[How is the sense of smell connected? Cellular and molecular mechanisms guiding the development of the synaptic connections from the nose to the cortex (I)]

The physiological particularities that occur during the development of the olfactory system make it one of the most fascinating parts of the central nervous system and one of models that has been most widely studied in order to understand the mechanisms related with axonal growth and guidance towards the right targets. A variety of mechanisms are known, some mediated by contact (laminins, cell adhesion molecules, ephrins, etc.) and others that are secreted (semaphorins, slits, growth factors, etc.), to play diverse roles in establishing the synaptic interactions among the olfactory epithelium, the olfactory bulb and the olfactory cortex. In relation to this, other specific mechanisms for this system have also been proposed, including the incredible family of close to 1000 different olfactory receptors. In recent years, different reviews have focused on the partial elements of this system, especially on the mechanisms involved in the formation of the olfactory nerve. However, no detailed review of those related with the development of the connections between the different olfactory structures (epithelium, bulb and cortex) has been put forward to date. In this first part of the review, we address this topic from the following perspective: the different cellular and molecular mechanisms that guide the formation of the olfactory nerve and the lateral olfactory tract.

Chemical properties of type 1 and type 2 periglomerular cells in the mouse olfactory bulb are different from those in the rat olfactory bulb

We analyzed the cellular composition of the juxtaglomerular region in the main olfactory bulb of C57B/6J strain mice, focusing on 1) the compartmental organization of the glomerulus and the presence of type 1 and 2 periglomerular cells, 2) the colocalization relationships among the 4 major chemically identified groups of periglomerular cells, glutamic acid decarboxylase (GAD)/gamma-aminobutyric acid (GABA), tyrosine hydroxylase, calretinin and calbindin D28k positive periglomerular cells, and 3) the chemical properties of the nitric oxide synthase (NOS)-positive juxtaglomerular cells. We confirmed the compartmental organization of the glomerulus and the presence of both type 1 and 2 periglomerular cells in the mice. Similar to rat periglomerular cells, the tyrosine hydroxylase-positive cells were type 1 and GAD/GABA-positive. On the other hand, both the calbindin D28k-positive and calretinin-positive cells were type 2 periglomerular cells, but in contrast to those in rats, which are GAD/GABA-negative, all of the calbindin D28k-positive periglomerular cells and 65% of the calretinin-positive periglomerular cells were GAD/GABA-positive. The GAD/GABA-positive cells thus included both type 1 and type 2 periglomerular cells. Juxtaglomerular NOS-positive cells have been proposed as a subgroup of type 1 periglomerular cells that are separate from the calretinin-positive and calbindin D28k-positive cells in rats. However, in the mice, about 70% of the NOS-positive cells were calretinin-positive, and 50% of the calretinin-positive cells were NOS-positive. We herein reveal the significant species differences in the chemical properties of periglomerular cells and suggest that the cellular organization of the mouse main olfactory bulb cannot be extrapolated from that of rats.

The olfactory route for cerebrospinal fluid drainage into the peripheral lymphatic system

Drainage of the cerebrospinal fluid through the olfactory nerves into the nasal lymphatics has been suggested repeatedly. To investigate precisely the morphology of this pathway, India ink was injected into the subarachnoidal space of the rat brain, and samples including the olfactory bulbs, olfactory tracts and the nasal mucosa were observed by light and electron microscopy. Under the dissecting microscope, ink particles were found within the subarachnoid space and along the olfactory nerves. At the nasal mucosa, a lymphatic network stained in black was identified near the olfactory nerves, which finally emptied into the superficial and deep cervical lymph nodes. Light microscopically, ink particles were found in the subarachnoid space, partially distributed around the olfactory nerves and within the lymphatic vessels. By electron microscopy, the subarachnoid space often formed a pocket-like space in the entrance of the fila olfactoria. The olfactory nerves were partially surrounded by ink particles within the space between perineurial cells and epineurial fibroblasts. At the nasal mucosa, the lymphatics were frequently located close to the nerves. These results indicate that the cerebrospinal fluid drains from the subarachnoid space along the olfactory nerves to the nasal lymphatics, which in turn, empties into the cervical lymph nodes. This anatomical communication, thus, allows the central nervous system to connect with the lymphatic system. The presence of this route may play an important role in the movement of antigens from the subarachnoidal space to the extracranial lymphatic vessels, resulting in inducement of an immune response of the central nervous system.

Abnormalities of Axon Growth in Human Olfactory Mucosa

OBJECTIVES/HYPOTHESIS

Random biopsies of the human adult olfactory mucosa often demonstrate degenerative changes in the olfactory epithelium (OE) in both dysosmic and normosmic patients and, consequently, have limited diagnostic usefulness. However, detailed analysis of the subepithelial tissue with specific attention to the fascicles of the olfactory nerve and abnormalities of axonal growth may improve the correlation of histopathology with sensory function.

STUDY DESIGN

Retrospective review of human OE biopsies.

METHODS

Mucosal biopsies from the olfactory area obtained from 27 subjects were examined by light and electron microscopy, with particular attention to the olfactory nerve fascicles; results were correlated with clinical status. Immunohistochemical analysis was used to characterize the extent of axonal depletion, relative maturity of the parent population, and aberrant axonal growth.

RESULTS

As expected, there are areas of respiratory metaplasia and neuronal depletion in normosmic as well as dysosmic patients. The degree of axon degeneration within the fascicles correlates better with individual olfactory status. Immature neurons predominate, and re-entrant neuromas develop in patients with olfactory loss caused by disconnection from the olfactory bulb. Individuals with olfactory loss caused by epithelial damage as with chronic rhinosinusitis display evidence of nerve fascicle degeneration and intraepithelial neuromas.

CONCLUSION

The status of olfactory axons provides useful information on the overall condition of the olfactory periphery and improves the diagnostic usefulness of mucosal biopsies. In addition to an assessment of the epithelium per se, the fascicles of the olfactory nerve need to be characterized for a complete analysis of the olfactory mucosa.

Characterization of somatostatin- and cholecystokinin-immunoreactive periglomerular cells in the rat olfactory bulb

Periglomerular cells (PG) are interneurons of the olfactory bulb (OB) that modulate the first synaptic relay of the olfactory information from the olfactory nerve to the dendrites of the bulbar principal cells. Previous investigations have pointed to the heterogeneity of these interneurons and have demonstrated the presence of two different types of PG. In the rat OB, type 1 PG receive synaptic contacts from the olfactory axons and are gamma-aminobutyric acid (GABA)-ergic, whereas type 2 PG do not receive synaptic contacts from the olfactory axons and are GABA immunonegative. In this study, we analyze and characterize neurochemically a group of PG that has not been previously classified either as type 1 or type 2. These PG are immunoreactive for the neuropeptides somatostatin (SOM) or cholecystokinin (CCK). By using double immunocytochemistry, we demonstrate that neither the SOM- nor the CCK-immunoreactive PG contain GABA immunoreactivity, which is a neurochemical feature of type 1 PG. Moreover, they do not contain the calcium-binding proteins calbindin D-28k and calretinin, which are neurochemical markers of the type 2 PG. Electron microscopy demonstrates that the dendrites of the SOM- and CCK-containing PG are distributed in the synaptic and sensory subcompartments of the glomerular neuropil and receive synaptic contacts from the olfactory axons. Therefore, they should be included in the type 1 group rather than in the type 2. Altogether, these data indicate that the SOM- and the CCK-containing PG may constitute a group of GABA-immunonegative type 1 PG that has not been previously described. These results further extend the high degree of complexity of the glomerular circuitry.

synaptic organization of the glomerulus in the main olfactory bulb: compartments of the glomerulus and heterogeneity of the periglomerular cells

According to the combinatorial receptor and glomerular codes for odors, the fine tuning of the output level from each glomerulus is assumed to be important for information processing in the olfactory system, which may be regulated by numerous elements, such as olfactory nerves (ONs), periglomerular (PG) cells, centrifugal nerves and even various interneurons, such as granule cells, making synapses outside the glomeruli. Recently, structural and physiological analyses at the cellular level started to reveal that the neuronal organization of the olfactory bulb may be more complex than previously thought. In the present paper, we describe the following six points of the structural organization of the glomerulus, revealed by confocal laser scanning microscopy and electron microscopy analyses of rats, mice and other mammals: (i) the chemical heterogeneity of PG cells; (ii) compartmental organization of the glomerulus, with each glomerulus consisting of two compartments, the ON zone and the non-ON zone; (iii) the heterogeneity of PG cells in terms of their structural and synaptic features, whereby type 1 PG cells send their intraglomerular dendrites into both the ON and non-ON zones and type 2 PG cells send their intraglomerular dendrites only into the non-ON zone, thus receiving either few synapses from the ON terminals, if present, or none at all; (iv) the spatial relationship of mitral/tufted cell dendritic processes with ON terminals and PG cell dendrites; (v) complex neuronal interactions via chemical synapses and gap junctions in the glomerulus; and (vi) comparative aspects of the organization of the main olfactory bulb.

Ultrastructural localization of connexins (Cx36, Cx43, Cx45), glutamate receptors and aquaporin-4 in rodent olfactory mucosa, olfactory nerve and olfactory bulb

Odorant/receptor binding and initial olfactory information processing occurs in olfactory receptor neurons (ORNs) within the olfactory epithelium. Subsequent information coding involves high-frequency spike synchronization of paired mitral/tufted cell dendrites within olfactory bulb (OB) glomeruli via positive feedback between glutamate receptors and closely-associated gap junctions. With mRNA for connexins Cx36, Cx43 and Cx45 detected within ORN somata and Cx36 and Cx43 proteins reported in ORN somata and axons, abundant gap junctions were proposed to couple ORNs. We used freeze-fracture replica immunogold labeling (FRIL) and confocal immunofluorescence microscopy to examine Cx36, Cx43 and Cx45 protein in gap junctions in olfactory mucosa, olfactory nerve and OB in adult rats and mice and early postnatal rats. In olfactory mucosa, Cx43 was detected in gap junctions between virtually all intrinsic cell types except ORNs and basal cells; whereas Cx45 was restricted to gap junctions in sustentacular cells. ORN axons contained neither gap junctions nor any of the three connexins. In OB, Cx43 was detected in homologous gap junctions between almost all cell types except neurons and oligodendrocytes. Cx36 and, less abundantly, Cx45 were present in neuronal gap junctions, primarily at "mixed" glutamatergic/electrical synapses between presumptive mitral/tufted cell dendrites. Genomic analysis revealed multiple miRNA (micro interfering RNA) binding sequences in 3'-untranslated regions of Cx36, Cx43 and Cx45 genes, consistent with cell-type-specific post-transcriptional regulation of connexin synthesis. Our data confirm absence of gap junctions between ORNs, and support Cx36- and Cx45-containing gap junctions at glutamatergic mixed synapses between mitral/tufted cells as contributing to higher-order information coding within OB glomeruli.

Defining the role of olfactory ensheathing cells in facilitating axon remyelination following damage to the spinal cord

Olfactory ensheathing cells (OECs) are unique cells that are responsible for the successful regeneration of olfactory axons throughout the life of adult mammals. More than a decade of research has shown that implantation of OECs may be a promising therapy for damage to the nervous system, including spinal cord injury. Based on this research, several clinical trials worldwide have been initiated that use autologous transplantation of olfactory tissue containing OECs into the damaged spinal cord of humans. However, research from several laboratories has challenged the widely held belief that OECs are directly responsible for myelinating axons and promoting axon regeneration. The purpose of this review is to provide a working hypothesis that integrates several current ideas regarding the mechanisms of the beneficial effects of OECs. Specifically, OECs promote axon regeneration and functional recovery indirectly by augmenting the endogenous capacity of host Schwann cells to invade the damaged spinal cord. Together with Schwann cells, OECs create a 3-dimensional matrix that provides a permissive microenvironment for successful axon regeneration in the adult mammalian central nervous system.

Olfactory ensheathing cells: their role in central nervous system repair

The olfactory system is an unusual tissue in that it can support neurogenesis throughout life; permitting the in-growth and synapse formation of olfactory receptor axons into the central nervous system (CNS) environment of the olfactory bulb. It is thought that this unusual property is in part due to the olfactory glial cells, termed olfactory ensheathing cells (OECs), but also due to neuronal stem cells. These glial cells originate from the olfactory placode and possess many properties in common with the glial cells from the peripheral nervous system (PNS), Schwann cells. Recent data has suggested that olfactory ensheathing cells are a distinct glial cell type and possess properties, which might make them more suitable for transplant-mediated repair of central nervous system injury models. This paper reviews the biological properties of these cells and illustrates their use in central nervous system repair.

Olfactory ensheathing cells and olfactory nerve fibroblasts maintain continuous open channels for regrowth of olfactory nerve fibres

The ensheathing cells of the olfactory nerves are arranged end-to-end to form a continuous channel enclosing the olfactory axons from their origin in the olfactory mucosa to their termination in the olfactory bulb. On their outer surface, the olfactory ensheathing cell channels have a basal lamina and an outer encirclement of olfactory nerve fibroblasts. We present an anatomical model of the ensheathing arrangements for the entire transit of the olfactory axons from the horizontal basal cells of the mucosa through the nerves to the superficial astrocytes of the bulb. We used intracranial section of the olfactory nerves to induce a rapid retrograde loss of olfactory neurons and degeneration of their axons, followed by replacement of the neurons from stem cells in the mucosa and growth of the newly formed axons along the olfactory nerves. The olfactory ensheathing cells survive and play a vital role in this process. Unlike Schwann cells in damaged peripheral nerve, the olfactory ensheathing cells neither divide nor migrate. They are actively phagocytic for removal of the degenerating axons, and provide continuous stable open channels along which adventitious cells such as erythrocytes and macrophages can travel, and along which the newly formed axons can regenerate. We suggest that the persistence of these open channels is an important element in the effectiveness of the regeneration. These properties, which the olfactory ensheathing cells exert in collaboration with olfactory nerve fibroblasts, may also be involved in the reparative effects of these cells when transplanted into lesions of the spinal cord.

Intercellular interactions in the mammalian olfactory nerve

The small, unmyelinated axons of olfactory sensory neurons project to the olfactory bulb in densely packed fascicles, an arrangement conducive to axo-axonal interactions. We recently demonstrated ephaptic interactions between these axons in the olfactory nerve layer, the layer of the olfactory bulb in which the axon fascicles interweave and rearrange extensively. In the present study, we hypothesized that the axons, which express connexins, may have another mode of communication: gap junctions. Previous transmission electron microscopy (TEM) studies have failed to demonstrate such junctions. However, the definitive method for detecting gap junctions, freeze fracture, has not been used to examine the interaxonal connections of the olfactory nerve layer. Here, we apply a combined approach of TEM and freeze fracture to determine if gap junctions are present between the olfactory axons. Gap junctions involving olfactory axons were not found. However, by freeze fracture, P faces of both the axons and ensheathing cells (glia that surround the axon fascicles) contained distinctive linear arrays of particles, aligned along the small columns of extracellular space. In axons, few intramembranous particles were present outside of these arrays. Multi-helix proteins, including ion channels and connexin hemichannels, have been shown to be visible as particles by freeze fracture. This suggests that the proteins important for signal transmission are confined to the linear arrays. Such an arrangement would facilitate ephaptic transmission, calcium waves, current oscillations, and paracrine communication and may be important for olfactory neural code processing.

Nitric oxide synthase containing periglomerular cells are GABAergic in the rat olfactory bulb

In the olfactory glomeruli of the rat olfactory bulb, there is a population of periglomerular cells (PG) that contains the neuronal isoform of the nitric oxide synthase (nNOS). To date, these PG have not been characterized neurochemically and it has not been determined whether they are type 1 (GABAergic PG that receive synaptic contacts from the olfactory axons) or type 2 PG (non-GABAergic PG that do not receive synapses from the olfactory axons). Combining pre-embedding NADPH-diaphorase histochemistry and post-embedding immunoperoxidase detection of GABA, we demonstrate that nNOS-containing PG are GABAergic and therefore, belong to the type 1 PG. The possible actions of nitric oxide in the olfactory glomeruli are discussed.

Ensheathment of the olfactory nerves in the adult rat

The ensheathment of the olfactory nerve fibres is achieved by cooperation of two cell types. The olfactory ensheathing cells have a rounded outer surface enclosed in a continuous single basal lamina, and enclose an inner compartment from which overlapping processes of the same and adjacent cells enwrap interweaving territories of tightly apposed aligned axons. The olfactory nerve fibroblasts are highly flattened, dense cells generating multiple layers of very thin processes encircling individual or groups of olfactory ensheathing cells. This paper illustrates the unique ultrastructural features of this ensheathment.

Microglia in the olfactory bulb of rats during postnatal development and olfactory nerve injury with zinc sulfate: a lectin labeling and ultrastrucutural study

Using isolectin (GSA I-B4) as a marker, this study examined the possible alterations of lectin-labeled membranous glycoproteins in microglial cells in the olfactory bulb of normal development and under experimentally induced degeneration. In light microscopy, several morphological types of microglial cells representing different degrees of cell differentiation were distributed in the bulb laminae. A gradient of microglial differentiation extending from the intermediate to superficial and intermediate to deep occurs in the bulb layers. The differentiation gradient and lectin labeling pattern of microglial cells in the developing bulb resembled those in other areas of the brain tissues. Differentiating microglia showed a gradual diminution of lectin staining when the nascent round cells transformed into the mature ramified cells. Microglia in the external plexiform layer of the olfactory bulb were the first to mature and the cells expressed very weak lectin reactivity. In mature or adult rats, some microglial cells showing intense lectin labeling were observed in the olfactory nerve layer, granule cell layer and subependymal layer. Ultrastructurally, lectin labeling was localized at the trans saccules of the Golgi apparatus. Microglial cells in other bulb laminae, however, exhibited a negative reaction for the isolectin at the Golgi apparatus. Following intranasal irrigation of zinc sulfate, some microglial cells in the olfactory nerve layer and glomerular layer were activated to become phagocytic cells with increased lectin labeling at their ramified processes. GSA I-B4 staining was also localized at their trans saccules of the Golgi apparatus. The lectin labeling pattern of these phagocytic cells resembled that of differentiating microglia in postnatal bulbs, suggesting that bulb microglia in the lesioned sites were activated through cell dedifferentiation into macrophages.

Microvasculature of the olfactory organ in the Japanese monkey (Macaca fuscata fuscata)

Olfaction is an important and primitive sense. As its importance has changed with evolution, anatomic adjustments have occurred in its structure and vasculature. Primates are a family of vertebrates that have had to develop their visual system to adapt to the arboreal environment and have evolved from a macrosmatic to a microsmatic species as the optic system has enlarged. This has resulted in anatomic changes of a small but critical area at the base of the brain. This paper describes the three-dimensional vascular anatomy of the olfactory organ of the Japanese monkey (Macaca fuscata fuscata). This is best understood by dividing the organ into three parts: the olfactory tract, olfactory bulb, and olfactory nerves in the nasal mucosa. The bulb can be partitioned into an outer or cortical part and inner or medullary part. The vasculature and tissue were examined grossly and with light microscopy and scanning electron microscopy of vascular corrosion casts. The olfactory tract and bulb were supplied by an arteriole from the anterior cerebral artery on each side. The tract was supplied by capillaries running spirally with a coarse network. At the olfactory bulb, the arteriole ramified into the intracortical and medullary branches that formed capillary networks. The bulbar intracortical capillaries were divided into two layers with different densities and vascular patterns. The capillaries of the superficial layer had a ladder-like pattern. The branches that ran into the medulla of the olfactory bulb were more widely spaced. Twigs from the posterior ethmoidal artery ran along the nerve fiber and formed intra- and extrafascicular networks. Each region of the olfactory organ had characteristic three-dimensional vascular patterns that were related to their cellular architecture.

Chemically defined neuron groups and their subpopulations in the glomerular layer of the rat main olfactory bulb--IV. Intraglomerular synapses of tyrosine hydroxylase-immunoreactive neurons

Synapses of intraglomerular processes of tyrosine hydroxylase-immunoreactive neurons in the rat main olfactory bulb were examined by electron microscopic immunocytochemistry. Prominent characteristics of intraglomerular synapses of tyrosine hydroxylase-immunoreactive elements were that the vast majority (about 80%) of their synaptic inputs were asymmetrical synapses from olfactory nerve terminals and, though far smaller in proportion, one half of the remaining were asymmetrical synapses from mitral/tufted cell dendrites and the other half were symmetrical synapses from gamma-aminobutyric acid-like immunoreactive elements. So far, we have observed no typical reciprocal synapses between tyrosine hydroxylase-immunoreactive processes and mitral/tufted dendrites; however, we have often identified serial synapses; that is, asymmetrical synapses from olfactory nerve terminals or mitral/tufted cell dendrites to tyrosine hydroxylase-immunoreactive processes, and then symmetrical synapses from the latter to different mitral/tufted cell dendrites. These synaptic connections of tyrosine hydroxylase-immunoreactive neurons were very different from those of Calbindin-D(28k)-immunoreactive neurons, which received no synaptic contact directly from olfactory nerve terminals but formed reciprocal synapses with mitral/tufted cells as we analysed previously.Thus, our present and previous electron microscopic studies combined with confocal laser scanning light microscopy clearly indicated for the first time the heterogeneity of periglomerular neurons, not only in their chemical and morphological features, but also in their synaptic organization in the olfactory glomerulus.

Ultrastructural characteristics and conduction velocity of olfactory receptor neuron axons in the olfactory marker protein-null mouse

Olfactory receptor neuron (ORN) axon diameters and the conduction velocity of the compound action potential along ORN axons were studied in olfactory marker protein (OMP)-null mice and genotypically matched controls. The compound action potential was distinguished from postsynaptic field potentials by its shorter latency, its persistence following application of cobalt or kynurenic acid that blocked postsynaptic responses, and its ability to follow paired-pulse stimulation at 300 Hz. Blockade of the postsynaptic field responses by kynurenic acid indicates that in the mouse, as in the rat, glutamate is the olfactory nerve transmitter. The mean conduction velocity of ORNs in wild-type control mice was 0. 47+/-0.19 (S.E.M.) m/s (n=5), similar to the conduction velocity reported for other mammals. The mean diameter of ORN axons in control mice was 0.202+/-0.005 and 0.261+/-0.006 microm in the OMP-null mice. This increase in fiber diameter in the OMP-nulls predicts an increase in impulse conduction velocity. However, the mean conduction velocity of OMP-null mice, 0.38+/-0.03 m/s (n=6), was not significantly different from control (P>0.1). The conduction velocity predicted by the increase in fiber diameter in OMP-null mice was within the 95% confidence interval of the measured value. Thus, OMP-null ORNs are normal with respect to the conduction velocity of their axons. The number of axodendritic synapses in the glomeruli of OMP-null mice is higher than in congenic wild-type mice.

Serotonergic nerve fibers in the primary olfactory pathway of the larval sea lamprey, Petromyzon marinus

In this study, serotonin (5-hydroxytryptamine; 5HT)-immunoreactive (5HT-IR) neuronal fibers were identified in the primary olfactory pathway of the sea lamprey. These neurons are likely part of a nonolfactory neural system that innervates the olfactory sac. Cell bodies with 5HT immunoreactivity predominated in the lamina propria of the rostral portion of the nasal cavity and were less prevalent adjacent to the olfactory epithelium. The 5HT-IR fibers were parallel to axons of the olfactory receptor neurons in the lamina propria of the olfactory mucosa and in the olfactory nerve. Serotonergic fibers crossed from the olfactory nerve into the olfactory bulb or branched in the caudal portion of the olfactory nerve and terminated at the junction of the olfactory nerve with the olfactory bulb. In the dorsal olfactory bulb, 5HT-IR fibers coursed along the layer of olfactory fibers. Throughout the layer with glomeruli and mitral cells, 5HT-IR fibers were seen along the border of glomerular units. Experimental lesion of the olfactory nerve was used to determine the origin of 5HT-IR fibers rostral to the olfactory bulb. The loss of these fibers and their reappearance during outgrowth of olfactory receptor neurons inferred that they emanate from the cell bodies in the olfactory sac. The results from this study suggest that axons of olfactory receptor neurons in larval lampreys receive modulation by 5HT from these neuronal fibers.

Transplantation of human olfactory ensheathing cells elicits remyelination of demyelinated rat spinal cord

Human olfactory ensheathing cells (OECs) were prepared from adult human olfactory nerves, which were removed during surgery for frontal base tumors, and were transplanted into the demyelinated spinal cord of immunosuppressed adult rats. Extensive remyelination was observed in the lesion site: In situ hybridization using a human DNA probe (COT-1) indicated a similar number of COT-1-positive cells and OEC nuclei within the repaired lesion. The myelination was of a peripheral type with large nuclei and cytoplasmic regions surrounding the axons, characteristic of Schwann cell and OEC remyelination. These results provide evidence that adult human OECs are able to produce Schwann cell-like myelin sheaths around demyelinated axons in the adult mammalian CNS in vivo.

Olfactory mucosa of the South American armadillo Chaetophractus villosus: an ultrastructural study

The sense of olfaction in armadillos plays an important role, suggested by the great development of the nasal structures, olfactory bulbs, and related brain regions. The mammalian olfactory mucosa is a privileged site of neuronal death and regeneration during the whole life span. A detailed knowledge of its ultrastructure is convenient for gaining insight into the factors controlling those phenomena. We performed this work in species not previously studied in order to provide a firm basis for further research on those factors. No information is available on the histology and ultrastructure of the olfactory mucosa in the order Xenarthra to which armadillos belong. Samples from the endoturbinals of the armadillo Chaetophractus villosus were prepared for light and electron microscopic examination by the usual conventional means. The olfactory epithelium of Chaetophractus villosus shows the classical three types of cells: supporting cells, olfactory receptor neurons, and basal cells. The olfactory neurons and the basal cells were similar to that described in other species. Two different types of supporting cells are described. An outstanding characteristic of the supporting cells is the normal presence of abundant phagosomes, apical secretory granules, apocrine-like protrusions, and highly developed smooth endoplasmic reticulum. Apoptotic bodies are frequently found in the infranuclear cytoplasm of supporting cells. The ductular epithelium of Bowman's glands reveals secretory activity. The lamina propria shows mixed Bowman's glands. Great development of smooth endoplasmic reticulum is observed in the mucous acinar cells. Evidence for merocrine and apocrine mechanisms in the Bowman's glands is presented. The presence of apoptotic bodies and phagosomes in supporting cells suggests a participation in the cellular events induced by cell death and proliferation of the olfactory epithelium. The variety of characteristics exhibited by the supporting cells of the olfactory mucosa may contribute to a deeper understanding of their scarcely known functions.

Localization of calcitonin gene-related peptide mRNA in developing olfactory axons

During development of the olfactory pathway, calcitonin gene-related peptide (CGRP) expression is regulated both temporally and spatially. We had previous evidence that between E13 and E19 CGRP mRNA was present at the level of olfactory axons but the resolution of light-microscope in situ hybridization did not permit the axons to be distinguished from the closely apposed ensheathing cells. In this study, the localization of CGRP mRNA was studied at early developmental stages (E13-15) through in situ hybridization at the transmission electron-microscope (TEM) level. CGRP transcripts were observed exclusively in axons and not in ensheathing cells. The distribution of transcripts in the axons suggests that they are associated with intermediate filaments rather than microtubules. In addition, a careful ultrastructural analysis provided evidence that polysomes and membrane-bound ribosomes are present in such axons, suggesting that the peptide could be synthesized locally.

Retinoid signaling distinguishes a subpopulation of olfactory receptor neurons in the developing and adult mouse

We asked whether retinoic acid (RA) influences olfactory receptor neurons (ORNs) in the developing and mature mouse olfactory epithelium (oe). The distribution of retinoid receptors and binding proteins in the oe changes between embryonic days 11.5 and 13.5, the period when ORNs first differentiate and send axons into the nascent olfactory nerve. Coincident with this change, RA, which is produced in the frontonasal mesenchyme at these ages, begins to activate gene expression in a bilaterally symmetric subset of ORNs in the dorsolateral oe, as judged by the expression of an RA-responsive transgene. Axons from these RA-activated ORNs are segregated in the olfactory nerve as it extends through the frontonasal mesenchyme toward the forebrain. In vitro, RA potentiates ORN neurite growth on laminin, which, in the embryo, is found in a stripe of frontonasal mesenchyme directly associated with the olfactory nerve. RA does not modify growth on fibronectin, type IV collagen, or L1, which olfactory axons encounter in different regions of the territory between the olfactory epithelium and the brain. The pattern of RA-mediated transcriptional activation and axon segregation persists in early postnatal mice, and RA signaling can be recognized in a subset of adult ORNs in the dorsolateral oe. Thus, RA-mediated gene expression distinguishes a subpopulation of ORNs in a distinct region of the oe during the early development of the olfactory pathway, and may influence differentiation and axonal projections of ORNs in this region throughout life.

Chemically defined neuron groups and their subpopulations in the glomerular layer of the rat main olfactory bulb--II. Prominent differences in the intraglomerular dendritic arborization and their relationship to olfactory nerve terminals

In the glomerular layer of the rat main olfactory bulb, we previously reported three chemically defined interneuron groups: GABA-like immunoreactive, calretinin-immunoreactive and Calbindin-D28k-immunoreactive groups [Kosaka K. et al. (1995) Neurosci. Res. 23, 73-88]. In the present study, we analysed the structural features of these three neuron groups using confocal laser scanning light microscopy, focusing on their dendritic arborization pattern, especially on their close apposition to olfactory receptor terminals labeled by olfactory marker protein. Each glomerulus consisted of two zones, the olfactory nerve zone and the non-olfactory nerve zone. The former was mainly occupied by olfactory nerve preterminals and terminals as well as their targets, postsynaptic fine dendritic portions of intrinsic neurons. The latter non-olfactory nerve zone was occupied mainly by olfactory marker protein-negative profiles. Processes of GABAergic neurons and those of one of their subpopulations, tyrosine hydroxylase-immunoreactive neurons, were numerous both in the olfactory nerve and non-olfactory nerve zones, resulting in their frequent close apposition to olfactory marker protein-immunoreactive elements. Combined confocal laser scanning light microscopic electron microscopic examination revealed synaptic contacts from olfactory nerve terminals on tyrosine hydroxylase-immunoreactive processes at these sites of close apposition. In contrast, calretinin-immunoreactive and Calbindin-D28k-immunoreactive processes, particularly Calbindin-D28k-immunoreactive ones, were distributed almost exclusively in the non-olfactory nerve zone, as if they avoided the olfactory nerve zone, showing a net or honeycomb pattern. Thus, calretinin-immunoreactive and Calbindin-D28k-immunoreactive processes were not or very rarely closely apposed to olfactory nerve terminals. These findings suggested that there might be some differences among chemically defined interneuronal groups in their synaptic contacts from olfactory nerves. Further quantitative image analysis clearly exhibited the prominent differences among these neuron groups in their intraglomerular dendritic arborization in relation with the olfactory nerve zone, i.e. the percentages of the area in the olfactory nerve zone occupied by GABAergic and tyrosine hydroxylase-immunoreactive processes were about 10%, respectively, whereas those of calretinin-immunoreactive and Calbindin-D28k-immunoreactive processes were only about 1% and 0.3%, respectively. These findings suggested that so-called periglomerular cells in glomeruli might be heterogeneous not only in their chemical nature, but also in their dendritic arborization pattern and synaptic contacts from olfactory nerve terminals.

Differential adhesion and the initial assembly of the mammalian olfactory nerve

During the initial assembly of the olfactory pathway, the behavior of olfactory axons changes as they grow from the olfactory epithelium toward the telencephalic vesicle. The axons exit the epithelium singly or in small fascicles, and their growth cones are simple and bullet-shaped. Outside the epithelium, they make a sharp dorsal turn and fasciculate into a single nerve; the growth cones remain simple. Upon entering the ventromedial telencephalon, the axons defasciculate, branch extensively, and end in complex, lamellate growth cones which extend toward the ventrolateral aspect of the telencephalic vesicle. The distribution of laminin, collagen-IV, and fibronectin varies in register with these changes in olfactory axon and growth cone behavior. Each of these extracellular matrix molecules influences olfactory neurite outgrowth and growth cone morphology in vitro consistent with its distribution in vivo. The distribution of E-cadherin, L1, neural cell adhesion molecule (NCAM) and the polysialated form of NCAM also varies in register with changes in olfactory axon behavior. In vitro, L1 modulates embryonic olfactory neurite outgrowth and growth cone morphology consistent with its distribution in vivo. Thus, olfactory axon trajectory, fasciculation, and growth cone morphology change within distinct adhesive environments in the nascent olfactory pathway, and some of the molecules that characterize these environments have differential effects upon olfactory neurite growth and growth cone morphology. Consequently, the patterned expression and activity of extracellular matrix and cell surface adhesion molecules may contribute to the initial assembly of the olfactory pathway.

Expression of extracellular matrix molecules in the embryonic rat olfactory pathway

Primary olfactory neurons arise from placodal neuroepithelium that is separate from the neuroepithelial plate that forms the neural tube and crest. The axons of these neurons course along a stereotypical pathway and invade the rostral telencephalic vesicle where they induce the formation of the olfactory bulb. In the present study we examined the expression of several extracellular matrix constituents during formation of the olfactory nerve pathway in order to identify putative developmentally significant molecules. Double-label immunofluorescence was used to simultaneously map the trajectory of growing primary olfactory axons by expression of growth associated protein 43 (GAP-43) and the distribution of either laminin, heparan sulfate proteoglycans (HSPG), or chondroitin sulfate proteoglycans (CSPG). At embryonic day 12.5 (E12.5) primary olfactory axons have exited the olfactory neuroepithelium of the nasal pit and formed a rudimentary olfactory nerve. These axons together with migrating neural cells form a large mass outside the rostral surface of the telencephalon. This nerve pathway is clearly defined by a punctate distribution of laminin and HSPG. CSPG is selectively present in the mesenchyme between the olfactory nerve pathway and the nasal pit and in the marginal zone of the telencephalon. At E14.5 primary olfactory axons pierce the telencephalon through gaps that have emerged in the basement membrane. At this age both laminin and HSPG are colocalized with the primary olfactory axons that have entered the marginal zone of the telencephalon. CSPG expression becomes downregulated in this same region while it remains highly expressed in the marginal zone adjacent to the presumptive olfactory bulb. By E16.5 most of the basement membrane separating the olfactory nerve from the telencephalon has degraded, and there is direct continuity between the olfactory nerve pathway and the central nervous system. This strict spatiotemporal regulation of extracellular matrix constituents in the olfactory nerve pathway supports an important role of these molecules in axon guidance. We propose that laminin and HSPG are expressed by migrating olfactory Schwann cells in the developing olfactory nerve pathway and that these molecules provide a conducive substrate for axon growth between the olfactory neuroepithelium and the brain. CSPG in the surrounding mesenchyme may act to restrict axon growth to within this pathway. The regional degradation of the basement membrane of the telencephalon and the downregulation of CSPG within the marginal zone probably facilitates the passage of primary olfactory axons into the brain to form the presumptive nerve fiber layer of the olfactory bulb.

Ultrastructural and cytochemical identification of apoptotic cell death accompanying development of the fetal rat olfactory nerve layer

It has been previously shown that the embryonic olfactory nerve contains, in addition to glial ensheathing cells, a large population of differentiated neurons that migrate from the developing olfactory epithelium, in close association with the olfactory axon fascicles. The purpose of our study was to verify the hypothesis according to which a process of physiological cell death might be involved in the progressive disappearance of these migrating neurons that has been reported during late embryonic stages in several immunocytochemical studies. To do so, we have investigated the development of the olfactory nerve layer in rat embryos by using light and electron microscopy, with special reference to the presence of cell death processes within this structure. We have also applied the histochemical TUNEL method allowing in situ visualization of cells degenerating by apoptosis. In order to determine if neurons were present among dying cells, a procedure of double-labeling was performed by combining the DNA-specific bisbenzimide with two neuronal markers, the protein B-50/GAP-43 and the lectin Ulex europaeus I. Results brought out the precise temporal and spatial patterns of programmed cell death accompanying the morphogenesis of the olfactory nerve layer. A cell death process was observed within the olfactory nerve layer from its onset at embryonic day 13 (E13). While only few pycnotic cells were observed in E13 and E14 embryos, their number increased from E15 to reach a maximum at E16 and then diminished. Few dying cells were also observed along the olfactory axon fascicles when they penetrated the olfactory nerve layer. Degenerating cells appeared strongly TUNEL-labeled and exhibited morphological features of cell death by apoptosis. Double-labeling experiments revealed that some of the apoptotic cells were neurons. These observations indicate that apoptosis may account for the progressive decrease in the number of migrating neurons present within the embryonic olfactory nerve layer. Otherwise, a zone of massive cell death by apoptosis was observed at E14 within the nasal mesenchyme located ventrally and caudally to the olfactory nerve layer. Double-labeling experiments showed that apoptotic cells present within this zone were not neurons. Our findings strongly suggest that apoptotic cell death of migrating neurons may allow the elimination of non-functional cells whereas that of mesenchymal cells may facilitate outgrowth of the newly formed olfactory axon fascicles by pathway formation.

An in vitro evaluation of the effects of local magnetic-susceptibility-induced gradients on anisotropic water diffusion in nerve

It has been suggested that the anistropy of the water-diffusion coefficient measured in nerve and in white matter could arise from locally anisotropic background gradients induced by the static field, B0. By utilizing 1) pulse sequences, which minimize the effects of background gradients, and 2) changes in sample orientation, which would maximize the change in the magnitude of these gradients if present, it is shown that in four excised nerves the background gradients do not play a measurable role in the anisotropy of the water-diffusion coefficient at a field strength of 2.35 T. The excised nerves evaluated were the olfactory, trigeminal, and optic nerves of the garfish and the sciatic nerve of the frog.

Cell surface-associated extracellular distribution of a neural proteoglycan, 6B4 proteoglycan/phosphacan, in the olfactory epithelium, olfactory nerve, and cells migrating along the olfactory nerve in chick embryos

The immunocytochemical and immuno-electron microscopic distribution of a neural proteoglycan (PG) was investigated with a monoclonal antibody, MAb 6B4, in the olfactory epithelium, the olfactory nerve, and the cells originating the epithelium and migrating along the olfactory nerve toward the forebrain in chick embryos. The PG recognized by MAb 6B4, that is 6B4 PG, in the brain of early postnatal rats, is identical to phosphacan. In chick embryos, immunoreactivity to 6B4 PG appeared on embryonic day (ED) 3-3.5 in a thin layer beneath the olfactory epithelium. It disappeared immediately, then becoming apparent in the bundles of the olfactory nerve. The immunoreactivity in the nerve bundles gradually increased during ED 5-11. On the other hand, cell surface-associated extracellular localization of the immunoreactivity was seen in the olfactory epithelium on ED 6 and afterwards. Immunofluorescent double-labeling of 6B4 PG and gonadotropin-releasing hormone (GnRH) revealed that the cell bodies of both GnRH-containing cells and other cells migrating along the olfactory nerve were surrounded by a rim immunoreactive to the PG. Under an electron microscope, the surfaces of the cell bodies and of the neurites in the nerve bundles were surrounded by deposits immunoreactive to 6B4 PG. These results indicate that 6B4 PG in chick embryos is one type of cell surface-associated extracellular matrix molecule, and that 6B4 PG covered the surfaces of migrating cells and of elongating olfactory nerve. The cell surface-associated extracellular localization of 6B4 PG found in the nasal region, taken together with the binding properties of this PG with cell adhesion molecules shown in rat brains, suggested that 6B4 PG played a role in guiding the migration of cells along the olfactory nerve in chick embryos.

Tangential migration of luteinizing hormone-releasing hormone (LHRH) neurons in the medial telencephalon in association with transient axons extending from the olfactory nerve

During embryonic development, luteinizing hormone-releasing hormone (LHRH) neurons migrate to the brain from the medial olfactory epithelium through the olfactory nerve. LHRH neurons enter the brain and migrate tangentially along the medial edge of the telencephalon in close association with a neural cell adhesion molecule (N-CAM) enriched fiber bundle. In the current work we wished to determine whether this N-CAM enriched fiber bundle is an extension of the olfactory nerve. Ablation experiments, immunocytochemistry and diI implants all suggest that LHRH neurons migrate in association with a very small subset of transient N-CAM enriched neuronal processes which extend out of the olfactory nerve proper to the septal-preoptic area.

Response of olfactory Schwann cells to intranasal zinc sulfate irrigation

The response of olfactory Schwann cells was assessed at 2, 4, and 7 days following intranasal zinc sulfate irrigation in 1-month-old mice. Ultrastructural and immunohistochemical observations showed dramatic differences between experimental and control mice which had been washed with saline intranasally. Two days after zinc sulfate treatment, many olfactory nerve bundles contained patchy areas of axonal degeneration, while the cell bodies of the olfactory Schwann cells appeared to have increased in electron density and to have shifted peripherally. Some of the cell bodies protruded from the surface of the axon fascicle, suggesting that the olfactory Schwann cells were in the initial process of migrating away. On the fourth day when most of the olfactory axons had degenerated, some olfactory Schwann cells were aligned immediately beneath the basal lamina of the olfactory epithelium. These cells were immunopositive for the S-100 protein and possessed an expanded perinuclear space. Many olfactory Schwann cells were present in the region beneath the cribriform plate, while some appeared to have passed through the gaps between the bony plates to reach the olfactory bulb. Hence, the results showed that many olfactory Schwann cells migrated towards the olfactory bulb following loss of axonal contact. Furthermore, on the seventh day following zinc sulfate treatment, some olfactory Schwann cells in the vicinity of the olfactory bulb appeared phagocytic, as indicated by their extension of processes around fragments of cell debris and the presence of lysosome-like organelles in the perikaryon. The control mice which had been intranasally irrigated with saline did not demonstrate massive olfactory axonal degeneration, and the morphology of the nasal cavity region was similar to that of normal mice.

Regeneration of olfactory sensory neurons and reconnection in the aging hamster central nervous system

Olfactory neuron recovery and axon growth was studied in 12-24 month old hamsters after unilateral olfactory nerve transection. At recovery times ranging from 4 to 126 days olfactory nerve regeneration and axon reconnection to the olfactory bulb were examined by anterograde horseradish peroxidase (HRP) neurohistochemistry and electron microscopy. Nerve transection produced immediate retrograde neuron degeneration and there was no HRP reaction product in the bulb at 4 days post transection. By day 35, centrally growing olfactory axons had reached the bulb. Axons formed glomeruli smaller than those in the unoperated control bulb and they were not always confined to the glomerular layer of the bulb. Some animals showed robust fiber growth with axon fascicles penetrating the different layers of the bulb and forming ectopic glomeruli along their path. Second order bulb neurons contained wheat germ agglutinin HRP reaction product, indicating that transneuronal transport had occurred. Electron microscopy confirmed transneuronal transport between olfactory axons and second order bulbar neurons. These results show that the capacity for olfactory neuron recovery and reconnection persists in the hamster well into old age.

Host primary olfactory axons make synaptic contacts in a transplanted olfactory bulb

Previous light microscopic studies have shown that host olfactory neurons are able to grow into a transplanted fetal olfactory bulb, and behavioral studies have shown that animals with transplanted olfactory bulbs recover functional olfactory abilities. We examined the olfactory bulb transplant at the ultrastructural level to determine whether synaptic contacts are reestablished between host olfactory neurons and donor olfactory bulb. Mature rats that, as neonates, had received embryonic olfactory bulb transplants following olfactory bulb removal were studied. An antibody specific for olfactory marker protein was used to identify the primary olfactory neurons; it was bound by a gold-conjugated secondary antibody for visualization. To preserve the antigenicity of the olfactory marker protein for immunolabeling, Lowicryl K4M hydrophilic resin was used. Synaptic contacts were unmistakable between labeled axons of host olfactory neurons and unlabeled processes within glomerulus-like areas of the transplanted olfactory bulb. The surrounding neuropil contained other elements similar to those found in normal tissue, including synaptic contacts between unlabeled profiles. We clearly show that the transplanted olfactory bulb exhibits sufficient plasticity to form an array of normal synaptic contacts, including the contacts from host primary olfactory neurons.

Numbers of olfactory receptor cells and fine structure of olfactory nerves in various birds

The numbers of olfactory receptor cells from electron micrographs in various species of birds were counted and the fine structure of their olfactory nerves was observed using electron micrographs. The birds were domestic ducks, a slay-backed gull, quails, budgerigars and bengalees. Data from pigeons obtained from a previous study were also included for comparison. The approximate numbers of olfactory cells on each side were 5,800,000 in the duck, 2,700,000 in the gull, 570,000 in the quail, 130,000 in the budgerigar, and 110,000 in the bengalee. From a cross section of an olfactory nerve, the nerve was observed to be divided roughly into several fascicles by perineurium. Each fascicle was composed of many small bundles which were surrounded by endoneurium. Each small bundle was separated into several divisions by mesaxon which originated from Schwann cells. In the anterior part of the nerve, the number of axons surrounded by mesaxon ranged between one and several dozen. The number in the middle part was much larger than in the anterior part. In the posterior part the number decreased again. The diameter of an axon was 0.21-0.26 microns on average. The axon contained neurotubules, neurofilaments, mitochondria and axonal smooth ER. The fine structure of the olfactory nerve and the numbers of olfactory cells in these birds are discussed and compared with those of other vertebrates.

Mitral cell dendrites: a comparative approach

Phylogenetically persistent structures such as the mitral cells of the vertebrate olfactory bulb undergo changes in their dendritic arbor in the course of evolution. The morphology of mitral cells and the main elements of the olfactory bulb circuit in all classes of vertebrates are reviewed in this paper. Most of the neuronal elements found in the mammalian olfactory bulb are present in anamniotes. However, in contrast to those of amniotes, the mitral cells of most anamniotes lack basal dendrites, and periglomerular cells are absent in fish. This suggests a different circuitry and therefore drastic changes in the processing of olfactory information within the olfactory bulb. Lateral inhibition, conferred by basal dendrites in anamniotes, must then utilize other mechanisms in anamniotes. Moreover, the marked segregation of olfactory inputs onto mammalian mitral cells is less obvious in mitral cells of anamniotes that lack basal dendrites. The general role of dendrites, including those of mitral cells, is discussed in the light of increasing evidence for dendritic excitability. The evolutionary significance of mitral cell basal dendrites is also discussed.

Ultrastructural demonstration of salmon-type gonadotropin-releasing hormone (sGnRH) in the olfactory system of masu salmon (Oncorhynchus masou)

Immunocytochemical and immunoelectron microscopic localization of salmon-type gonadotropin-releasing hormone (sGnRH) were observed in the olfactory system (olfactory epithelium, olfactory nerve and olfactory bulb) of masu salmon (Oncorhynchus masou) to investigate its possible involvement in the olfactory functions. sGnRH-immunoreactive (ir) bipolar neuron, which might be related to the terminal nerve, was located in dorsal portion of the olfactory nerve. sGnRH-ir gold particles were concentrated on electron-dense granule-like structures of 50 nm in diameter in fibers of the olfactory nerve close both to the olfactory epithelium and to the olfactory bulb. These findings suggest that sGnRH might participate in neurotransmission and/or neuromodulation in the olfactory system of masu salmon. sGnRH would become a useful molecular marker for studying the olfactory imprinting and homing mechanisms in salmonids.

Ulex europaeus I and glycine max bind to the human olfactory bulb

The distribution of binding sites for the fucose-selective lectin Ulex europaeus I and the terminal N-acetylgalactosamine-selective lectin glycine max in the human olfactory bulb were studied. These lectins bound to primary olfactory axons in the olfactory nerve layer and the glomerular layer. They also bound to fibers located in the deeper layers such as the external plexiform layer and the granular layer. Furthermore they projected to the olfactory stalk but not in the cerebrum. The deeper projections of the lectin binding fibers may affect the function of the olfactory bulb in humans.

Effects of chronic low-level copper exposure on ultrastructure of the olfactory system in rainbow trout (Oncorhynchus mykiss)

This study investigated the effects of a chronic exposure to a low level of copper on cell populations of the olfactory system in yearling rainbow trout. Fish were sacrificed after 15, 30 and 60 days of copper exposure. Transmission electron microscopy was used to describe the sequence of subcellular changes occurring in three tissues, the sensory epithelium, the olfactory nerve and the olfactory bulb. Data show that a 15-day exposure to 20 micrograms/l of copper causes specific degeneration of all mature receptor cells as well as numerous immature neurons. Moreover, degenerating receptor cells exhibited morphological features of a cell death by apoptosis. After 30 days, and more specifically after 60 days of exposure, numerous clusters of cells were observed in the basal region of the epithelium, suggesting a great mitotic activity in this area. In parallel, an increased number of maturing receptor cells and goblet cells were observed, but no fully mature neurons were noted even after 60 days of exposure. In both the olfactory nerve and the olfactory bulb, the number of degenerating axons and terminals, which was high at 15 days, decreased with time and some process of glomerular reinnervation was detected after 60 days. A reactive hypertrophy of supporting, ensheathing and astrocytic cells was also observed in exposed fish, which demonstrates that these cell types are actively involved in the process of tissue scarring. Even though some signs of neuronal regeneration were reported during the time-course of exposure, indicating some fish acclimation, results raise the question of the olfactory function during such environmental stress.

Synaptophysin and synaptoporin expression in the developing rat olfactory system

The expressions of two closely related synaptic vesicle antigens synaptophysin and synaptoporin were examined in the olfactory system of the adult rat and during pre- and postnatal development. In the adult, immunocytochemistry showed that the continuously regenerating olfactory receptor neurons (primary neurons) produce both synaptophysin and synaptoporin which were localized in the cell bodies of the receptor neurons in the olfactory epithelium, their dendrites, axonal processes in the olfactory nerve and their terminals in the olfactory bulb glomeruli. Furthermore, ultrastructural analysis revealed synaptophysin- and synaptoporin-immunoreactivities associated with synaptic vesicles in most olfactory receptor axonal terminals impinging on dendrites of the mitral and tufted neurons (secondary neurons in the olfactory bulb circuitry) in the olfactory glomeruli. In like manner, tufted neurons, granule and periglomerular neurons (interneurons in the olfactory bulb circuitry) express both synaptophysin and synaptoporin. In contrast, mitral neurons expressed only the synaptophysin antigen which was likewise associated with mitral axonal terminals in their target the olfactory cortex. The patterns of synaptophysin and synaptoporin expressions in mitral neurons (synaptophysin only) and tufted neurons (synaptophysin and synaptoporin) were similar in prenatal, postnatal and adult rats as revealed by immunocytochemistry and in situ hybridization. However, the biosynthesis of synaptophysin and synaptoporin by granule and periglomerular neurons, olfactory bulb interneurons, occurred mainly postnatally.

Morphological study of the effects of intranasal zinc sulfate irrigation on the mouse olfactory epithelium and olfactory bulb

The effects of intranasal zinc sulfate (ZnSO4) irrigation on the morphology of the olfactory epithelium and olfactory bulb were studied in mice with short survival times (as early as 1 day) and with long survival times (up to 593 days) after the irrigation procedure. As in several previous studies, the olfactory epithelium was completely destroyed within a few days after the ZnSO4 treatment. Within 2-4 days, the septum and turbinates were covered by a new, cuboidal epithelium, the cells of which differed significantly from any cells normally seen in the olfactory epithelium. Slowly, over several months, small areas of the olfactory epithelium regenerated in many of the animals. The ultrastructural changes occurring in the olfactory bulb from 1 to 25 days (the reactive stage) were characterized by degenerating olfactory axons and axon terminals, hypertrophy of astroglial cell processes, and proliferation of or extravasation by phagocytic cells. By 25 days after intranasal ZnSO4 irrigation, the number of reactive glial processes and phagocytic cells returned to normal. In some mice with survival times of 150 days or longer, there was reinnervation of small areas of the olfactory bulb by regenerated olfactory axons. These new olfactory axons innervated only superficial glomeruli or the outer portions of deeper glomeruli, but they formed synaptic contacts with mitral/tufted cells and periglomerular cells that did not differ from control animals. These findings were supported by tract-tracing experiments with 3H-amino acids and by behavioral analysis. In summary, the ultrastructural changes observed in the olfactory bulb in this study were not significantly different from those observed after surgical lesions of the olfactory epithelium or nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Glial cells in the nerve fiber layer of the main olfactory bulb of embryonic and adult mammals

This article provides a detailed description of the glial cell types in the nerve fiber layer of the main olfactory bulb during embryonic development, in adult mammals, and at the nerve entry zone of the first cranial nerve. In adult mammals, the glial cell types of the olfactory nerve fiber layer include intrafascicular ensheathing cells, which have the exclusive role of ensheathing the olfactory axons in both the PNS and CNS, and interfascicular astrocytes, which occupy the spaces between adjacent olfactory fascicles. The ensheathing cells are particularly interesting because they possess a mixture of Schwann cell and astrocytic phenotypic features, are more likely to be of placodal than of CNS origin, and have the exclusive role of forming the glia limitans at the PNS-CNS transitional zone. It is proposed that one important function of ensheathing cells is to modulate the growth of olfactory axons within the CNS; this modulation is probably mediated by selective cell adhesion molecules, extracellular matrix molecules, and chemotropic agents.

Ultrastructural studies of microtubules and microtubule organizing centers of the vertebrate olfactory neuron

The olfactory neuron is specialized along its length into highly determined morphological regions. These regions include the dendritic cilia, dendritic vesicle, dendritic shaft proper, perikaryon, axon, zone of transition where the axon widens as it approaches its termination, and the axon terminal. Except for the zone of transition and the terminal, characteristic populations of microtubules occur in these compartments. In the olfactory vesicle, three discrete microtubule organizing centers (MTOCs) nucleate microtubules: the basal body, the lateral foot associated with the body, and dense masses of nearby material. Little is known about MTOCs elsewhere in the neuron, although the polarity of the axonal microtubules indicate that they originate at or near the perikaryon. An attempt is made to summarize what is known of the origin, structure, distribution, and function of microtubules in vertebrate olfactory neurons, which are useful model systems in which to study microtubules. Information about olfactory neuron microtubules may be applicable to neurons in general (e.g., the discovery that axons contain microtubules of uniform polarity was first made in the olfactory neuron) or to microtubules in other eukaryotic cells.

Comparative study of the olfactory epithelium of the three-spined stickleback (Gasterosteus aculeatus) and the nine-spined stickleback (Pungitius pungitius)

The olfactory epithelium of the three-spined stickleback (Gasterosteus aculeatus) and the nine-spined stickleback (Pungitius pungitius) has been studied with a conventional histochemical and a novel immunological staining technique. In both species, the sensory epithelium is arranged in folds separated by non-sensory epithelial tissue. In the nine-spined stickleback, intrinsic folds consisting of non-sensory cells are found in the apical part of the sensory epithelium where they divide the surface of the sensory epithelium into small islets. These non-sensory cells are non-ciliated, flattened and piled on top of each other; they contain numerous electron-translucent vesicles. The intrinsic folds are absent from the sensory epithelium of the three-spined stickleback. In both species, axons of receptor cells form a layer of fibers in the sensory epithelium immediately above the basal cells. In the three-spined stickleback, thick branches of the olfactory nerve are frequently found in this layer. These branches are only occasionally observed in the sensory epithelium of the nine-spined stickleback. Thus, the three-spined stickleback and the nine-spined stickleback show considerable differences in the organization of the sensory regions of the olfactory epithelium.

Development of the olfactory nerve in the clawed frog, Xenopus laevis: II. Effects of hypothyroidism

Quantitative and morphological data were obtained on developing olfactory axons in normal and hypothyroid larvae of the African clawed frog Xenopus laevis. Hypothyroid larvae were produced by rearing the animals, beginning at stage 48, in a 0.01% solution of propylthiouracil (PTU), a treatment that blocks synthesis of thyroid hormone. These PTU-treated larvae were compared to their age-matched siblings when these siblings reached stage 52 (premetamorphic larvae; prior to synthesis of thyroid hormone), stage 57 (late premetamorphic larvae; after the onset of thyroid hormone synthesis), or stage 58 (larvae at the onset of metamorphic climax; thyroid hormone levels continue to rise). The number of olfactory axons did not differ between stage 52 control animals and the age-matched, PTU-treated animals, but there were only about half the number of axons in the PTU-treated animals that were age-matched to the stage 57 or 58 controls. Thus, PTU had no effect on olfactory axon number prior to the normal rise in thyroid hormone levels. But PTU significantly reduced the normal increase in olfactory axon number compared to stage 58 control larvae, whose thyroid hormone levels are high. While PTU also produced some changes in several other body measurements, the effect on the olfactory axons was the most consistent and prominent. The results presented here support our previous findings that thyroid hormone plays a significant role in the development of the olfactory system in Xenopus.

Nerve growth factor receptor expression in the young and adult rat olfactory system

Nerve growth factor (NGF) and its receptor (NGFR) are proteins that have a role in the normal development and survival of neurons in the peripheral and central nervous systems. During development, NGF is necessary for outgrowth of axons and establishment of synapses, and NGFR is the transmembrane protein that binds NGF and brings it into the cell. NGF and NGFR expression in the rat olfactory system have been studied previously, and age differences in NGFR are explored further in this study, using immunocytochemistry and immunoelectron microscopy to determine the changes in two different ages: postnatal day 5 and the adult. Dramatic differences were found in the distribution of NGFR immunoreactivity in the olfactory system of each of the two ages studied. Electron microscopy revealed that glial cells were responsible for this immunoreactivity.

PNS-CNS transitional zone of the first cranial nerve

This study examined the ultrastructure of the region of transition where fascicles of olfactory axons leave the peripheral nervous system (PNS) to enter the central nervous system (CNS), the so-called PNS-CNS transitional zone. Adult rats were transcardially perfused with a solution of 1% glutaraldehyde and 1% paraformaldehyde, decapitated, and the heads decalcified over a period of several weeks in a solution of 1% glutaraldehyde in 0.1 M tetrasodium ethylenediamine tetraacetic acid; the latter solution was changed daily. It was found that astrocytes did not form the glia limitans at the nerve entry zone, unlike the situation that exists in other cranial and spinal nerves. Rather, the glia limitans in this region of the olfactory bulb was formed by a special type of glial cell, referred to as an ensheathing cell. Ensheathing cells are found only in the nerve fiber layer of the olfactory bulb. They possess a mixture of Schwann cell and astrocytic features and are more likely to be of placodal than of CNS origin. The meningeal coverings of the olfactory nerve rootlets and of the olfactory bulb are also described and the functional implications of the findings discussed.

Calbindin-D-28k-like immunoreactive structures in the olfactory bulb and anterior olfactory nucleus of the human adult: distribution and cell typology--partial complementarity with parvalbumin

Calbindin-D-28k and parvalbumin are calcium-binding proteins. The laminar distribution and morphological features of calbindin-D-28k-like immunoreactive structures were studied in 60-microns-thick sections of the human olfactory bulb. Except for the olfactory nerve layer, immunoreactive neurons were present in all layers of the olfactory bulb. They reached highest densities in the external plexiform layer and internal granule cell layer. Considerable numbers of calbindin-like nerve cells were also found in the olfactory tract and in distal portions of the anterior olfactory nucleus. When comparing the distribution of calbindin-positive structures to that of parvalbumin-positive ones a partially complementary distribution pattern was found. Calbindin-like immunoreactive portions of the anterior olfactory nucleus and olfactory tract were mirrored by immunonegative areas in adjacent sections stained for parvalbumin. Using the combined pigment-Nissl procedure we observed the presence of lipofuscin deposits in nearly 80% of all the calbindin-immunoreactive neurons analysed. Moreover, analysis of their lipofuscin deposits rendered the further differentiation of morphologically similar neuronal subpopulations possible. In contrast, all parvalbumin-like immunoreactive neurons remained free of lipofuscin granules.

Ultrastructural organization of receptor cell axons in frog olfactory nerve

Repeated odorant or electrical stimulation of the frog olfactory nerve leads to long-lasting reduction of excitability of the receptor neurons. In the turtle olfactory nerve electrical stimulation causes an increase in extracellular potassium concentration due to the efflux of potassium from active axons. Elevated potassium concentration depolarizes an axon membrane and inactivates it. If axons travel parallel to each other in the nerve for extended distances, changes in ionic concentration due to activity in one axon will reduce excitability of its neighbors. This phenomenon may have effects like those of lateral inhibition among neighbors first described in Limulus and may act as a filter with a long time constant, for the nervous message. The amphibian olfactory nerve consists of densely packed, small-diameter, unmyelinated fibers. In this study we have examined the ultrastructure of the frog (Rana pipiens) olfactory nerve in longitudinal and cross-sections to determine whether axons follow a sinuous course and change neighbors often along the length of the olfactory nerve, or whether they follow parallel trajectories and thus tend to stay close to the same neighbors. We have found that axons have a very straight course within the nerve and that axons tend to remain adjacent to the same individual axons over long distances. We think that the anatomical substrate of the olfactory nerve favors strong inhibitory axon-axon interaction.

Early neurogenesis of the mouse olfactory nerve: Golgi and electron microscopic studies

The early neurogenesis of the mouse olfactory nerve, from its exist at the nasal epithelium to its entrance into the embryonic telencephalon, has been investigated by using the rapid Golgi method and electron microscopy. Previously unrecognized anatomical and possible functional interrelationships between developing olfactory nerve axons and their sheath cells have been observed: 1) at their exit from sensory epithelium (nasal compartment), 2) at their contact with the CNS surface (intracranial compartment), and 3) at their entrance into the embryonic telencephalon (central nervous tissue compartment). Based on these observations the anatomy of the mouse olfactory nerve is herein redefined. Exiting olfactory nerve axons and sheath cells from the same regions of the nasal epithelium establish an early association which is maintained up to their terminal glomerular neuropile. No disruptions have been found in either the olfactory nerve axons or in the continuity of their sheath cells from exit at the nasal epithelium to entrance into the developing olfactory bulb. Corresponding olfactory nerve axons with their sheath cells enter together and become incorporated into the developing olfactory bulb as units. Consequently, the cellular envelope of the olfactory glomerulus must be composed of olfactory sheath cells rather than of glial (astroglial) cells from the CNS. With this simple anatomical arrangement, a topographic map of the sensory epithelium could be established progressively in the developing olfactory bulb. Eventually, "regenerating" olfactory nerve axons from different nasal regions could be guided by their specific sheath cell conduits toward their target glomeruli; hence, the olfactory message may be maintained undisturbed throughout the life span of the animal. In addition, olfactory nerve axons establish synaptic-like contacts with their corresponding sheath cells prior to or during the perforation of the CNS surface. Reciprocal recognition between corresponding axons and their sheath cells at this crucial stage in their neurogenesis may play a significant role in the establishment of their terminal glomerulus. This new concept of the anatomy of the mammalian olfactory nerve should provide insights helpful in clarifying some of the still-unresolved questions regarding the structural and functional organizations of this primitive system.

Post-traumatic anosmia. Ultrastructural correlates

Five patients suffering post-traumatic anosmia were studied at the University of Colorado Health Sciences Center, Denver. Each patient underwent psychophysical testing, clinical evaluation, and olfactory biopsy. The biopsy specimens were examined ultrastructurally and were found to vary from normal tissues. The overall appearance of the olfactory epithelium in the post-traumatic patient is disrupted and the receptor cells are distorted. Large numbers of axons are located near the basement membrane and can often be found in bundles throughout the epithelium, extending even to the mucosal surface. Olfactory cilia are rarely seen in epithelia obtained from post-traumatic patients. Bald olfactory vesicles, often containing basal bodies, are frequently observed. We postulate that in these cases, the olfactory epithelium regenerates following head trauma and the receptor cells attempt to send axons centrally. However, the cribriform plate has undergone fibrotic healing and the axons are unable to penetrate it and make contact with olfactory bulb neurons.

Atypical olfactory glomeruli contain original olfactory axon terminals: an ultrastructural horseradish peroxidase study in the rat

The labelling of olfactory bulb glomeruli following horseradish peroxidase lavage of the nasal cavity has been studied in the rat. In such conditions, atypical glomeruli, previously described according to their high acetylcholinesterase content, display a strong tracer accumulation. The course of afferent olfactory fibres could be followed along the lateral and dorsal surface of the olfactory bulbs. The primary olfactory axons ending in atypical glomeruli have been identified with horseradish peroxidase in electron microscopy. They differ significantly from classical olfactory terminals owing to the presence of large dense-cored vesicles accompanying small clear ones. Moreover, the olfactory terminals do not gather in dark nodules as they do classically in olfactory glomeruli. The study demonstrates that a subset of olfactory neuroreceptors displaying original ultrastructural characteristics projects selectively into atypical olfactory glomeruli. Ultrastructural features indicate that olfactory information processing taking place in the neuropil might be similar to that which occurs in typical glomeruli. Considered together, the atypical olfactory neuroreceptors, glomeruli and acetylcholinesterase-containing centrifugal fibres could constitute a new olfactory subsystem. This hypothesis is discussed by taking into account previous demonstration of other olfactory subsystems devoted to the processing of olfactory cues of fundamental biological importance.