Electron microscopic immunocytochemical localization of nerve growth factor in developing mouse olfactory neurons

Spinal cord injury

A spinal cord injury is a devastating, life-changing neurologic event that challenges patients, families, and caregivers. A myriad of neurologic and medical sequelae occur subsequent to the original insult. This article discusses epidemiology, primary and secondary injuries, acute therapy, and neuroprotective agents as well as the exciting areas of spinal cord recovery and regeneration, with an emphasis on cellular transplantation. Neurologic neurorehabilitation techniques and equipment are also reviewed, with a focus on their relation to increasing the independence and functional capacity of the patient. The article concludes with the clinical presentation and management of common spinal cord injury complications.

Neurotrophin expression in the adult olfactory epithelium

Published reports of neurotrophin expression in the olfactory system are incomplete because of missing data and conflicting results. Previous studies used a variety of fixation procedures and antibodies on different species and different ages. The aim of the present study was to examine expression of neurotrophins and their receptors using optimized methodologies: five methods of fixation, multiple antibodies, a variety of immunochemical protocols, and RT-PCR. We show here that (i) transcripts for all neurotrophins and their receptors are found in the adult olfactory epithelium; (ii) all neurotrophins are expressed in the supporting cells and the neuronal layers of the undisturbed adult olfactory epithelium while NT4 is found additionally in the horizontal basal cells; (iii) neurotrophin immunoreactivity required a fixative that included parabenzoquinone (not used in previous studies of olfactory tissue); (iv) TrkB and TrkC are restricted to the globose basal cell and neuron layers while TrkA is found in the horizontal basal cells and in the supporting cells where it co-localizes with the low affinity receptor for NGF (p75NTR). These findings confirm that neurotrophins are produced within the olfactory epithelium, suggesting autocrine and paracrine regulation of olfactory neurogenesis.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

Use of a cell line to investigate olfactory ensheathing cell-enhanced axonal regeneration

Olfactory ensheathing cells (OECs), a unique type of macroglia required for normal olfactory axonal regeneration throughout the lifetime of an individual, have been shown to have regeneration-enhancing properties when used to treat various neuronal injuries. Availability of OECs is a hurdle facing future clinical use of the cells for spinal cord injury (SCI) therapy. The number of OECs that can realistically be harvested from each animal is limited, and ensuring a pure cell population is difficult. We have begun to characterize a nonsyngeneic strain of OECs, i.e., from a homogenous OEC clonal cell line (nOECs). The purpose of this study was to determine whether nOECs have the same properties and provide the same functional recovery after SCI, as primary cultures of OECs. The results indicate that nOECs survive in vivo, produce growth-promoting proteins, and possess regeneration-promoting capabilities. Spinal cord injured rats that were treated with nOECs performed significantly better on functional tests than injured control animals beginning at 5 weeks after operation. In summary, evidence of nOEC regeneration-promoting capabilities suggests that this cell line can be used as potential therapy in SCI research.

Nerve growth factor promotes olfactory axonal elongation

An explant culture system was used to test the effect of nerve growth factor (NGF) on olfactory axonal elongation. Statistical analysis showed that exogenously applied NGF (50 ng/ml) significantly enhanced olfactory neurite elongation from E14 rat olfactory epithelial explants (p = 0.025). Immunostaining showed that the neurites expressed active TrkA receptors and that S-100-positive ensheathing cells were also present. In a separate experiment, immunoassay confirmed that following a growth period of 72 h, E14 presumptive olfactory bulb expressed and secreted NGF into the culture medium. The results indicate that during ontogeny, the olfactory bulb secretes NGF which binds to olfactory axons and facilitates their elongation.

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.

Cultured olfactory ensheathing cells express nerve growth factor, brain-derived neurotrophic factor, glia cell line-derived neurotrophic factor and their receptors

In the primary olfactory pathway axons of olfactory neurons (ONs) are accompanied by ensheathing cells (ECs) as the fibres course towards the olfactory bulb. Ensheathing cells are thought to play an important role in promoting and guiding olfactory axons to their appropriate target. In recent years, studies have shown that transplants of ECs into lesions in the central nervous system (CNS) are able to stimulate the growth of axons and in some cases restore functional connections. In an attempt to identify a possible mechanism underlying EC support for olfactory nerve growth and CNS axonal regeneration, this study investigated the production of growth factors and expression of corresponding receptors by these cells. Three techniques immunohistochemistry, enzyme linked immunosorbent assay (ELISA) and reverse transcriptase-polymerase chain reaction (RT-PCR) were used to assess growth factor expression in cultured ECs. Immunohistochemistry showed that ECs expressed nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and glial cell-line derived neurotrophic factor (GDNF). ELISA confirmed the intracellular presence of NGF and BDNF and showed that, compared to BDNF, about seven times as much NGF was secreted by ECs. RT-PCR analysis demonstrated expression of mRNA for NGF, BDNF, GDNF and neurturin (NTN). In addition, ECs also expressed the receptors trkB, GFRalpha-1 and GFRalpha-2. The results of the experiments show that ECs express a number of growth factors and that BDNF in particular could act both in a paracrine and autocrine manner.

Immunohistochemical localization of interleukin-2 and its receptor subunits alpha, beta and gamma in the main olfactory bulb of the rat

Endogenous interleukin-2 (IL-2) was found in the adult rat brain, however, it has not been reported whether this cytokine is present in the olfactory bulb. Immunohistochemical techniques were used to examine the cellular localization of IL-2 and its receptor subunits in the main olfactory bulb of the rat. Strong IL-2 immunoreactivity was localized in glial cells, specifically in the olfactory nerve layer, glomerular layer and external plexiform layer. IL-2 mRNA was detected in the olfactory bulb by RT-PCR. All three IL-2 receptor subunits also showed distinct laminar distributions. The IL-2Ralpha and IL-2Rbeta immunoreactivity was found both in neurons and glial cells, whereas IL-2Rgamma imunoreactivity was found in glial cells, and thus resembled IL-2 immunostaining. The present results demonstrated a wide distribution of IL-2 and its receptor subunits in the main olfactory bulb of the rat, suggesting that IL-2 might play a role in the olfactory function through autocrine or paracrine pathways. The exclusive high expression of IL-2 in glial cells in distinct laminar structures, where neuron-glia interactions are closely associated with olfactory nerve regeneration, imply that IL-2 might be involved in the process of nerve regeneration in the olfactory bulb.

Mechanisms of axonal plasticity: lessons from the olfactory pathway

The olfactory pathway has emerged recently as an effective model for studying general principles of axon extension and regeneration. A variety of both trophic as well as repulsive molecules are found in the olfactory pathway and are being characterized for their roles in promoting the high capacity for plasticity and growth in olfactory receptor cell axons. In addition, olfactory ensheathing cells, which line the olfactory nerve, have been shown to promote axon extension not only in the olfactory pathway but also in the injured spinal cord. This review summarizes some of our current knowledge of these mechanisms and how they may function collectively to promote axon plasticity.

Neurotrophic factors in the primary olfactory pathway

The number of identified growth factors continues to increase rapidly with many being implicated in the development of the nervous system, although for most of them the autocrine and paracrine pathways of cellular regulation still remain to be elucidated. The primary olfactory pathway, consisting of the olfactory epithelium and olfactory bulb, is presented here as a very useful model for the analysis of growth factor function. Review of the available literature suggests that a large proportion of neuroactive growth factors and their receptors are present in the olfactory epithelium or olfactory bulb. Furthermore, the primary olfactory pathway is one of the most plastic in the nervous system with neurogenesis continuing to contribute new sensory neurones in the olfactory epithelium and new interneurones in the olfactory bulb throughout adult life. The rich diversity of growth factors and their receptors in the olfactory system indicates that it will be useful in elucidating how these molecules regulate the formation of the nervous system. The olfactory epithelium in particular is proving useful as a model for the actions of growth factors in directing the neuronal lineage from stem cell to mature neurone.

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.

Growth factor regulation of neurogenesis in adult olfactory epithelium

Neurogenesis continues throughout adult life in the mammalian olfactory epithelium. This process is a dynamic state of proliferation, differentiation and cell death, probably regulated by autocrine and paracrine signals such as peptide growth factors. Previous investigations have demonstrated roles for some growth factors in olfactory neurogenesis in vitro, but the assay systems used make it difficult to be certain of their effects (proliferation, differentiation, enhanced survival) or their target cells. The present study investigated the effects of growth factors in cultures of purified olfactory epithelium comprising only basal cells and supporting cells in serum-free media. The advantage of this culture system is that proliferation, differentiation and survival of the basal cells and neurons can be examined separately. Under these conditions, three growth factors exerted well-defined effects: (i) fibroblast growth factor-2 stimulated proliferation of the globose basal cells; (ii) transforming growth factor-beta2 induced these cells to differentiate into neurons; and (iii) platelet-derived growth factor promoted survival of the differentiated neurons. We conclude that fibroblast growth factor-2, transforming growth factor-beta2 and platelet-derived growth factor act sequentially on precursor cells and immature neurons during neurogenesis in the adult olfactory epithelium.

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.

FGF2 promotes neuronal differentiation in explant cultures of adult and embryonic mouse olfactory epithelium

Neurogenesis in the adult olfactory epithelium is highly regulated in vivo. Little is known of the molecular signals which control this process, although contact with the olfactory bulb or with astrocytes has been implicated. Explants of mouse olfactory epithelium were grown in the presence or absence of several peptide growth factors. Basic fibroblast growth factor (FGF2) stimulated differentiation of sensory neurons in adult and embryonic olfactory epithelium. Other growth factors tested were ineffective. FGF2-stimulated neurons were born in vitro and expressed neurofilament, neural cell adhesion molecule, and beta-tubulin. The cells also expressed olfactory marker protein, a marker for mature olfactory sensory neurons in vivo. These bipolar neurons did not express glial fibrillary acidic protein or low-affinity nerve growth factor receptor. These results indicate that neither astrocytes nor olfactory bulb are necessary for differentiation of olfactory sensory neurons in vitro.

The telencephalic vesicles are innervated by olfactory placode-derived cells: a possible mechanism to induce neocortical development

During early embryonic development, the olfactory placode is the source of different cell types migrating toward the telencephalic vesicle. Among these cell types are the ensheathing cells, the luteinizing hormone-releasing hormone-producing cells and the olfactory marker protein-immunoreactive cells. We have identified a novel group of olfactory placode-derived migratory cells using an antibody against beta-tubulin to label neurons and acetylcholinesterase histochemistry to label posmitotic cells. In this paper we describe the morphology, migration and fate of this novel group of cells. The first neurons detected in the rostral prosencephalon with acetylcholinesterase and anti-beta-tubulin antibody are localized in the olfactory placodes at embryonic day 11 in the rate. At embryonic day 12, anti-beta-tubulin antibody-positive cells were observed in the mesenchymal tissue between the olfactory pit and the rostral pole of the telencephalic vesicle. Anti-beta-tubulin antibody-positive cells were seen running superficially over the pial (dorsal) side of the telencephalic vesicle at embryonic day 13. The majority of these cells have a bipolar profile with short leading and trailing processes, suggesting that they are migratory elements. However, some of these cells showed elaborate processes extending for quite long distances, overlying the pial surface of the telencephalic vesicle. A mass of cells extending over the telencephalic vesicle from the developing olfactory epithelium were observed at embryonic day 13 using acetylcholinesterase histochemistry. Some of these acetylcholinesterase-positive cells were identified as neurons with the specific neuronal marker anti-beta-tubulin antibody. On embryonic day 12, neurons from the olfactory epithelium send axonal fibers toward the telencephalic vesicles. Most of these fibers spread over the anteroventral pole of the vesicles but others entered deep into the telencephalon, reaching the germinal ventricular zone. We also show that fibers run rostrocaudally over the surface of the telencephalic vesicles. We suggest that these cells and fibers, apparently originating in the olfactory placode and migrating through non-conventional routes, might play a significant role in the earliest stages of telencephalic vesicle development.

Olfactory bulb ensheathing glia: a unique cell type with axonal growth-promoting properties

The olfactory bulb (OB) is a structure of the central nervous system (CNS) in which axonal growth occurs throughout the lifetime of the organism. A major difference between the OB and the remaining CNS is the presence of ensheathing glia in the first two layers of the OB. Ensheathing glia display properties that might be involved in the process of regeneration and they appear to be responsible for the permissibility of the adult OB to axonal growth. In fact, transplants of ensheathing glia can be used as promoters of axonal regeneration within the adult CNS. The axonal growth-promoting properties of ensheathing glia make the study of this cell type interesting for understanding the mechanisms underlying axonal regeneration. Several groups have studied OB ensheathing cells extensively in an attempt to classify them within any of the known glial groups. However, this cell type does not exhibit the phenotypic features of any glial population described thus far. In this article we review the characteristics that differentiate ensheathing glia from other peripheral and central glial populations as well as the properties that involve them in axonal regeneration. The evidence suggests that ensheathing glia are unique, have their own identity, and do not belong to any previously described glial type.

An improved procedure for the immunohistochemical localization of nerve growth factor-like immunoreactivity

Nerve growth factor (NGF) is a survival factor required by a number of neuronal populations including most post-ganglionic sympathetic neurones. NGF has been detected and quantified in many tissues but there is little information regarding its cellular localization. Although it has been argued that histological detection has proven difficult due to the low levels of NGF present, other factors may contribute to prevent its identification. In the present study, we report a method for the histological detection of NGF-like immunoreactivity in the rat superior cervical ganglia (SCG). Adult Wistar-Kyoto rats were perfused briefly with either a high or low pH buffer prior to fixation and routine immunohistochemistry. Polyclonal antibodies to native mouse NGF used in the present study recognized mouse NGF but not recombinant human neurotrophin 3 (rhNT3) or brain-derived neurotrophic factor (rhBDNF) by immunoblot analysis. NGF-like immunoreactivity was localized to most sympathetic neurones. Immunoreactivity was detected in the cytoplasm with dense labelling around nuclei. No stain was seen in sections incubated with normal sheep IgG or from animals perfused with phosphate buffer (pH 7.4) prior to fixation. In addition, axotomy resulted in the disappearance of NGF immunoreactivity which was confirmed by biochemical quantification. Finally, no NGF immunoreactivity was found in neurones of rats treated systemically with NGF antiserum 3 days earlier. Possible mechanisms underlying the improvement of NGF immunohistochemistry by pH manipulation before fixation are discussed.

In vitro enfolding of olfactory neurites by p75 NGF receptor positive ensheathing cells from adult rat olfactory bulb

Secondary cultures of adult rat olfactory bulb (OB) contained three different types of cell: (i) process-bearing cells; (ii) macrophage-like cells and (iii) fusiform cells. The immunohistochemical properties of process-bearing cells closely corresponded to those described for ensheathing glia in vivo. The most distinctive feature of these cells was their immunoreactivity for low affinity nerve growth factor receptor (NGFR). Process-bearing cells also shared the ultrastructural properties of ensheathing glia in vivo, as well as the ability to ensheath olfactory axons. In contrast, macrophage-like cells had the immunostaining properties of microglia, and fusiform cells were likely capillary endothelial cells. Neurites outgrowing from olfactory epithelium explants, when co-cultured with adult OB cells, grew preferentially over NGFR positive cells. Olfactory neurites exhibited NGFR immunoreactivity and were enfolded by NGFR positive cells. After ensheathment, this immunoreactivity decreased from the neurite and disappeared from the glial membrane in contact with the neurite. However, NGFR immunoreactivity was maintained in the portion of the glial membrane not involved in ensheathing. In summary, ensheathing cells in vitro retained both the ultrastructure shown in vivo and the ability to ensheath olfactory neurites. The Schwann cell-like properties of ensheathing glia, could partially explain the permissibility of adult OB to axonal growth.

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.

Molecular and cellular properties of mammalian primary olfactory axons

How are the axonal projections of olfactory and vomeronasal receptor neurons to the olfactory bulb formed during development? How are the primary olfactory axonal connections functionally organized? With progress in molecular biological techniques and histochemical methods, it became possible to study cellular strategies and molecular mechanisms which guide the primary olfactory axons of the main and accessory olfactory systems to the target glomeruli in the bulb. In addition, new methodologies have begun to elucidate various subsets of the primary olfactory axons with distinctive central connections. The aim of the present paper is to review (1) the characteristic organization of the projection of the primary olfactory axons, (2) projection patterns of histochemically defined subsets of primary olfactory axons, and (3) information on molecules expressed by the surface membrane of the primary olfactory axons. This knowledge gives insight into the functional organization of the primary olfactory axon projection, which is indispensable for understanding signal processing in the olfactory system. This knowledge also underscores the notion that the primary olfactory axon projection provides an excellent model system in which to study axonal guidance and the formation of specific synaptic connections.

Nerve growth factor (NGF) and its receptor in rat olfactory epithelium

Nerve Growth Factor (NGF) is a bioactive protein that plays an important role in the genesis, differentiation, growth, maintenance and aging of the central and peripheral nervous systems. The purpose of this study was to explore the significance of NGF in the turnover, regeneration and maintenance of the olfactory epithelium. We tried to detect NGF and NGF-receptor in the olfactory epithelium of young and mature rats with an immunohistochemical technique. Immunoreactivity to NGF and NGF-receptor was detected in all specimens of olfactory epithelium. Immunoreactivity to NGF was positive in the olfactory nerve cells, especially in the cytoplasm and dendrites, in the basal cell layer, and on fibers piercing the olfactory epithelium. Immunoreactivity to NGF-receptor was positive in the basal cell layer and on fibers piercing the olfactory epithelium. These results suggest that NGF plays a role in the maintenance of olfactory nerve cells, and in the differentiation from the basal cell to the mature olfactory nerve cell, in both the young and the mature rat. In addition, the differences in the density of immunoreactive cells between different parts of the olfactory epithelium support the concept of an active and a quiescent zone in olfactory epithelium turnover.

Immunocytochemical localization of cell adhesion molecules in the developing and mature olfactory system

The localization of Ca+(+)-independent cell adhesion molecules (CAMs) in the developing and mature olfactory epithelium and bulb is reviewed. The CAMs included in this article are the neural cell adhesion molecule (N-CAM), the 180 kD component of N-CAM (N-CAM 180), the embryonic form of N-CAM (E-N-CAM), L1 glycoproteins, J1 glycoproteins, and the adhesion molecule on glia (AMOG). In addition, the expression of the L2-HNK-1 carbohydrate epitope, shared by N-CAM, L1, J1 and myelin-associated glycoprotein (MAG) in the adult olfactory epithelium and bulb has also been documented. For the localization of these molecules at the light and electron microscopic levels, immunocytochemical techniques were used and are described in detail. During development and organogenesis, the olfactory system exhibits a pattern of CAM expression similar to the general pattern described for the developing nervous system. In the adult olfactory system, however, a significant retention of CAMs characteristic for developmental and morphogenetic processes, such as E-N-CAM, AMOG, as well as the high molecular weight components of J1 glycoproteins, can be observed. The retention of these embryonic features are most likely associated with the cell turnover and high plasticity of this system. Moreover, the predominance of N-CAM 180 with respect to other components of N-CAM, as well as the absence of the L2/HNK-1 carbohydrate epitope, are also particular traits of the primary olfactory system which could be associated with its exceptional properties.

Regulation of the low affinity receptor for nerve growth factor, p75NGFR, in the olfactory system of neonatal and adult rat

Using MAb192, a monoclonal antibody to the rat low affinity receptor for nerve growth factor (p75NGFR), we determined the expression of p75NGFR in rat neonatal and adult olfactory system. In neonates and adults, we observed discrete p75NGFR-immunoreactivity (p75NGFR-ir) in the glomerular layer of the main olfactory bulb. The intensity and organization of glomerular p75NGFR-ir increased with age. This was in keeping with the general ontogeny of the main olfactory bulb. Generally, granule cells, mitral cells and periglomerular cells of the main olfactory bulb were not specifically stained. However, in early neonates, granule cells close to the lateral olfactory tract exhibited p75NGFR-ir. Additional specific staining was found in the olfactory receptor neurons of neonatal and adult olfactory neuroepithelium, the olfactory fascicles and in the glomeruli of the accessory olfactory bulb. The intensity, but not the organization, of specific staining in the accessory olfactory bulb increased as the animal matured. We believe that p75NGFR-ir in the olfactory system is associated with its unique capacity to regenerate its peripheral input to the main olfactory bulb. The presence of p75NGFR-ir in the accessory olfactory bulb would suggest a broader role for this protein. Here we discuss the implications of these findings with regards to nerve growth factor, other trophic molecules, and their receptors. The data presented provide a foundation for studies involving manipulation of regenerative phenomena while monitoring the expression of neurotrophic factors and their receptors.

Monoclonal antibody immunohistochemistry of a temporal relationship between axonal elimination of aberrant olfactory nerves and synaptogenesis in the rabbit olfactory bulb during middle embryonic periods

Immunostaining using olfactory nerve- and synaptic vesicle protein-specific monoclonal antibodies revealed their characteristic appearance in the rabbit olfactory bulb during prenatal development. Prior to formation of glomeruli, olfactory nerve fibers extended beyond their target region deep into the bulb zones. Subsequently the aberrant axons decreased in number, and correspondingly, synaptic vesicle protein occurred in the innermost region of the olfactory nerve layer. It is concluded that the lack of synaptogenesis causes axonal elimination of aberrant olfactory nerves. Present results support the hypothesis that supernumerary axons degenerate unless synaptic contacts are secured; the olfactory nerves must arrive at the appropriate terminal zone in the glomeruli.

Carnosine, nerve growth factor receptor and tyrosine hydroxylase expression during the ontogeny of the rat olfactory system

The localizations of carnosine, nerve growth factor (NGF) receptor and tyrosine hydroxylase (TH) were studied in the embryonic and postnatal rat olfactory bulb and epithelium by means of single- and double-immunostaining methods. Tyrosine hydroxylase ontogeny was also evaluated at the mRNA level by in situ hybridization. All these molecules were expressed in the olfactory bulb but with different developmental patterns and cellular localization: carnosine immunoreactivity is seen from embryonic day 17 in primary olfactory neurons scattered in the nasal cavity and in fibres projecting from them to the olfactory bulb. Nerve growth factor-receptor immunoreactivity associated with small glial-like cells is visible in some glomeruli starting from the second day of postnatal life. At postnatal day 10 NGF-receptor immunoreactivity is extended to all glomeruli. Periglomerular neurons expressing TH mRNA and protein are present prenatally and their number sharply increases during the early postnatal development. Double-staining methods show that TH and NGF-receptor immunoreactivity do not overlap in cell bodies and processes. In addition, NGF-receptor immunoreactivity is not colocalized with carnosine. These findings definitely exclude NGF-receptor expression in periglomerular and primary olfactory neurons, suggesting that at least part of NGF-receptor expression in the olfactory bulb is associated with glial cells. In addition, they provide the first immunohistochemical data on carnosine ontogeny and confirm at the mRNA level previous studies on the ontogeny of TH protein.

Glial influences on axonal growth in the primary olfactory system

Neurogenesis in the olfactory epithelium continues throughout the entire life of mammals, and it is the axons of these newly formed olfactory receptor neurons that grow into the target tissue after the first cranial nerve is injured, not the regenerating axons of mature cells. These axons are able to enter and grow within the CNS of adult animals, unlike regenerating axons in injured dorsal roots, the majority of which are prevented from penetrating very far into the spinal cord. One reason why the olfactory axons are so successful in entering the CNS may be due, at least partially, to the fact that they are ensheathed by a type of glial cell (the ensheathing cell) that expresses phenotypic features of both astrocyte and Schwann cells. The presence of both L1/Ng-CAM and N-CAM in the plasma membranes of both ensheathing cells and immature olfactory receptor neurons would enable the olfactory axons to use the glial cell surfaces as a substratum on which to grow. It is probably also true that ensheathing cells synthesize and secrete laminin, thus providing an additional adhesive substrate for the olfactory axons, as well as glia-derived nexin and nerve growth factor, both of which are neurite-promoting agents.