Three-dimensional scanning electron microscopic study of the normal hamster olfactory epithelium

Anatomical, histochemical and surface ultrastructural characteristics of cavum nasi of the common quails (Coturnix coturnix. Erlangeri)

Ten normal, mature and common quails were used to study in detail the gross anatomy, histochemical and surface ultrastructural characteristics of the nasal cavity. The relationship between the structure and function of the nasal cavity also were assessed. The quail nasal cavity was divided into the vestibule, nasal cavity proper and fundus. The nasal cavity began rostrally by two slit-like external nares located laterally in the middle third of the upper beak. A previous authors stated that no rostral concha but the current study record that the rostral nasal concha was located opposite the nostrils and exhibited a C-shaped appearance in transverse section and was 5 mm long and 3 mm wide. The middle nasal concha was narrow and elongated. The caudal nasal concha was spherical, located caudodorsal to the rostral nasal concha and measured 2 mm in diameter. The infraorbital sinus was a roughly triangular cavity situated immediately rostral to the orbit. The histological and surface ultrastructural study of the nasal cavity of common quail did not studied previously. Histologically, the cavum nasi was composed of three regions: vestibule, respiratory and olfactory. The vestibule was lined with stratified squamous epithelium that was keratinized rostrally and non-keratinized caudally. The respiratory region was covered by pseudostratified columnar epithelium. Intra-epithelial mucous glands were present in the respiratory region and displayed a strong reaction with Alcian blue. The lining epithelium in the olfactory region was pseudostratified and contained olfactory, supporting and basal cells.

Visualizing in deceased COVID-19 patients how SARS-CoV-2 attacks the respiratory and olfactory mucosae but spares the olfactory bulb

Anosmia, the loss of smell, is a common and often the sole symptom of COVID-19. The onset of the sequence of pathobiological events leading to olfactory dysfunction remains obscure. Here, we have developed a postmortem bedside surgical procedure to harvest endoscopically samples of respiratory and olfactory mucosae and whole olfactory bulbs. Our cohort of 85 cases included COVID-19 patients who died a few days after infection with SARS-CoV-2, enabling us to catch the virus while it was still replicating. We found that sustentacular cells are the major target cell type in the olfactory mucosa. We failed to find evidence for infection of olfactory sensory neurons, and the parenchyma of the olfactory bulb is spared as well. Thus, SARS-CoV-2 does not appear to be a neurotropic virus. We postulate that transient insufficient support from sustentacular cells triggers transient olfactory dysfunction in COVID-19. Olfactory sensory neurons would become affected without getting infected.

Modification of the Peripheral Olfactory System by Electronic Cigarettes

Electronic cigarettes (e-cigs) are used by millions of adolescents and adults worldwide. Commercial e-liquids typically contain flavorants, propylene glycol, and vegetable glycerin with or without nicotine. These chemical constituents are detected and evaluated by chemosensory systems to guide and modulate vaping behavior and product choices of e-cig users. The flavorants in e-liquids are marketing tools. They evoke sensory percepts of appealing flavors through activation of chemical sensory systems to promote the initiation and sustained use of e-cigs. The vast majority of flavorants in e-liquids are volatile odorants, and as such, the olfactory system plays a dominant role in perceiving these molecules that enter the nasal cavity either orthonasally or retronasally during vaping. In addition to flavorants, e-cig aerosol contains a variety of by-products generated through heating the e-liquids, including odorous irritants, toxicants, and heavy metals. These harmful substances can directly and adversely impact the main olfactory epithelium (MOE). In this article, we first discuss the olfactory contribution to e-cig flavor perception. We then provide information on MOE cell types and their major functions in olfaction and epithelial maintenance. Olfactory detection of flavorants, nicotine, and odorous irritants and toxicants are also discussed. Finally, we discuss the cumulated data on modification of the MOE by flavorant exposure and toxicological impacts of formaldehyde, acrolein, and heavy metals. Together, the information presented in this overview may provide insight into how e-cig exposure may modify the olfactory system and adversely impact human health through the alteration of the chemosensory factor driving e-cig use behavior and product selections. © 2021 American Physiological Society. Compr Physiol 11:2621-2644, 2021.

Phylogenic outline of the olfactory system in vertebrates

Phylogenic outline of the vertebrate olfactory system is summarized in the present review. In the fish and the birds, the olfactory system consists only of the olfactory epithelium (OE) and the olfactory bulb (B). In the amphibians, reptiles and mammals, the olfactory system is subdivided into the main olfactory and the vomeronasal olfactory systems, and the former consists of the OE and the main olfactory bulb (MOB), while the latter the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB). The subdivision of the olfactory system into the main and the vomeronasal olfactory systems may partly be induced by the difference between paraphyletic groups and monophyletic groups in the phylogeny of vertebrates.

A large-scale model of the locust antennal lobe

The antennal lobe (AL) is the primary structure within the locust's brain that receives information from olfactory receptor neurons (ORNs) within the antennae. Different odors activate distinct subsets of ORNs, implying that neuronal signals at the level of the antennae encode odors combinatorially. Within the AL, however, different odors produce signals with long-lasting dynamic transients carried by overlapping neural ensembles, suggesting a more complex coding scheme. In this work we use a large-scale point neuron model of the locust AL to investigate this shift in stimulus encoding and potential consequences for odor discrimination. Consistent with experiment, our model produces stimulus-sensitive, dynamically evolving populations of active AL neurons. Our model relies critically on the persistence time-scale associated with ORN input to the AL, sparse connectivity among projection neurons, and a synaptic slow inhibitory mechanism. Collectively, these architectural features can generate network odor representations of considerably higher dimension than would be generated by a direct feed-forward representation of stimulus space.

Regeneration of olfactory receptor cells

The vertebrate olfactory system has become an important model for the study of neural regeneration. The most remarkable feature of this system is its unique capacity for neurogenesis and replacement of degenerating receptor neurons. This replacement is made possible by a persistent neurogenesis among basal cells. Basal cells differentiate, develop into sensory neurons and grow axon processes. Receptor cell axons project back to the olfactory bulb where they reestablish connections with the central nervous system. When mature receptors reach a critical age, are damaged by nerve injury, or are exposed to environmental agents that enter the nasal cavity, they degenerate and are subsequently replaced by newly regenerated receptor cells. Recent experiments demonstrate that olfactory neurogenesis is not simply an extension of growth and development but is a unique capacity for cell replacement that persists beyond maturity and well into old age. Even more remarkable is the finding that replacement receptor cells re-establish connections with the CNS and restore sensory function. It is expected that further studies of olfactory neurogenesis using cell and tissue culture methods will provide important advances for the field of neural regeneration.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Cytoarchitecture of the normal rat olfactory epithelium: Light and scanning electron microscopic studies

The three-dimensional cytoarchitecture of the normal rat olfactory epithelium was examined by scanning electron microscopy (SEM) of KOH digested tissues as well as by light and transmission electron microscopy of plastic sections. Observations specimens from the lateral side of the olfactory epithelium allowed identification of four cell types by their surface structure: olfactory neurons, supporting cells, basal cells, and duct cells of the Bowman's gland. The olfactory neurons were characterized by the presence of a thick apical process (i.e., dendrite) and a thin basal process (i.e., axon). These olfactory neurons tended to be aligned along the vertical axis of the epithelium. Immature olfactory neurons were present at the basal part of the epithelium and had a pear-shaped cell body with a thin and long axon and a short dendrite which failed to reach the epithelial surface. Supporting cells were roughly columnar in shape and occupied the full length of the epithelium. They became thinner in the basal two thirds of their length but had branched foot processes spreading on the basal surface of the epithelium. Basal cells located in the basal epithelial region were oval, round or cuboidal and present among the foot processes of the supporting cells. The ducts of the Bowman's gland entered the epithelium from the lamina propria and took straight, perpendicular courses within the epithelium. These intraepithelial ducts were composed of several slender cells. The acinar cells are sometimes present in the epithelium and appeared as a globular bulge of the duct at the basal part of the epithelium. SEM observation of the basal surface of the olfactory epithelium also clearly showed that axon bundles were surrounded by the sheet-like processes of Schwann cells, the investment being found at the base of the epithelium just before axon bundles leave the epithelium.

Olfactory processing in a changing brain

The perception of odorant molecules provides the essential information that allows animals to explore their surrounding. We describe here how the external world of scents may sculpt the activity of the first central relay of the olfactory system, i.e., the olfactory bulb. This structure is one of the few brain areas to continuously replace one of its neuronal populations: the local GABAergic interneurons. How the newly generated neurons integrate into a pre-existing neural network and how basic olfactory functions are maintained when a large percentage of neurons are subjected to continuous renewal, are important questions that have recently received new insights. Furthermore, we shall see how the adult neurogenesis is specifically subjected to experience-dependent modulation. In particular, we shall describe the sensitivity of the bulbar neurogenesis to the activity level of sensory inputs from the olfactory epithelium and, in turn, how this neurogenesis may adjust the neural network functioning to optimize odor information processing. Finally, we shall discuss the behavioral consequences of the bulbar neurogenesis and how it may be appropriate for the sense of smell. By maintaining a constitutive turnover of bulbar interneurons subjected to modulation by environmental cues, we propose that adult ongoing neurogenesis in the olfactory bulb is associated with improved olfactory memory. These recent findings not only provide new fuel for the molecular and cellular bases of sensory perception but should also shed light onto cellular bases of learning and memory.

Making scents of olfactory neurogenesis

Olfaction was long considered to belong more to the realm of art than to that of science. As a result, how the brain perceives, discriminates, and recognizes odorant molecules is still a mystery. Recent progress has nonetheless been made at early stages of the olfactory pathway when olfactory studies entered into the molecular era to elucidate the first contact of an odor molecule with a receptor. Our group focuses on the analysis of odor information in the olfactory bulb, the first processing relay in the mammalian brain. Using this model, we are attempting to decipher the code for odorant information. Furthermore, the olfactory bulb also provides an attractive model to investigate neuronal proliferation, differentiation, migration, and neuronal death, processes involving an interplay between genetic and epigenetic influences. Finally, our goal is to explore the possible consequences of the olfactory bulb plasticity, in olfactory performance. For these purposes, we aim to combine morphological, electrophysiological and behavioral approaches to investigate: (1) how the olfactory bulb processes odor molecule information, (2) how neural precursors differentiate into olfactory bulb interneurons, (3) how these newly-generated neurons integrate into an operational neural network, (4) what role they play in the adult olfactory bulb, and (5) how are basic olfactory functions maintained in such a sensory system subjected to continuous renewal of a large percentage of its neuronal population. These questions should provide new fuel for the molecular and cellular bases of sensory perception and shed light onto cellular bases of learning and memory.

An olfactory sensory neuron line, odora, properly targets olfactory proteins and responds to odorants

The site for interactions between the nervous system and much of the chemical world is in the olfactory sensory neuron (OSN). Odorant receptor proteins (ORPs) are postulated to mediate these interactions. However, the function of most ORPs has not been demonstrated in vivo or in vitro. For this and other reasons, we created a conditionally immortalized cell line derived from the OSN lineage, which we term odora. Odora cells, under control conditions, are phenotypically similar to the OSN progenitor, the globose basal cell. After differentiation, odora cells more closely resemble OSNs. Differentiated odora cells express neuronal and olfactory markers, including components of the olfactory signal transduction pathway. Unlike other cell lines, they also efficiently target exogenous ORPs to their surface. Strikingly, differentiated odora cells expressing ORPs respond to odorants, as measured by an influx of calcium. In particular, cells expressing one ORP demonstrate a specific response to only one type of tested odorant. Odora cells, therefore, are ideal models to examine the genesis and function of olfactory sensory neurons.

Globose basal cells are neuronal progenitors in the olfactory epithelium: a lineage analysis using a replication-incompetent retrovirus

We have used a replication-incompetent retrovirus to analyze the lineage of olfactory receptor neurons in young rats. At 5-40 days after infection, clusters of infected cells comprised two major types: one consisted of 1-2 horizontal basal cells, and a second consisted of variable numbers of globose basal cells and immature and mature sensory neurons. Olfactory nerve lesion (which enhances neuronal turnover) increased the frequency of the globose-sensory neuron clusters as well as the number of cells in such clusters. No clusters contained both horizontal and globose basal cells, and none contained sustentacular cells. These data suggest, at least in young rats, that horizontal basal cells are not precursors of olfactory neurons, that there is a lineage path from globose cells to mature neurons, and that sustentacular cells may arise from a separate lineage.

Protein kinase C sensitizes olfactory adenylate cyclase

Effects of neurotransmitters on cAMP-mediated signal transduction in frog olfactory receptor cells (ORCs) were studied using in situ spike recordings and radioimmunoassays. Carbachol, applied to the mucosal side of olfactory epithelium, amplified the electrical response of ORCs to cAMP-generating odorants, but did not affect unstimulated cells. A similar augmentation of odorant response was observed in the presence of phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC). The electrical response to forskolin, an activator of adenylate cyclase (AC), was also enhanced by PDBu, and it was attenuated by the PKC inhibitor Goe 6983. Forskolin-induced accumulation of cAMP in olfactory tissue was potentiated by carbachol, serotonin, and PDBu to a similar extent. Potentiation was completely suppressed by the PKC inhibitors Goe 6983, staurosporine, and polymyxin B, suggesting that the sensitivity of olfactory AC to stimulation by odorants and forskolin was increased by PKC. Experiments with deciliated olfactory tissue indicated that sensitization of AC was restricted to sensory cilia of ORCs. To study the effects of cell Ca2+ on these mechanisms, the intracellular Ca2+ concentration of olfactory tissue was either increased by ionomycin or decreased by BAPTA/AM. Increasing cell Ca2+ had two effects on cAMP production: (a) the basal cAMP production was enhanced by a mechanism sensitive to inhibitors of calmodulin; and (b) similar to phorbol ester, cell Ca2+ caused sensitization of AC to stimulation by forskolin, an effect sensitive to Goe 6983. Decreasing cell Ca2+ below basal levels rendered AC unresponsive to stimulation by forskolin. These data suggest that a crosstalk mechanism is functional in frog ORCs, linking the sensitivity of AC to the activity of PKC. At increased activity of PKC, olfactory AC becomes more responsive to stimulation by odorants, forskolin, and cell Ca2+. Neurotransmitters appear to use this crosstalk mechanism to regulate olfactory sensitivity.

Fine structural aspects of secretion and extrinsic innervation in the olfactory mucosa

The mucus at the surface of the olfactory mucosa constitutes the milieu in which perireceptor events associated with olfactory transduction occur. In this review, the ultrastructure of olfactory mucus and of the secretory cells that synthesize and secrete olfactory mucus in the vertebrate olfactory mucosa is described. Bowman's glands are present in the olfactory mucosa of all vertebrates except fish. They consist of acini, which may contain mucous or serous cells or both, and ducts that traverse the olfactory epithelium to deliver secretions to the epithelial surface. Sustentacular cells are present in the olfactory epithelium of all vertebrates. In fish, amphibia, reptiles, and birds, they are secretory; in mammals, they generally are considered to be "non-secretory," although they may participate in the regulation of the mucous composition through micropinocytotic secretion and uptake. Goblet cells occur in the olfactory epithelium of fish and secrete a mucous product. Secretion from Bowman's glands and vasomotor activity in the olfactory mucosa are regulated by neural elements extrinsic to the primary olfactory neurons. Nerve fibers described in early anatomical studies and characterized by immunohistochemical studies contain a variety of neuroactive peptides and have several targets within the olfactory mucosa. Ultrastructural studies of nerve terminals in the olfactory mucosa have demonstrated the presence of adrenergic, cholinergic and peptidergic input to glands, blood vessels, and melanocytes in the lamina propria and of peptidergic terminals in the olfactory epithelium. The neural origins of the extrinsic nerve fibers and terminals are the trigeminal, terminal, and autonomic systems.

The human primary olfactory pathway: fine structural and cytochemical aspects during development and in adults

Despite increasing knowledge about the biophysiology of the human olfactory system, understanding of the development of this pathway in humans lags considerably behind that of other vertebrates. Developmental studies have largely concentrated on the generation of cell types in the olfactory epithelium during the first trimester, while detailed ultrastructural observations usually describe the adult morphology. In this review, we have shown that contrary to what has been generally assumed, the surface of the human olfactory epithelium is heterogeneous and that its olfactory nerves differ ultrastructurally from those of other vertebrates studied. The development of the human primary olfactory pathway is discussed in terms of the appearance of olfactory bulb laminae, synaptogenesis and the expression of specific cell markers, such as the S-100 protein and olfactory marker protein (OMP). Positive immunohistochemical staining for N-cadherin in human fetuses suggests that growth of olfactory axons to their target may be mediated by cell adhesion molecules. The overall data presented here indicate that this pathway develops more precociously in humans than in rodents. Whether this translates also to earlier functional maturity remains to be elucidated.

Olfactory epithelium in young adult and aging rats as seen with high-resolution scanning electron microscopy

The present study uses mainly scanning electron microscopy to demonstrate the three-dimensional internal cell structures of rat olfactory epithelial cells. The aldehyde-prefixed osmium-DMSO-osmium (AODO) method devised by Tanaka and Mitsushima (1984) was applied to the present study to disclose intracellular structures such as endoplasmic reticulum, mitochondria, Golgi apparatus, and lysosomes. The spatial distribution pattern of these structures in olfactory and supporting cells is discussed, paying special attention to the formation of lipofuscin-like granules present in aged rats.

Morphology of olfactory epithelium in humans and other vertebrates

Human olfactory epithelium is similar in organization and cell morphology to that of most vertebrate species. The epithelium has a pseudostratified columnar organization and consists of olfactory neurons, supporting and basal cells. Near the mucosal surface there are also microvillar cells. These cells have neuron-like features and may be chemoreceptors. Human olfactory epithelium is not a uniform sensory sheet. Patches of non-sensory tissue often appear in what was thought to be a purely olfactory region. The significance of these patches has not been determined, but they could reflect exposure to environment agents or changes that occur during the normal aging process. In order to better understand the human olfactory system, further knowledge of the normal structure is necessary. This review addresses the morphology of the human olfactory epithelium and the remarkable plasticity of the vertebrate olfactory system.

Intracellular injection of vital dyes into single cells in the salamander olfactory epithelium

Intracellular vital dye injection was used to examine the morphology of single sustentacular and receptor cells and the developmental fate of individual basal cells in the olfactory epithelium of the tiger salamander. In acute experiments, Lucifer yellow injections were used to identify single basal, receptor or sustentacular cells on the basis of their overall morphology. Dye-coupling between a number of the different epithelial cells was observed. Progeny of basal cells were examined by following labeled cells for up to 2 weeks using intracellular injection of rhodamine-labeled dextran. These experiments indicate that some olfactory epithelial cells are dye-coupled and that dye-filled basal cells can undergo division and migration.

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.

Ultrastructure of the human vomeronasal organ

Virtually all vertebrates have a vomeronasal system whose involvement in pheromone detection plays a crucial role in reproduction. In humans, the vomeronasal organ has been assumed to be vestigial or absent and without functional significance. In the present study involving over 400 subjects, vomeronasal pits were observed in all individuals except those with pathological conditions affecting the septum. Electron microscopy of the adult human vomeronasal organ indicates the presence of two potential receptor elements in the pseudostratified epithelial lining: microvillar cells, and unmyelinated, intraepithelial axons. In addition, unmyelinated axons are common in the lamina propria surrounding the organ. They appear to constitute the components essential for a functional chemosensory system, and may thus provide the basis for a pheromone detection system as in other animals.

An immunocytochemical study of the development of the olfactory system in the three-spined stickleback (Gasterosteus aculeatus L., Teleostei)

Antisera against a variety of substances have been found to produce an identical immunoreaction in the developing olfactory system of a teleost, the three-spined stickleback (Gasterosteus aculeatus). The label is localized in the olfactory placode, the olfactory nerve and those parts of the secondary olfactory tracts which constitute the dorsal descending fascicles and the ventral descending fibers of the medial olfactory tract. The label was first detected 3 days after fertilization (3D) in the olfactory placode where labeled supporting cells were observed. At 4D, the label was observed at the site of the developing olfactory bulbs. At 7D, the olfactory placode lost the direct contact with the brain and the labeled olfactory nerve became visible. At the same time, the medial olfactory tract emerged from the bulbs, and contacts with cells in the nucleus of the terminal nerve were observed. The development of the medial olfactory tract proceeded caudally, and by the end of 10D, the olfactory tract reached the periventricular hypothalamus. Pre-absorption of the antisera with the respective antigens did not abolish the capacity of the antisera to produce the label. The immunoreaction is thus not specific for the antigens against which the antisera have been raised. Yet the label produced by the immunoreaction is an extremely reliable marker for the primary olfactory tract, and the only existing marker by which secondary olfactory tracts can be visualized.

Identification of sugar residues in secretory glycoconjugates of olfactory mucosae using lectin histochemistry

Lectin histochemistry at the light microscope level was used to determine the distribution of sugar residues in secretory cells of the olfactory mucosae of salamander, hamster, and mouse. Differences in sugar composition and distribution of glycoconjugates found in sustentacular cells and acinar cells of Bowman's glands of these three animals were characterized. Oligosaccharides in secretory products of sustentacular cells in salamander olfactory mucosa contained sialic acid, galactose (Gal), N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc), fucose, and mannose residues. Glycoconjugates of these cells lacked terminal galactosyl-beta-(1,3)N-acetylgalactose (Gal beta 1,3GalNAc) residues. The sequences Gal beta 1,3GalNAc, N-acetyllactosamine (Gal beta 1,4GlcNAc), and GalNAc were penultimate to sialic acid residues. Sustentacular cells of mouse and hamster did not appear to contain O-linked oligosaccharides but stained for mannose-containing N-linked oligosaccharides. Glycoconjugates of acinar and duct cells of Bowman's glands in the salamander, hamster, and mouse contained variable amounts of beta(1,4)GlcNAc residues, and terminal N-acetyllactosamine, Gal beta 1,3GalNAc, and GalNAc residues. In the salamander, glycoconjugates of acinar cells possessed terminal GlcNAc residues but were not sialylated, while those of hamster and mouse generally stained for sialic acid but did not possess terminal GlcNAc residues. Secretory products of a subpopulation of rodent acinar cells also contained penultimate Gal beta 1,3GalNAc residues. Staining for sialic acid, Gal, GalNAc, and GlcNAc in glycoconjugates of rodents was often limited to a sub-population of Bowman's glands. This was especially noticeable in the mouse.

Morphology of the human olfactory epithelium

The human olfactory epithelium has been previously studied with scanning electron microscopy; however, most studies have been limited to examining the epithelial surface. In an attempt to examine structures below the surface, we scanned epithelial fractures that occurred during tissue preparation. This made it possible to obtain unique three-dimensional images of cell profiles from the mucosal surface through the full depth of the epithelium. We examined supporting cells, olfactory neurons, basal cells, and a fourth cell type, the microvillar cell. Supporting cells had a microvillar surface and were in close contact with olfactory neurons and their processes. Olfactory neurons were primarily located in the middle and lower epithelial regions. Basal cells occurred alone or in clusters adjacent to the basal lamina. Microvillar cells were always observed in the upper epithelial region. They were flask- or pear-shaped, had a tuft of microvilli that extended into the nasal cavity, and a thin axon-like process that passed basally towards the lamina propria. This study represents the first comprehensive scanning electron microscopy examination of the human olfactory epithelium. Three-dimensional images obtained for each epithelial cell type allowed us to examine cell processes and their close contacts, especially between supporting cells and olfactory neurons. These results also revealed the irregular and patchy distribution of olfactory receptors within the human nasal cavity. Further studies that examine the detailed morphology of the human olfactory epithelium should provide a better understanding of the physiological mechanism and clinical disorders that affect olfactory function in humans.

Scanning electron microscopic study of degeneration and regeneration in the olfactory epithelium after axotomy

The olfactory epithelium of the adult hamster (Mesocricetus auratus) was examined with the scanning electron microscope following olfactory nerve axotomy. Axotomy results in retrograde degeneration of mature olfactory neurons. Maximum degeneration was observed around day 4. During the degeneration period the epithelium consists primarily of supporting and basal cells. Microvillar columnar supporting cells were observed to have fine cellular processes extending from their lateral border to neighbouring cells. Supporting cells extended to the basal lamina where they terminated in foot-like processes of variable shapes (club, splay and hook). Basal cells which gave rise to new replacement olfactory neurons were observed near the basal lamina. They had a rough cellular surface covered with small granules and fine cellular extensions. Bowman's gland duct cells extended unbranched through the epithelium where they formed funnel duct openings covered with microvilli. During early recovery periods (5-30 days) the number of olfactory neurons in the lower epithelium region increased. We observed olfactory neurons with developing axon and dendritic processes. Specialized growth cone structures were seen at the tips. Olfactory neuron growth cones were elongated or club-shaped and had a ruffled membrane surface. Several thin filopodia extended from the growth cone and made contact with adjacent cells. At late recovery periods (35-120 days) there was a marked increase in the number of olfactory neurons within the middle and lower epithelium regions. Numerous dendritic processes extended to the epithelial surface and terminated in knob-like ciliated structures. Olfactory axons passed basally, forming small intra-epithelial bundles that penetrated the basal lamina then fasciculated into larger bundles within the lamina propria. This study provides detailed three-dimensional observations of the olfactory epithelium following neuron injury, and describes neural degenerative changes, replacement of olfactory neurons, development and maturation. In addition, we describe the structure and basal attachment of supporting cells and their glial-like relation with olfactory neurons.