters experienced birth stress as shown by a significant 2-fold reduction in their levels. During the first two postnatal weeks, a progressive increase in the glutamate content was detected diminishing slightly in the adult stage. The aspartate concentrations showed a maximal value at P3 and then decreased gradually until the second postnatal week; in the young adult age, its concentration was comparable with that of glutamate. The postnatal GABA contents increased progressively from birth to maturity, showing maximal levels at P3, P11 and in the adult. Throughout the studied developmental period, the concentration of glycine remained relatively low. With regard to taurine, very low concentrations were detected during the prenatal period but after birth, the taurine content gradually increased with age, and in the adult animal, its concentration was comparable with those of GABA and glutamate. Our data demonstrate the predominance of GABA and glutamate during olfactory bulb synaptogenesis, however, in the adult animal, both glutamate and aspartate exert the same influence in the excitatory synaptic transmission; in the adult inhibitory synaptic transmission, taurine appears to play an important neuromodulatory or neurotransmitter role as that of GABA. To determine the intrinsic neurotransmitter production, primary histotypic olfactory bulb cultures were prepared from mice at P10. The comparative analysis of in vitro neurotransmitter contents with those in in situ adult animal showed higher levels of endogenously produced glutamate, glycine and GABA in the olfactory bulb than the extrinsic ones coming from olfactory nerve axons and higher olfactory brain centers. On the other hand, most of aspartate and taurine neurotransmitters apparently come from extrinsically located neurons.

Adhesion molecule expression and phenotype of glial cells in the olfactory tract

These data illustrate that OBECs have a highly plastic nature in keeping with their need to respond rapidly to changing environmental cues. This relates to their required function in supporting axonal extension throughout life. Future studies using antibodies to PSA-NCAM and L-NGFr together with FACS sorting to purify the two types of OBEC should give us a clearer understanding of the lineage relationship of the two phenotypes. With purified populations of the astrocyte-like and Schwann cell-like OBEC we should be able to determine if these cells have different functions in vivo, using several approaches namely: i) identifying the growth factors that regulate their growth and differentiation, ii) measuring the ability of the purified cells to remyelinate the experimentally-created CNS lesions and iii) carry out more detailed cellular and molecular comparisons of the two phenotypes.

Development of P2 olfactory glomeruli in P2-internal ribosome entry site-tau-LacZ transgenic mice

Primary olfactory neurons project their axons to the olfactory bulb, where they terminate in discrete loci called glomeruli. All neurons expressing the same odorant receptor appear to terminate in a few glomeruli in each olfactory bulb. In the P2-IRES-tau-LacZ line of transgenic mice, LacZ is expressed in the perikarya and axons of primary olfactory neurons that express the P2 odorant receptor. In the present study, we examined the developmental appearance of P2 neurons, the topographical targeting of P2 axons, as well as the formation of P2 glomeruli in the olfactory bulb. P2 axons were first detected in the olfactory nerve fiber layer at embryonic day 14.5 (E14.5), and by E15.5 these axons terminated in a broad locus in the presumptive glomerular layer. During the next 5 embryonic days, the elongated cluster of axons developed into discrete glomerulus-like structures. In many cases, glomeruli appeared as pairs, which were initially connected by a fascicle of P2 axons. This connection was lost by postnatal day 7.5, and double glomeruli at the same locus were observed in 85% of adult animals. During the early postnatal period, there was considerable mistargeting of P2 axons. In some cases P2 axons entered inappropriate glomeruli or continued to grow past the glomerular layer into the deeper layers of the olfactory bulb. These aberrant axons were not observed in adult animals. These results indicate that olfactory axons exhibit errors while converging onto a specific glomerulus and suggest that guidance cues may be diffusely distributed at target sites in the olfactory bulb.

Development of a glia-rich axon-sorting zone in the olfactory pathway of the moth Manduca sexta

Olfactory receptor cells (ORCs) of a particular odor tuning are dispersed in the olfactory epithelium, but their axons converge on distinct glomeruli in primary olfactory centers. As a consequence, axon associations must change to bring axons of ORCs with the same odor specificity together. Studies in Manduca sexta have indicated that just before they enter the antennal lobe (AL), ORC axons undergo extreme reorganization, finally entering the AL in fascicles destined for subsets of glomeruli. This axon-sorting zone is heavily populated by glial cells, and ORC axon growth cones often are in close physical contact with the glia. In moths rendered glia deficient, ORC axons fail to fasciculate in this region. Using propidium iodide to label nuclei and 5-bromo-2'-deoxyuridine to monitor proliferation, we found that the glia in the sorting zone arise from the AL, appearing shortly after the first ORC axons arrive. Experimental removal of some or all of the sensory innervation revealed that proliferation of sorting-zone glia is triggered by ORC axons. A second set of glia arises in the antenna and migrates along the antennal nerve toward the brain, populating the nerve after the establishment of the sorting zone. Development of this type of glial cell is independent of contact of the ORC axons with their central targets. We conclude that the sorting zone arises from CNS glia in response to ingrowth of ORC axons, and a critical number of glia must be present in the sorting zone for axons to correctly establish new neighbor-neighbor associations.

Expression of galectin-1 in the olfactory nerve pathway of rat

The olfactory neuroepithelium is characterised by the mosaic distribution of primary olfactory neurons that express different odorant receptors and cell surface glycoconjugates. Carbohydrates are believed to form a glycocode that mediates sorting out and fasciculation of primary olfactory axons through interactions with carbohydrate-binding proteins such as galectin-1. In the present study, we describe in detail the expression pattern of galectin-1 in the developing and adult rat olfactory system. We demonstrate that galectin-1 is expressed by olfactory ensheathing cells both in olfactory nerve and within the nerve fibre layer of the olfactory bulb of the embryonic and adult rat. In the adult rat, galectin-1 was preferentially expressed by olfactory ensheathing cells in the nerve fibre layer of the ventromedial and lateral surfaces of the olfactory bulb. Galectin-1 was also expressed by subsets of periglomerular cells and granule cells, particularly in the ventromedial region of the olfactory bulb. In adult rat, the galectin-1 ligand, N-acetyl-lactosamine, was expressed by primary olfactory axons that terminated in glomeruli present in the ventromedial and lateral olfactory bulb. These results suggest that expression of galectin-1 may provide a mechanism for the sorting of subpopulations of axons in the nerve fibre layer of the olfactory bulb during development as well as play a role in the postnatal maintenance of specific glomerular connections.

Olfactory systems: common design, uncommon origins?

In both vertebrates and invertebrates, odorant molecules reach the dendrites of olfactory receptor cells through an aqueous medium, which reflects the evolutionary origin of these systems in a marine environment. Important recent advances, however, have demonstrated striking interphyletic differences between the structure of vertebrate and invertebrate olfactory receptor proteins, as well as the organization of the genes encoding them. While these disparities support independent origins for odor-processing systems in craniates and protostomes (and even between the nasal and vomeronasal systems of craniates), olfactory neuropils share close neuroanatomical and physiological characters. Whereas there is a case to be made for homology among members of the two great protostome clades (the ecdysozoans and lophotrochozoans), the position of the craniates remains ambiguous.

Temporal and spacial relationships between PSA-NCAM-expressing, newly generated granule cells, and radial glia-like cells in the adult dentate gyrus

The granule cell layer of the adult dentate gyrus possesses two characteristics of an immature nervous system. The first is that granule cells continue to be generated in the innermost region of the granule cell layer, and newly generated and developing granule cells in the adult express highly polysialylated neural cell adhesion molecule (PSA-NCAM). PSA-NCAM-expressing apical dendrites have dynamically unstable processes such as irregular shafts and many stick-like or fan-shaped fine processes. The second is that radial glia-like cells expressing glial fibrillary acidic protein (GFAP) remain in a similar region of the granular layer. The numbers of PSA-NCAM-expressing granule cells and GFAP-expressing radial glia-like cells show a parallel age-dependent decrease during aging. Moreover, by using confocal laser scanning microscopy and immunoelectron microscopy, we demonstrated that PSA-NCAM-expressing dendrites and GFAP-expressing radial processes are partly in contact with each other, and occasionally the radial glial processes envelop the PSA-NCAM-positive dendritic processes. The temporal and spatial relationship between the two immature elements suggests that the processes of the radial glia-like cells are closely associated with the dendritic growth of the newly generated granule cells in the adult dentate gyrus and that these two immature features of neurons and glia in the dentate gyrus diminish with age.

Dendritic pattern development of the honeybee antennal lobe neurons: a laser scanning confocal microscopic study

The processing of odorant signals is performed, in the olfactory bulb of vertebrates or in the antennal lobe of insects, by different types of neurons which display specific morphological and functional features. The present work characterizes the morphogenesis of the main neuronal types which participate in olfactory discrimination in the adult honeybee (Apis mellifera). Neurons were stained intracellularly with Lucifer yellow at different stages of pupal development and in the adult, and imaged by laser scanning confocal microscopy. Attending to branching patterns, all pupal neurons could be attributed to morphological types previously established in the adult. Given the functional importance of intraglomerular dendritic arbors in the processing of olfactory information, the study focused on their development. The two main classes, dense and sparse intraglomerular arbors, display adultlike features as early as the second day of pupal development. However, morphometric measurements and confocal observations show that their general pattern undergoes continuous maturation processes until late pupal stages and after emergence of the adult. Among these, the results point out a pruning of dendritic branches in sparse arbors, but not in dense arbors.

Developmental analysis of the peripheral olfactory organ of the opossum Monodelphis domestica

The gray, short-tailed opossum, Monodelphis domestica, is born in a very immature state after a brief (14-day) gestation. As a result, the species provides a unique opportunity to examine very early periods of mammalian development. The present study provides the first detailed morphometric analysis of the development of the olfactory mucosa and the nasal cavity in Monodelphis. The extent of the sensory mucosa increases dramatically across development, covering a growing nasal cavity and increasingly elaborate turbinates. Both nasal cavity convolution (a measure of turbinate complexity) and mucosal surface area show extensive growth between birth and adulthood. These measurements are greatest in the central portion of the mucosa (in the caudal portion of the nose) at all ages examined. A developmental BrdU study reveals a robust decrease in cellular proliferation with age; proliferation decreases to near adult-like patterns by postnatal day (P) 40. Results from these studies show that there is dramatic structural and cellular postnatal growth in the opossum peripheral olfactory organ.

High glucose-6-phosphate dehydrogenase activity contributes to the structural plasticity of periglomerular cells in the olfactory bulb of adult rats

Glucose-6-phosphate dehydrogenase (G6PD) activity, assayed spectrophotometrically, was found to be higher in the olfactory bulb (OB) than in other brain areas of adult rats [P. Ninfali, G. Aluigi, W. Balduini, A. Pompella, Glucose-6-phosphate dehydrogenase is higher in the olfactory bulb than into other brain areas, Brain Res. 744 (1997) 138-142]. Histochemical demonstration of G6PD activity in cryostat sections of OB, analyzed with optical microscopy, revealed a marked and well defined line of formazan deposition in the internal part of the glomerular layer (Glm), indicating that G6PD was much higher in cells distributed along the glomeruli. Electron microscope analysis showed that G6PD activity was mainly concentrated in cytoplasm and dendrites of periglomerular cells, the interneurons which span glomeruli and connect olfactory nerves with mitral/tufted cells. Since G6PD regulates the flux through the hexose monophosphate shunt (HMS) pathway, which provides NADPH for reductive biosynthesis and pentose phosphates for nucleic acid formation, it can be concluded that high G6PD activity in periglomerular neurons is functional to their differentiating capability. This result is consistent with the occurrence of structural plasticity events in the OB of adult rats.

Development of the vertebrate main olfactory system

Olfactory receptor neurons project from the sensory epithelium to stereotyped targets within the olfactory bulb. Recent studies suggest that the generation of this precise spatial map probably involves a hierarchy of guidance events, as receptor neurons integrate information present in the epithelium and bulb to reach their target.

Adult rat olfactory nerve ensheathing cells are effective promoters of adult central nervous system neurite outgrowth in coculture

A coculture method is described for ensheathing glial cells from adult rat olfactory nerve, serving as a substrate for the regrowth of neurites from adult rat retinal ganglion cells. Immunocytochemically identified phenotypes present in primary cultures of olfactory nerve cells are described, and their ability to promote neurite outgrowth is compared with neonatal astrocytes and Schwann cells, with other nonglial cells, and with laminin. Ensheathing cell cultures were more effective than any other substrate tested and also directed the orientation of regrowing neurites. In comparison with cultured Schwann cells, which released neurotrophic factors into the culture medium, there was no evidence of a similar activity in ensheathing cell cultures. Combinations of ensheathing cell-conditioned medium and substrates of laminin, merosin, or 3T3 cells also failed to show the release of factors enhancing either survival or neurite outgrowth from retinal ganglion cells. Evidence is presented for a partial inhibition of neurite outgrowth in the presence of calcium channel antagonists or an intracellular calcium-chelating reagent. This provides evidence for a contribution from an intracellular calcium signaling mechanism, possibly implicating ensheathing cell adhesion molecules in promoting neurite outgrowth.

An olfactory sensory map develops in the absence of normal projection neurons or GABAergic interneurons

Olfactory sensory neurons expressing a given odorant receptor project to two topographically fixed glomeruli in the olfactory bulb. We have examined the contribution of different cell types in the olfactory bulb to the establishment of this topographic map. Mice with a homozygous deficiency in Tbr-1 lack most projection neurons, whereas mice with a homozygous deficiency in Dlx-1 and Dlx-2 lack most GABAergic interneurons. Mice bearing a P2-IRES-tau-lacZ allele and deficient in either Tbr-1 or Dlx-1/Dlx-2 reveal the convergence of axons to one medial and one lateral site at positions analogous to those observed in wild-type mice. These observations suggest that the establishment of a topographic map is not dependent upon cues provided by, or synapse formation with, the major neuronal cell types in the olfactory bulb.

Molecular development of the olfactory nerve pathway

There are, at least, two major questions concerning the molecular development of the olfactory nerve pathway. First, what are the molecular cues responsible for guiding axons from the nasal cavity to the olfactory bulb? Second, what is the molecular basis of axon targeting to specific glomeruli once axons reach the olfactory bulb? Studies in the primary olfactory pathway have focused on the role of extracellular matrix and ensheathing cells in establishing an initial substrate for growth of pioneer axons between the periphery and brain. The primary axons also express a multitude of cell adhesion molecules that regulate fasciculation of axons and hence may play a role in fascicle formation in the olfactory nerve. Although the olfactory neuroepithelium principally consists of a morphologically homogeneous class of primary olfactory neurons, there are numerous subpopulations of olfactory neurons expressing chemically distinct phenotypes. In particular, numerous subpopulations have been characterized by expression of unique carbohydrate residues and olfactory receptor proteins. Some of these molecules have recently been implicated in axon guidance and targeting to specific glomeruli.

Two novel monoclonal antibodies (1.9.E and 4.11.C) against olfactory bulb ensheathing glia

We produced and characterized two monoclonal antibodies, termed 1.9.E and 4.11.C, that specifically recognize olfactory bulb ensheathing glia. Both antibodies were generated using the olfactory nerve layer (ONL) of newborn rat olfactory bulbs (P0, P1) as immunogens. The specificity of these antibodies was tested by immunofluorescence techniques on tissue sections and cultures of adult and neonatal rat olfactory bulbs, and by Western blot analysis. 1.9.E labeled the ONL and glomerular layer of the olfactory bulb (OB) of adult rats. In newborn rats, 1.9.E immunostained ensheathing cells from the ONL and peripheral olfactory fascicles. Furthermore, 1.9.E reacted with some processes of the radial glia in the periventricular germinal layer of the newborn rat. Although 4.11.C also specifically labeled ensheathing cells in the adult OB, it did not stain any cell type in the ONL of newborn rats. The lack of double labeling with either 1.9.E or 4.11.C and anti-olfactory marker protein (OMP) antibody, a specific marker for olfactory axons, indicated that none of the monoclonals recognized olfactory axons. Double immunostaining of adult OB cultures with 1.9.E or 4.11.C and anti-p75-nerve growth factor receptor revealed that both antibodies specifically recognized ensheathing glia in those cultures. Filaments were strongly labeled throughout the entire cytoplasm of ensheathing cells, suggesting that 1.9.E and 4.11.C immunoreacted with ensheathing glia cytoskeleton. 4.11.C stained a few Schwann cells in adult sciatic nerve sections. Moreover, 4.11.C immunostained cortical astrocyte cultures from newborn rats (P1). In Western blot analysis both antibodies recognized a major component, migrating with an apparent molecular weight of 60 kDa, from olfactory nerve and glomerular layer (ONGL) extracts of adult and neonatal rats. The pattern of immunoreactivity of 1.9.E and 4.11.C antibodies suggest that both antibodies are specific markers for olfactory ensheathing glia in the adult rat central nervous system (CNS).

Glomerulus development in the absence of a set of mitral-like neurons in the insect olfactory lobe

Mitral cells are the first neurons in the mammalian olfactory bulb to synapse with olfactory receptor axons during glomerulus development, and in an invertebrate, the moth Manduca sexta, mitral-like neurons overlap very early with olfactory receptor axons as they begin to form protoglomeruli. The possibility for early interaction between receptor neurons and mitral-like neurons led us to ask whether such an interaction plays an essential role in glomerulus development. In the current study in the moth, we surgically removed a major class of these mitral-like neurons before glomeruli began to form and asked: (a) Is the formation of the array of olfactory glomeruli triggered by an interaction of the first-arriving receptor axons with the dendrites of mitral-like neurons? (b) At the level of individual glomeruli, must the mitral-like dendrites be in place either to maintain receptor axons in a glomerular arrangement, or to guide later-growing dendrites of other types into the developing glomeruli? Our results indicate that even without the participation of this group of mitral-like neurons, the array of sexually isomorphic ordinary glomeruli forms and the basic substructure of individual glomeruli develops apparently normally. We conclude that the mitral-like neurons in Manduca are not essential for the formation of ordinary olfactory glomeruli during development.

Spatiotemporal patterns of expression of extracellular matrix molecules in the developing and adult rat olfactory system

Using immunocytochemical methods, we have examined extensively the spatial and temporal patterns of expression of three extracellular matrix molecules-laminin, fibronectin, and type IV collagen-in the embryonic, postnatal (days 2 and 11) and adult rat olfactory system. The study started at embryonic day 14 when olfactory fibres and their associated migrating cells course through the nasal mesenchyme. From embryonic day 14 to the adult, a sheet-like pattern of labelling for laminin, fibronectin and type IV collagen was observed along the basal surface of the olfactory epithelium and around the telencephalon. This type of labelling was continuous around the telencephalic vesicle, whereas it appeared disrupted in the basal lamina of the olfactory epithelium to permit exit of the olfactory axons and their associated migrating cells into the mesenchyme. From embryonic day 14 to day 20, punctate labelling for the three molecules studied was observed along the mesenchymal olfactory pathway, the ventral part of the olfactory bulb, the olfactory nerve layer and the presumptive glomerular layer, respectively. By embryonic day 17, the punctate labelling initially detected in the mesenchymal olfactory pathway was replaced by a sheet-like pattern related to the mature basal lamina surrounding the olfactory axon fascicles. Punctate labelling for laminin and type IV collagen persisted in the olfactory nerve layer and around the glomeruli through adult life whereas that of fibronectin declined and disappeared by postnatal day 2. The spatiotemporal distribution of the punctate pattern for laminin, fibronectin and type IV collagen observed in the embryonic olfactory system suggests a role in delineating the pathway for olfactory axon elongation. The continuous expression of laminin and type IV collagen in the adult olfactory bulb may be related to the regenerative activity and high plasticity of the olfactory system.

Olfactory ensheathing glia: properties and function

The failure of regenerating axons to grow within the adult mammalian central nervous system (CNS) does not apply to the olfactory bulb (OB). In this structure, normal and transected olfactory axons are able to enter, regenerate, and reestablish lost synaptic contacts with their targets, throughout the lifetime of the organism. A remarkable difference between an axonal growth-permissive structure such as the OB and the remaining CNS resides in the presence of ensheathing glia in the former. These cells exhibit phenotypic and functional properties known to be involved in the process of axonal elongation that may explain the permissibility of the OB to axonal growth. In addition, transplants of ensheathing glia were successfully used to promote axonal regeneration within the injured adult CNS. The axonal growth-promoting properties of ensheathing glia make the study of this cell type interesting to provide an insight into the mechanisms underlying the process of axonal regeneration. Therefore, in this article we review the developmental, morphologic, immunocytochemical, and functional properties presented by this unique glial cell type, and correlate them with the axonal growth-promoting ability of ensheathing glia. In addition, we provide some evidence of the potentiality that ensheathing glia might have as a promoter of axonal regeneration within the injured nervous system.

Spatio-temporal patterns of ensheathing cell differentiation in the rat olfactory system during development

An immunocytochemical approach with specific glial markers was used to investigate the temporal and spatial patterns of differentiation of ensheathing glia wrapping axon fascicles along the primary olfactory pathway of the rat during development. The two glial markers tested, the proteins S-100 and glial fibrillary acidic protein, are known to be expressed at different stages of maturation in glial cells. The S-100 protein was first weakly expressed in cells accompanying the olfactory axons at embryonic day 14 (E14), while a first faint glial fibrillary acidic protein staining was detected along the olfactory axons at E15 and along the vomeronasal nerves at E16. A strong S-100 immunoreactivity was already present from E16 onwards along the axon fascicles through their course in both the nasal mesenchyme and the subarachnoid space before entering the olfactory nerve layer of the olfactory bulb. A gradual increase in glial fibrillary acidic protein expression was observed along this part of the developing olfactory pathway from E16 up to E20, when an adult-like pattern of staining intensity was seen. By contrast, most of the ensheathing cells residing in the olfactory nerve layer exhibited some delay in their differentiation timing and also a noticeable delayed maturation. It was only from E20 onwards that a weak to moderate S-100 expression was detected in an increasing number of cells throughout this layer, and only few of them appeared weakly glial fibrillary acidic protein positive at postnatal days 1 and 5. The immunocytochemical data indicate that there is a proximodistal gradient of differentiation of ensheathing cells along the developing olfactory pathway. The prolonged immaturity of ensheathing cells in the olfactory nerve layer, which coincides with the formation of the first glomeruli, might facilitate the sorting out of olfactory axons leading to a radical reorganization of afferents before they end in specific glomeruli.

How simple is the organization of the olfactory glomerulus?: the heterogeneity of so-called periglomerular cells

Recent progress in the studies of the olfactory system, especially in the molecular biological studies, makes it one of the useful sensory model systems for understanding neural mechanisms for the information processing. In the olfactory bulb, the primary center of the olfactory system, glomeruli are regarded as important functional units in the transmission of odorant signals and in processing the olfactory information, but have been believed to be composed by only a small number of neuronal types and thus to be simple in their neuronal and synaptic organization. However, accumulating morphological data reveal that each type of neurons might further consist of several different subpopulations, indicating that the organization of glomeruli might not be so simple as it was believed. Here we describe an aspect of the structural organization of glomeruli, focusing on the heterogeneities of periglomerular neurons in mammalian main olfactory bulb.