Electron-microscopic demonstration of olfactory-marker protein with protein G-gold in freeze-substituted, Lowicryl K11M-embedded rat olfactory-receptor cells

Immunocytochemical Localization of Olfactory-signaling Molecules in Human and Rat Spermatozoa

Expression of olfactory receptors (ORs) in non-olfactory tissues has been widely reported over the last 20 years. Olfactory marker protein (OMP) is highly expressed in mature olfactory sensory neurons (mOSNs) of the olfactory epithelium. It is involved in the olfactory signal transduction pathway, which is mediated by well-conserved components, including ORs, olfactory G protein (Golf), and adenylyl cyclase 3 (AC3). OMP is widely expressed in non-olfactory tissues with an apparent preference for motile cells. We hypothesized that OMP is expressed in compartment-specific locations and co-localize with an OR, Golf, and AC3 in rat epididymal and human-ejaculated spermatozoa. We used immunocytochemistry to examine the expression patterns of OMP and OR6B2 (human OR, served as positive olfactory control) in experimentally induced modes of activation and determine whether there are any observable differences in proteins expression during the post-ejaculatory stages of spermatozoal functional maturation. We found that OMP was expressed in compartment-specific locations in human and rat spermatozoa. OMP was co-expressed with Golf and AC3 in rat spermatozoa and with OR6B2 in all three modes of activation (control, activated, and hyperactivated), and the mode of activation changed the co-expression pattern in acrosomal-reacted human spermatozoa. These observations suggest that OMP expression is a reliable indicator of OR-mediated chemoreception, may be used to identify ectopically expressed ORs, and could participate in second messenger signaling cascades that mediate fertility.

Effects of pyridine inhalation exposure on olfactory epithelium in mice

Olfactory neurons in the nasal mucosa have the capacity to regenerate continuously along the lifespan by neurogenesis processes starting with progenitor cells close to the basal lamina. The cellular turnover into olfactory neuroepithelium may be modified by environmental stimuli insofar as nasal mucosa is directly in contact with airborne chemicals. However, few studies have been focused on selective changes, especially those concerning mature olfactory neurons and basal cells during specific inhalation exposure. Among chemicals, solvents are known to induce changes in smell abilities and concomitant histological and cellular modifications related to the type of molecule, concentration and time of exposure. This study was designed to characterize smell sensitivity (using behavioral tests) and immunohistochemical effects on olfactory neuroepithelium induced by pyridine exposure in mice. Olfactory marker protein (OMP) and proliferating cell nuclear antigen (PCNA) were used to characterize respectively mature olfactory neurons and basal cells. Results showed that inhalation exposure to pyridine had no impact on smell sensitivity whatever the concentration used and the time of exposure. These findings were in agreement with immunohistochemical measurements showing the same cellular kinetic whatever the condition of exposition to pyridine. Indeed, OMP-positive cells increased and PCNA-positive cells decreased as early as the beginning of exposure and cell amounts remained stable at this level until the end of exposure. These findings suggest that pyridine could have the property to rapidly activate a cellular turnover from basal cell progenitors. Rather than toxic effects, the present findings suggest that the metabolites of pyridine might have cell cycle activation properties.

Site-specific population dynamics and variable olfactory marker protein expression in the postnatal canine olfactory epithelium

The main olfactory epithelium is a pseudostratified columnar epithelium that displays neurogenesis over the course of a lifetime. New olfactory neurons arise basally and are transferred to the middle third of the epithelium during maturation. It is generally believed that this pattern is present throughout the olfactory area. In the present study, we show that the postnatal canine olfactory epithelium is composed of two distinct types of epithelium, designated A and B, which not only differ in olfactory neuron morphology, marker expression and basal cell proliferation but also display a patchy distribution and preferential localization within the nasal cavity. Type A epithelium, abundant in the caudal part of the olfactory area, contains well-differentiated olfactory neurons positive for olfactory marker protein but low numbers of immature neurons and proliferating basal cells, as visualized by TrkB/Human Natural Killer-1 (HNK-1) glyco-epitope and Ki-67 immunostaining, respectively. In contrast, type B epithelium is mainly found in the rostral part and contains smaller and elongated neurons that display increased levels of TrkB/Human Natural Killer-1 (HNK-1) glyco-epitope immunoreactivity and a higher number of Ki-67-positive basal cells but lower and variable levels of olfactory marker protein. The vomeronasal organ displays a uniform distribution of molecular markers and proliferating basal cells. The observation that olfactory marker protein in type A and B epithelium is preferentially localized to the nucleus and cytoplasm, respectively, implies correlation between subcellular localization and olfactory neuron maturation and may indicate distinct functional roles of olfactory marker protein. Whether the site-specific population dynamics in the postnatal canine olfactory epithelium revealed in the present study are modulated by physiological parameters, such as airflow, has to be clarified in future studies.

Bestrophin 2: an anion channel associated with neurogenesis in chemosensory systems

The chemosensory neuroepithelia of the vertebrate olfactory system share a life-long ability to regenerate. Novel neurons proliferate from basal stem cells that continuously replace old or damaged sensory neurons. The sensory neurons of the mouse and rat olfactory system specifically express bestrophin 2, a member of the bestrophin family of calcium-activated chloride channels. This channel was recently proposed to operate as a transduction channel in olfactory sensory cilia. We raised a polyclonal antibody against bestrophin 2 and characterized the expression pattern of this protein in the mouse main olfactory epithelium, septal organ of Masera, and vomeronasal organ. Comparison with the maturation markers growth-associated protein 43 and olfactory marker protein revealed that bestrophin 2 was expressed in developing sensory neurons of all chemosensory neuroepithelia, but was restricted to proximal cilia in mature sensory neurons. Our results suggest that bestrophin 2 plays a critical role during differentiation and growth of axons and cilia. In mature olfactory receptor neurons, it appears to support growth and function of sensory cilia.

Carbon dioxide effects on olfactory functioning: behavioral, histological and immunohistochemical measurements

Most studies on toxic inhalation focus on solvent effects and few have dealt with gases on olfactory functioning. Among gases, the effects of carbon dioxide on general physiology have been well investigated contrary to the impact on olfactory neuroepithelium. Thus, this work was designed to evaluate in mice the possible effects of 3% CO(2) in two exposure periods: a 5h/day and a 12h/day conditions. Behavioral, histological and immunohistochemical observations were conducted every 2 weeks, i.e. before (W0), during (W2, W4) and after exposure (W6, W8). Firstly, behavioral evaluations of odor sensitivity showed differences in relation to the odor tested, i.e. no effect with congener urine odor and a reinforcement of 2,4,5-trimethythiazoline (TMT) (predator odor) repulsion. Secondly, histological evaluations showed a similar evolution of the epithelium thickness, i.e. a decrease along the exposure as well as during the post-exposure period and an increase of cell number (whatever the phenotype) although the kinetic appeared different in both experimental conditions. Thirdly, immunohistochemical quantification of olfactory marker protein (OMP)- and proliferating cell nuclear antigen (PCNA)-positive cells revealed that the number of mature olfactory neurons increased at the early beginning of exposure period in both conditions. While a decrease was observed in the following weeks (W4-W8) for the 12h/day condition, a stable amount of OMP-positive cells was maintained in the 5h/day condition. In contrast, the number of PCNA-positive cells followed a similar evolution, i.e. a constant decrease along the experiment. These findings indicate that the effects of CO(2) inhalation exposure are selectively dose-dependent.

Inhalation exposure to acetone induces selective damage on olfactory neuroepithelium in mice

Due to their specific position in the nasal cavity, the cells of olfactory neuroepithelium can be damaged by exposure to environmental airborne chemicals. However, few studies have been focused on selective damage, i.e. olfactory sensory neurons, basal cells, supporting and duct cells. As solvents are known to induce critical effects on olfactory neuroepithelium (OE), this study was designed to characterize histological and immunohistological effects induced by acetone exposure on OE in mice. Behavioral tests were conducted to evaluate olfactory sensitivity. Moreover, olfactory neuroepithelium was examined to evaluate the thickness and the total number of cells. Finally, different markers, olfactory marker protein (OMP) and proliferating cell nuclear antigen (PCNA), were used to characterize respectively olfactory sensory neurons and basal cells, and secondly to evaluate the dynamic of the tissue turnover. Results showed structural modifications, since the thickness and the number of cells in the OE were modified according to the time course of the exposure. Additionally, no changes for OMP-positive cells were observed whereas significant differences appeared for the density of PCNA-positive cells in relation to their location (main-body or basal layer of OE). These findings indicate that acetone exposure induces selective damage in olfactory neuroepithelium.

The interaction of Bex and OMP reveals a dimer of OMP with a short half-life

Olfactory marker protein (OMP) participates in the olfactory signal transduction pathway. This is evident from the behavioral and electrophysiological deficits of OMP-null mice, which can be reversed by intranasal infection of olfactory sensory neurons with an OMP-expressing adenovirus. Bex, brain expressed X-linked protein, has been identified as a protein that interacts with OMP. We have now further characterized the interaction of OMP and Bex1/2 by in vitro binding assays and by immuno-coprecipitation experiments. OMP is a 19 kDa protein but these immunoprecipitation studies have revealed the unexpected presence of a 38 kDa band in addition to the expected 19 kDa band. Furthermore, the 38 kDa form was preferentially co-immunoprecipitated with Bex from cell extracts. In-gel tryptic digestion, mass spectrometry, and two-dimensional gel electrophoresis indicate that the 38 kDa protein behaves as a covalently cross-linked OMP-homodimer. The 38 kDa band was also identified in western blots of olfactory epithelium demonstrating its presence in vivo. The stabilities and subcellular localizations of the OMP-monomer and -dimer were studied in transfected cells. These results demonstrated that the OMP-dimer is much less stable than the monomer, and that while the monomer is present both in the nuclear and cytosolic compartments, the dimer is preferentially located in a Triton X-100 insoluble cytoskeletal fraction. These novel observations led us to hypothesize that regulation of the level of the rapidly turning-over OMP-dimer and its interaction with Bex1/2 is critical for OMP function in sensory transduction.

Identification of members of the Bex gene family as olfactory marker protein (OMP) binding partners

Olfactory marker protein (OMP) expression is a hallmark of mature vertebrate olfactory receptor neurons (ORNs). Evidence for OMP function derives from altered behavioral and electrophysiological activities of OMP-KO mice. The molecular basis for the altered phenotype following the deletion of OMP is still unclear. Recent structural studies predict the involvement of OMP in protein-protein interaction. Here we report the identification of an OMP partner, Bex2, by phage-display screening of an olfactory mucosal cDNA-library. In situ hybridization demonstrates cellular co-localization of OMP mRNA with mRNAs for Bex1, Bex2, and Bex3 in ORNs of olfactory tissue of the mouse. The OMP/Bex interaction has been confirmed by demonstrating the chemical cross-linking of recombinant rat OMP with a synthetic peptide derived from the Bex amino acid sequence. The subcellular localization of Bex and OMP proteins was evaluated in transfected HEK293 cells. Bex is visualized in the nucleus and cytoplasm. Following co-transfection we observed the unexpected presence of some OMP in the nucleus along with Bex. Together, these data argue convincingly that we have identified Bex as an OMP partner whose further characterization will provide insight to the role of OMP and to the mechanism of the OMP/Bex interaction in ORN differentiation and function.

Immunocytochemical characteristics of cells and fibers in the nasal mucosa of young and adult macaques

The mammalian nasal cavity is lined by an olfactory mucosa (OM) and a respiratory mucosa (RM). The principal OM cell type is the olfactory receptor neuron (ORN). However, little is known about ORNs in the life histories of primates. The RM, similar to the RM in the tracheobronchial tract (TBT), is dominated by ciliated columnar cells. Neuroendocrine cells (NECs) are essential in the TBT; little is known about nasal NECs. This study examined the immunolabeling characteristics of primate OM and RM for three important proteins-calretinin (CR), olfactory marker protein (OMP), and protein gene product 9.5 (PGP). Tissues from newborn to 15-year-old macaques were analyzed to determine the expression of these proteins during various stages of development. Standard immunocytochemistry on aldehyde-fixed tissues was applied, utilizing the avidin-biotin peroxidase (ABC) method. Immuno-electron microscopy confirmed the immunoreactive cell types. ORNs were immunoreactive for CR, OMP, and PGP at all ages studied. Immunoreactivity for PGP also was displayed in a subset of ciliated, columnar epithelial cells in the RM and in an extensive network of subepithelial fibers spread throughout both mucosae. The results suggest that macaque ORNs express three important proteins over a wide life history, and that the macaque may be a reliable model for studying primate/human olfaction during aging. The PGP-labeling results also suggest that the macaque nasal peptidergic fibers express PGP and that the respiratory epithelium contains NECs with labeling characteristics similar to those in the TBT.

CaBPs and other immunohistochemical markers of the human vomeronasal system: a comparison with other mammals

After more than two centuries of almost sporadic inquiry as to the existence and function of the human vomeronasal system (VNS), the last decade has seen a resurgent interest in it. The principal question vexing many laboratories is whether adult humans retain the VNS that clearly develops during fetal growth. Additional questions are whether the structurally defined fetal VNS has any function role, and if this structure and function extend into postnatal life. One research tool that has been successfully used to identify key components of the mammalian VNS has been immunohistochemistry (IHC). This technique has clearly defined the vomeronasal receptor neurons in the vomeronasal organ, the vomeronasal nerve that projects into the central nervous system, and the target of this nerve, the accessory olfactory bulb. This review will discuss immunohistochemical studies that have identified these features in the mammalian VNS, and relate them to structural and IHC studies of the fetal and adult human VNS. Suggestions as to future studies to clarify the status of the human VNO also are offered.

Olfactory marker protein (OMP) gene deletion causes altered physiological activity of olfactory sensory neurons

Olfactory marker protein (OMP) is an abundant, phylogentically conserved, cytoplasmic protein of unknown function expressed almost exclusively in mature olfactory sensory neurons. To address its function, we generated OMP-deficient mice by gene targeting in embryonic stem cells. We report that these OMP-null mice are compromised in their ability to respond to odor stimull, providing insight to OMP function. The maximal electroolfactogram response of the olfactory neuroepithelium to several odorants was 20-40% smaller in the mutants compared with controls. In addition, the onset and recovery kinetics following isoamyl acetate stimulation are prolonged in the null mice. Furthermore, the ability of the mutants to respond to the second odor pulse of a pair is impaired, over a range of concentrations, compared with controls. These results imply that neural activity directed toward the olfactory bulb is also reduced. The bulbar phenotype observed in the OMP-null mouse is consistent with this hypothesis. Bulbar activity of tyrosine hydroxylase, the rate limiting enzyme of catecholamine biosynthesis, and content of the neuropeptide cholecystokinin are reduced by 65% and 50%, respectively. This similarity to postsynaptic changes in gene expression induced by peripheral olfactory deafferentation or naris blockade confirms that functional neural activity is reduced in both the olfactory neuroepithelium and the olfactory nerve projection to the bulb in the OMP-null mouse. These observations provide strong support for the conclusion that OMP is a novel modulatory component of the odor detection/signal transduction cascade.

Freeze-fracture, deep-etch, and freeze-substitution studies of olfactory epithelia, with special emphasis on immunocytochemical variables

Freeze-fracturing and deep-etching are a well-suited set of methods to study membrane and cytoplasmic features. Various approaches are available. Possible variables include tissue preparation, fracturing only or fracturing followed by etching, modes and materials of replication, and various ways of combining freeze-fracturing and/or deep-etching with (immuno)cytochemistry. Freeze-substitution, in particular combined with embedding in methacrylate resins such as the Lowicryls, is becoming rather widely accepted for purposes of ultrastructural (immuno)cytochemistry. Most investigators active in this field agree that this combination yields superior results compared to (immuno)cytochemistry combined with more conventional means of thin section transmission electron microscopy. Yet relatively little information is available on the variations that can occur with different approaches of freeze-substitution immunocytochemistry. This review deals with some of the variations in freeze-fracturing, freeze-etching, and freeze-substitution as applied to olfactory epithelial structures and with the effectiveness of observations obtained by application of the above sets of methods in relating the special morphology of olfactory epithelial cellular structures with those obtained by other approaches. Indeed, the data obtained continue to provide an integral image in which that morphology can be related to the special biochemistry, cell and molecular biology, and electrophysiology of olfactory epithelial structures.

Immunocytochemical study of the differentiation process of the septal organ of Masera in developing rats

The septal organ of Masera is a small patch of olfactory epithelium located near the base of the nasal septum. Using the growth-associated protein B-50/GAP-43 as neuronal marker, we have studied the differentiation process of this organ from the olfactory sheet in embryonic and newborn rats. Results show that the septal organ first appeared at embryonic day 16. Even though it was included in the olfactory sheet, the presumptive septal organ could be distinguished by a higher density of B-50/GAP-43-positive neurons. Concomitantly to its morphological development, the septal organ progressively isolated from the main olfactory epithelium. This isolation resulted from the extension of a transitional area which progressively lost its typical features of olfactory epithelium to become a putative respiratory epithelium in late embryonic stages. Results strongly suggest that the septal organ should be a proper chemosensory system with its own time-course of development.

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.

On the formation of neuromata in the primary olfactory projection

Olfactory axons have been shown to grow aberrantly and form dense collections of axons, termed neuromas, in the olfactory epithelium of rats in which the olfactory bulb was ablated. Likewise, in human olfactory mucosa, collections of neurites have been noted in a variety of disease states, including Alzheimer's disease. We report here an immunohistochemical and electron microscopic analysis of aberrant axonal growth in the rat olfactory mucosa induced by experimental lesion. In particular, we have used the monoclonal antibody 2G12, which binds to the phosphorylated form of GAP-43, as an extremely sensitive marker for neuromatous axons, because it does not label neuronal cell bodies. In unilaterally bulbectomized rats, neuromas form in posterior olfactory epithelium on the operated side. Several lines of evidence, including serial section reconstruction, indicate that olfactory axons are induced to grow back into the epithelium at a distance from their point of origin as a consequence of bulbectomy, and are accompanied by glial cells from the olfactory nerve. Avulsion of a part of the olfactory nerve has similar effects as destruction of the olfactory bulb. Intraepithelial neuromas also develop in the olfactory mucosa of rats simultaneously exposed to methyl bromide gas and injected with 3-methyl indole; this treatment severely damages the olfactory epithelium directly. Exposure to methyl bromide alone causes milder damage, and the neuromas that form are transient. The evidence indicates that neuromas form after the epithelium is directly damaged because axons are trapped in the epithelium. Both of the mechanisms identified here should be taken into account when considering the findings in the human olfactory mucosa.

Cryofixation, cryosubstitution, cryoembedding for ultrastructural, immunocytochemical and microanalytical studies

Cryofixation, cryosubstitution and cryoembedding are a set of low-temperature methods for immunocytochemical and microanalytical ultrastructural studies. This review covers the theoretical and practical aspects of these cryomethods, simple, low-cost, safe devices that provide reproducible results and a summary of recent results. Sections prepared by these three cryomethods can be used to determine elemental composition, molecular composition, functions and 3-D ultrastructure. The information obtained can be treated by multivariate statistical methods. Thus, each cellular compartment can be identified by its morphology, molecular and elemental composition and function and changes in these data during physiological and pathological processes can be monitored.

An immuno-electron microscopic comparison of olfactory marker protein localization in the supranuclear regions of the rat olfactory epithelium and vomeronasal organ neuroepithelium

Immuno-electron microscopy was used to examine olfactory marker protein (OMP) ultrastructural localization in the supranuclear regions of the rat olfactory epithelium (OE) and vomeronasal organ (VNO) neuroepithelium. In the OE, OMP immuno-reaction product was observed within the cytoplasm of olfactory chemoreceptor cell dendrites, vesicles and cilia. Reaction product was absent from olfactory microvillar cells and their unique microvillar projections. In the neuroepithelium of the VNO, immuno-reaction product was seen within the dendrites of the chemoreceptor cells, and in the bases of their attached microvilli; the remaining distal portions of these microvilli were unlabeled. These results demonstrate a difference in the distribution of OMP immunoreactivity over the surfaces of the rat OE and VNO. They also show that OMP immunoreactivity does not exist in the rat olfactory microvillar cells.

Freeze substitution after fast-freeze fixation in preparation for immunocytochemistry

As compared to classical chemical fixation, the physical immobilization of ultrastructures by fast-freeze fixation (FFF) and the subsequent exchange of water in its solid state by freeze substitution (FS) improve the preparation procedure for immunogold labeling (IGL). FFF-FS results in a morphological preservation of unchallenged quality, as well as in a better preservation of antigenic reactivity, thus allowing remarkable precision of labeling on sections. However, FFF, particularly over a cooled metal plate, requires a heavy and expensive machine. It is not suitable for all biological specimens and in the best conditions, which remain difficult to standardize, the thickness of the well-preserved portion of the specimen does not exceed a few microns for compact tissues, and exceptionally 30-40 microns for isolated cells. The FS procedure is long and must be adjusted empirically for every new specimen and antigenic detection. The preservation of a given antigen's reactivity in the presence of fixative agents and embedding resins remains unpredictable. The action of fixative agents is different and milder in FS than when they are used classically in chemical fixation. By chance, one of the best FS procedures for the preservation of both ultrastructure and antigenicity appears to be by using acetone alone, together with a molecular sieve to improve the water exchange process. A large choice of embedding resins usually allows us to find a compromise between ultrastructural and antigenic preservation.

Impact of freeze substitution on biological electron microscopy

Considering the increasing necessity for improved preparation techniques in biological electron microscopy as a basis for the identification and localization of cellular substances within the compartments of the cell, this review is focussed on the method of freeze substitution as an important link between the cryofixation (ultrarapid freezing) and resin embedding of biological specimens. The theory and practice of freeze substitution is summarized with particular interest in the physical and thermodynamic as well as in the chemical basis of this technique. A survey of practical aspects of the technical process of freeze substitution concerning the equipment and various protocols successfully applied in biological systems is also given. The main advantage of freeze substitution versus conventional chemical fixation is seen in the maintenance of the hydration shell of molecules and macromolecular structures. This results in an improved fine structural preservation, superior retention of the antigenicity of proteins and decreased loss of unbound, diffusible cellular components. Examples of excellent visualization of the ultrastructure of macromolecular complexes (nucleic acids, extracellular material, membranes etc.), small organisms (bacteria, algae, cyanobacteria and fungi) and large biological samples such as plant and animal tissue as well as the plant-pathogen (fungus) interface and infection structures are presented. Recent data on the molecular characterization of freeze-substituted biological tissue are exemplified with special emphasis on the subcellular detection of soluble components (elements, lipids, proteins and drugs) and the inter-/intracellular localization of proteins including foreign proteins in transgenic plants. The molecular analysis of freeze-substituted specimens is achieved by the combination of low temperature preparation techniques in biological electron microscopy with various detection methods such as X-ray microanalysis, immunocytochemistry and high resolution autoradiography.

Immunolocalization of B-50 (GAP-43) in the mouse olfactory bulb: predominant presence in preterminal axons

Because the growth-associated protein B-50 (GAP-43) has been implicated in neurite outgrowth as well as in synaptic plasticity, we studied its light and electron microscopical distribution in the mouse olfactory bulb, an area of the nervous system which exhibits a high degree of synaptic plasticity. Immunofluorescent staining with monospecific affinity-purified anti-B-50 antibodies revealed that B-50 is most abundantly expressed in the olfactory nerve fibre layer and the granule cell layer neuropil, while little staining was observed in the external plexiform layer and in cell bodies. B-50 is absent from dendrites and myelinated axons as indicated by double labelling with monoclonal antibodies against microtubule-associated protein 2 and the large neurofilament protein, respectively. Using post-embedding immunogold labelling on ultrathin Lowicryl sections, B-50 was found to be highly concentrated in presumed growth cones in the olfactory nerve fibre layer and in thin unmyelinated axons and presynaptic terminals in the granule cell layer neuropil. Near background immunolabelling was seen in perikarya, dendrites and myelinated axons. In view of the implication of B-50 in plasticity-related phenomena, its abundance in the thin unmyelinated preterminal axons suggests that these are potential sites of extrasynaptic plasticity.

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.

Ultrastructural localization of olfactory transduction components: the G protein subunit Golf alpha and type III adenylyl cyclase

Electron microscopy and postembedding immunocytochemistry on rapidly frozen, freeze-substituted specimens of rat olfactory epithelia were used to study the subcellular localization of the transduction proteins Golf alpha and type III adenylyl cyclase. Antibody binding sites for both of these proteins occur in the same receptor cell compartments, the distal segments of the olfactory cilia. These segments line the boundary between organism and external environment inside the olfactory part of the nasal cavity. Therefore, they are the receptor cell regions that most likely first encounter odorous compounds. The results presented here provide direct evidence to support the conclusion that the distal segments of the cilia contain the sites of the early events of olfactory transduction.