Anatomy of the olfactory mucosa

Early corticosteroid treatment enhances recovery from SARS-CoV-2 induced loss of smell in hamster

Among the numerous long COVID symptoms, olfactory dysfunction persists in ∼10 % of patients suffering from SARS-CoV-2 induced anosmia. Among the few potential therapies, corticoid treatment has been used for its anti-inflammatory effect with mixed success in patients. In this study, we explored its impact using hamster as an animal model. SARS-CoV-2 infected hamsters lose their smell abilities and this loss is correlated with damage of the olfactory epithelium and persistent presence of innate immunity cells. We started a dexamethasone treatment 2 days post infection, when olfaction was already impacted, until 11 days post infection when it started to recover. We observed an improvement of olfactory capacities in the animals treated with corticoid compared to those treated with vehicle. This recovery was not related to differences in the remaining damage to the olfactory epithelium, which was similar in both groups. This improvement was however correlated with a reduced inflammation in the olfactory epithelium with a local increase of the mature olfactory neuron population. Surprisingly, at 11 days post infection, we observed an increased and disorganized presence of immature olfactory neurons, especially in persistent inflammatory zones of the epithelium. This unusual population of immature olfactory neurons coincided with a strong increase of olfactory epithelium proliferation in both groups. Our results indicate that persistent inflammation of the olfactory epithelium following SARS-CoV-2 infection may alter the extent and speed of regeneration of the olfactory neuron population, and that corticoid treatment is effective to limit inflammation and improve olfaction recovery following SARS-CoV-2 infection.

Establishment of a consensus protocol to explore the brain pathobiome in patients with mild cognitive impairment and Alzheimer's disease: Research outline and call for collaboration

Microbial infections of the brain can lead to dementia, and for many decades microbial infections have been implicated in Alzheimer's disease (AD) pathology. However, a causal role for infection in AD remains contentious, and the lack of standardized detection methodologies has led to inconsistent detection/identification of microbes in AD brains. There is a need for a consensus methodology; the Alzheimer's Pathobiome Initiative aims to perform comparative molecular analyses of microbes in post mortem brains versus cerebrospinal fluid, blood, olfactory neuroepithelium, oral/nasopharyngeal tissue, bronchoalveolar, urinary, and gut/stool samples. Diverse extraction methodologies, polymerase chain reaction and sequencing techniques, and bioinformatic tools will be evaluated, in addition to direct microbial culture and metabolomic techniques. The goal is to provide a roadmap for detecting infectious agents in patients with mild cognitive impairment or AD. Positive findings would then prompt tailoring of antimicrobial treatments that might attenuate or remit mounting clinical deficits in a subset of patients.

The Pharmacokinetics of Drugs Delivered to the Upper Nasal Space

Pharmacokinetics (PK) includes how a drug is absorbed, distributed, metabolized and eliminated. The compartment providing this information is usually the plasma. This is as close to the tissue of interest that we can get, although biopsies may be obtained to give "tissue levels" of drugs. Ultimately, the goal of PK is to understand how long the drug is actually engaged with the target in the tissue of interest after a dose has been administered. Most drugs at some point in their development will have been administered intravenously (IV), which acts as the standard for 100% bioavailability. By comparing various routes of administration to IV, the percentage of drug delivered to the plasma, on a dose-normalized basis, can be calculated and is referred to as the "absolute bioavailability". As pharmacology has advanced and more drugs have become available, many older products have been reformulated to be given by routes other than those originally intended (often oral). As the drawbacks of oral (or IV) administration have become better appreciated, non-oral, non-IV formulations and methods of administration have become more popular. Nasal administration is one route that has historically been overlooked as an alternative to oral administration-particularly for products needing rapid and non-invasive access to the target tissue-mostly via the blood stream. But attention is now focused on nasal administration for direct access to the brain, as that has the potential to bypass the blood-brain-barrier (BBB), which not even IV administration can always achieve. Assessing PK for these drugs targeting the brain may require serial sampling of the cerebrospinal fluid (CSF), making PK assessments of CNS drugs more invasive and complex, but still possible in future product development. However, we are now seeing more drugs reformulated for nasal delivery to gain faster systemic levels than oral administration (especially in patients with known or suspected gastrointestinal dysmotility), while avoiding the use of needles. For example, in recent years several different formulations and delivery methods for an old drug, dihydroergotamine (DHE), have been developed and these show very different characteristics, suggesting that delivery to different parts of the nose may have very different PK profiles. This review summarizes the systemic PK of different nasal DHE options that have been, or are being, developed and suggests that delivery of drugs to the upper nasal space (UNS) may represent an optimal target. Further research is required to ascertain if this route could also be utilized for direct administration to the CNS (as an attractive alternative to intrathecal delivery) via the olfactory or trigeminal nerves-but already preclinical data (and some human data) suggest this is entirely possible.

Identity, lineage and fates of a temporally distinct progenitor population in the embryonic olfactory epithelium

We defined a temporally and transcriptionally divergent precursor cohort in the medial olfactory epithelium (OE) shortly after it differentiates as a distinct tissue at mid-gestation in the mouse. This temporally distinct population of Ascl1+ cells in the dorsomedial OE is segregated from Meis1+/Pax7+ progenitors in the lateral OE, and does not appear to be generated by Pax7+ lateral OE precursors. The medial Ascl1+ precursors do not yield a substantial number of early-generated ORNs. Instead, they first generate additional proliferative precursors as well as a distinct population of frontonasal mesenchymal cells associated with the migratory mass that surrounds the nascent olfactory nerve. Parallel to these in vivo distinctions, isolated medial versus lateral OE precursors in vitro retain distinct proliferative capacities and modes of division that reflect their in vivo identities. At later fetal stages, these early dorsomedial Ascl1+ precursors cells generate spatially restricted subsets of ORNs as well as other non-neuronal cell classes. Accordingly, the initial compliment of ORNs and other OE cell types is derived from at least two distinct early precursor populations: lateral Meis1/Pax7+ precursors that generate primarily early ORNs, and a temporally, spatially, and transcriptionally distinct subset of medial Ascl1+ precursors that initially generate additional OE progenitors and apparent migratory mass cells before yielding a subset of ORNs and likely supporting cell classes.

[Olfaction and respiratory viruses… A relationship revealed by Covid-19]

The sense of smell has been underestimated for a long time in humans. It has been brought to the fore by its sudden disappearance during the Covid-19 pandemic of which anosmia (complete loss of smell) is one of the major symptoms. However, respiratory viruses have long been associated with smell disorders, 25% of which are linked to a viral infection. Olfaction begins in the nose within the olfactory epithelium which has the particularity of containing neurons in direct contact with the environment. Several respiratory viruses are known for their replicative capacity within this epithelium. This is particularly the case for the flu virus (influenza) and bronchiolitis (respiratory syncytial virus) but their tropism for this tissue is much lower than SARS-CoV-2. The understanding of the SARS-CoV-2 pathophysiology in the nasal cavity makes it possible to reveal part of the links between viral infection and olfactory disorders.

The olfactory limbus of the red fox (Vulpes vulpes). New insights regarding a noncanonical olfactory bulb pathway

Introduction: The olfactory system in most mammals is divided into several subsystems based on the anatomical locations of the neuroreceptor cells involved and the receptor families that are expressed. In addition to the main olfactory system and the vomeronasal system, a range of olfactory subsystems converge onto the transition zone located between the main olfactory bulb (MOB) and the accessory olfactory bulb (AOB), which has been termed the olfactory limbus (OL). The OL contains specialized glomeruli that receive noncanonical sensory afferences and which interact with the MOB and AOB. Little is known regarding the olfactory subsystems of mammals other than laboratory rodents. Methods: We have focused on characterizing the OL in the red fox by performing general and specific histological stainings on serial sections, using both single and double immunohistochemical and lectin-histochemical labeling techniques. Results: As a result, we have been able to determine that the OL of the red fox (Vulpes vulpes) displays an uncommonly high degree of development and complexity. Discussion: This makes this species a novel mammalian model, the study of which could improve our understanding of the noncanonical pathways involved in the processing of chemosensory cues.

Mechanistic Understanding of the Olfactory Neuroepithelium Involvement Leading to Short-Term Anosmia in COVID-19 Using the Adverse Outcome Pathway Framework

Loss of the sense of smell (anosmia) has been included as a COVID-19 symptom by the World Health Organization. The majority of patients recover the sense of smell within a few weeks postinfection (short-term anosmia), while others report persistent anosmia. Several studies have investigated the mechanisms leading to anosmia in COVID-19; however, the evidence is scattered, and the mechanisms remain poorly understood. Based on a comprehensive review of the literature, we aim here to evaluate the current knowledge and uncertainties regarding the mechanisms leading to short-term anosmia following SARS-CoV-2 infection. We applied an adverse outcome pathway (AOP) framework, well established in toxicology, to propose a sequence of measurable key events (KEs) leading to short-term anosmia in COVID-19. Those KEs are (1) SARS-CoV-2 Spike proteins binding to ACE-2 expressed by the sustentacular (SUS) cells in the olfactory epithelium (OE); (2) viral entry into SUS cells; (3) viral replication in the SUS cells; (4) SUS cell death; (5) damage to the olfactory sensory neurons and the olfactory epithelium (OE). This AOP-aligned approach allows for the identification of gaps where more research should be conducted and where therapeutic intervention could act. Finally, this AOP gives a frame to explain several disease features and can be linked to specific factors that lead to interindividual differences in response to SARS-CoV-2 infection.

Comparative Neuroanatomical Study of the Main Olfactory Bulb in Domestic and Wild Canids: Dog, Wolf and Red Fox

Simple Summary

The study of the morphological, physiological and molecular changes associated with the domestication process has been one of the most interesting unresolved neuroanatomical issues. The olfactory system deserves special attention since both wild and domestic canids are macrosmatic mammals with very high olfactory capacities. Nevertheless, the question remains open as to whether domestication involuted the sense of smell in domestic dogs. Further, there is a lack of comparative morphological information on the olfactory bulb, the first structure integrating olfactory sensory information in the brain. To provide comparative information on the domestication process, we studied the olfactory bulb of dogs and their two most important wild ancestors: the wolf and the fox. The study was carried out by macroscopic dissection and histological and immunohistochemical techniques and has allowed us to verify, first of all, that the three species present olfactory bulbs corresponding to a macrosmatic animal, but that there are noticeable differences not only in size, which was already known, but also in the cellularity and intensity of the immunohistochemical pattern characteristic of each species. These variations point to a reduction of the olfactory system as a consequence of the selection pressure associated with the domestication of animals.

Abstract

The sense of smell plays a fundamental role in mammalian survival. There is a considerable amount of information available on the vomeronasal system of both domestic and wild canids. However, much less information is available on the canid main olfactory system, particularly at the level of the main olfactory bulb. Comparative study of the neuroanatomy of wild and domestic canids provides an excellent model for understanding the effects of selection pressure associated with domestication. A comprehensive histological (hematoxylin–eosin, Nissl, Tolivia and Gallego’s Trichrome stains), lectin (UEA, LEA) and immunohistochemical (Gαo, Gαi2, calretinin, calbindin, olfactory marker protein, glial fibrillary acidic protein, microtubule-associated protein 2) study of the olfactory bulbs of the dog, fox and wolf was performed. Our study found greater macroscopic development of the olfactory bulb in both the wolf and fox compared to the dog. At the microscopic level, all three species show a well-developed pattern of lamination and cellularity typical of a macrosmatic animal. However, greater development of cellularity in the periglomerular and mitral layers of wild canids is characteristic. Likewise, the immunohistochemical study shows comparable results between the three species, but with a noticeably higher expression of markers in wild canids. These results suggest that the reduction in encephalization experienced in dogs due to domestication also corresponds to a lower degree of morphological and neurochemical differentiation of the olfactory bulb.

Is There a Link between Vomeronasalitis and Aggression in Stable Social Groups of Female Pigs?

The vomeronasal organ (VNO) is a bilateral chemosensory structure strongly involved in animal behaviour, thanks to its sensory epithelium (VNSE) that detects pheromones. Experimental VNO lesions can impair social, reproductive and maternal behaviour, while feline spontaneous vomeronasalitis has been associated with aggression. This study aimed to describe vomeronasalitis in farm pigs and explore its association with intraspecific behavioural alterations. Using 38 six-month-old pigs, the skin lesion score based on Welfare Quality® protocols was obtained during the fattening period. The seventy-six VNOs from these pigs were stained in haematoxylin-eosin for histological examinations. VNSE inflammation was classified considering its intensity. Skin lesions data were compared to vomeronasalitis. There were 34% of pigs that showed unilateral VNSE inflammation, while 66% were bilaterally affected. The mean ± SD number of skin lesions/animal was 4.4 ± 2.82, and 34% of pigs scored 1 (moderately wounded animals) at least once during the fattening period. Statistical analysis showed an association between bilateral vomeronasalitis and skin lesion score (p < 0.05) and between bilateral moderate vomeronasalitis and skin lesions number (p < 0.01). This is the first report linking vomeronasalitis to social life in farm animals. Considering the role of social life in animal welfare, our data opens a research field linking pathology to animal behaviour.

Neuroanatomical and Immunohistological Study of the Main and Accessory Olfactory Bulbs of the Meerkat (Suricata suricatta)

Simple Summary

In wild mammals, chemical senses are crucial to survival, but sensory system information is lacking for many species, including the meerkat (Suricata suricatta), an iconic mammal with a marked social hierarchy that has been ambiguously classified in both canid and felid families. We studied the neuroanatomical basis of the meerkat olfactory and accessory olfactory bulbs, aiming to provide information on the relevance of both systems to the behaviors of this species and contributing to improving its taxonomic classification. The accessory olfactory bulb serves as the integration center of vomeronasal information. When examined microscopically, the accessory olfactory bulb of the meerkat presents a lamination pattern more defined than observed in dogs and approaching the pattern described in cats. The degree of lamination and development in the meerkat main olfactory bulb is comparable to the general pattern observed in mammals but with numerous specific features. Our study supports the functionality of the olfactory and vomeronasal integrative centers in meerkats and places this species within the suborder Feliformia. Our study also confirms the importance of chemical signals in mediating the social behaviors of this species and provides essential neuroanatomical information for understanding the functioning of their chemical senses.

Abstract

We approached the study of the main (MOB) and accessory olfactory bulbs (AOB) of the meerkat (Suricata suricatta) aiming to fill important gaps in knowledge regarding the neuroanatomical basis of olfactory and pheromonal signal processing in this iconic species. Microdissection techniques were used to extract the olfactory bulbs. The samples were subjected to hematoxylin-eosin and Nissl stains, histochemical (Ulex europaeus agglutinin, Lycopersicon esculentum agglutinin) and immunohistochemical labelling (Gαo, Gαi2, calretinin, calbindin, olfactory marker protein, glial fibrillary acidic protein, microtubule-associated protein 2, SMI-32, growth-associated protein 43). Microscopically, the meerkat AOB lamination pattern is more defined than the dog’s, approaching that described in cats, with well-defined glomeruli and a wide mitral-plexiform layer, with scattered main cells and granular cells organized in clusters. The degree of lamination and development of the meerkat MOB suggests a macrosmatic mammalian species. Calcium-binding proteins allow for the discrimination of atypical glomerular subpopulations in the olfactory limbus between the MOB and AOB. Our observations support AOB functionality in the meerkat, indicating chemosensory specialization for the detection of pheromones, as identified by the characterization of the V1R vomeronasal receptor family and the apparent deterioration of the V2R receptor family.

Can domestication shape Canidae brain morphology? The accessory olfactory bulb of the red fox as a case in point

BACKGROUND

The accessory olfactory bulb (AOB) is the first integrative center of the vomeronasal system (VNS), and the general macroscopic, microscopic, and neurochemical organizational patterns of the AOB differ fundamentally among species. Therefore, the low degree of differentiation observed for the dog AOB is surprising. As the artificial selection pressure exerted on domestic dogs has been suggested to play a key role in the involution of the dog VNS, a wild canid, such as the fox, represents a useful model for studying the hypothetical effects of domestication on the AOB morphology.

METHODS

A comprehensive histological, lectin-histochemical, and immunohistochemical study of the fox AOB was performed. Anti-Gαo and anti-Gαi2 antibodies were particularly useful, as they label the transduction cascade of the vomeronasal receptor types 1 (V1R) and 2 (V2R), respectively. Other employed antibodies included those against proteins such as microtubule-associated protein 2 (MAP-2), tubulin, glial fibrillary acidic protein, growth-associated protein 43 (GAP-43), olfactory marker protein (OMP), calbindin, and calretinin.

RESULTS

The cytoarchitecture of the fox AOB showed a clear lamination, with neatly differentiated layers; a highly developed glomerular layer, rich in periglomerular cells; and large inner cell and granular layers. The immunolabeling of Gαi2, OMP, and GAP-43 delineated the outer layers, whereas Gαo and MAP-2 immunolabeling defined the inner layers. MAP-2 characterized the somas of AOB principal cells and their dendritic trees. Anti-calbindin and anti-calretinin antibodies discriminated neural subpopulations in both the mitral-plexiform layer and the granular cell layer, and the lectin Ulex europeus agglutinin I (UEA-I) showed selectivity for the AOB and the vomeronasal nerves.

CONCLUSION

The fox AOB presents unique characteristics and a higher degree of morphological development compared with the dog AOB. The comparatively complex neural basis for semiochemical information processing in the fox compared with that observed in dogs suggests loss of AOB anatomical complexity during the evolutionary history of dogs and opens a new avenue of research for studying the effects of domestication on brain structures.

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.

Hormonal Contraception and Violent Death: The Physiological and Psychological Links

In the past decade, two large prospective cohort studies of British and American women have been conducted which found a statistically significant increase in the risk of violent death in ever-users of hormonal contraceptives. Research on the effects of hormonal contraceptives upon the behaviors of intimate partners and on the physiology of women using hormonal contraceptives has provided insight into the possible basis for the resulting increase in violent death. This review examines the changes that are potential contributors to the reported increase.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

SARS-CoV-2 infection in the mouse olfactory system

SARS-CoV-2 infection causes a wide spectrum of clinical manifestations in humans, and olfactory dysfunction is one of the most predictive and common symptoms in COVID-19 patients. However, the underlying mechanism by which SARS-CoV-2 infection leads to olfactory disorders remains elusive. Herein, we demonstrate that intranasal inoculation with SARS-CoV-2 induces robust viral replication in the olfactory epithelium (OE), not the olfactory bulb (OB), resulting in transient olfactory dysfunction in humanized ACE2 (hACE2) mice. The sustentacular cells and Bowman’s gland cells in the OE were identified as the major target cells of SARS-CoV-2 before invasion into olfactory sensory neurons (OSNs). Remarkably, SARS-CoV-2 infection triggers massive cell death and immune cell infiltration and directly impairs the uniformity of the OE structure. Combined transcriptomic and quantitative proteomic analyses revealed the induction of antiviral and inflammatory responses, as well as the downregulation of olfactory receptor (OR) genes in the OE from the infected animals. Overall, our mouse model recapitulates olfactory dysfunction in COVID-19 patients and provides critical clues for understanding the physiological basis for extrapulmonary manifestations of COVID-19.

Analysis of the vomeronasal organ transcriptome reveals variable gene expression depending on age and function in rabbits

The vomeronasal organ (VNO) is a chemosensory organ specialized in pheromone detection that shows a broad morphofunctional and genomic diversity among mammals. However, its expression patterns have only been well-characterized in mice. Here, we provide the first comprehensive RNA sequencing study of the rabbit VNO across gender and sexual maturation stages. We characterized the VNO transcriptome, updating the number and expression of the two main vomeronasal receptor families, including 128 V1Rs and 67 V2Rs. Further, we defined the expression of formyl-peptide receptor and transient receptor potential channel families, both known to have specific roles in the VNO. Several sex hormone-related pathways were consistently enriched in the VNO, highlighting the relevance of this organ in reproduction. Moreover, whereas juvenile and adult VNOs showed significant transcriptome differences, male and female did not. Overall, these results contribute to understand the genomic basis of behavioural responses mediated by the VNO in a non-rodent model.

Olfaction across the water-air interface in anuran amphibians

Extant anuran amphibians originate from an evolutionary intersection eventually leading to fully terrestrial tetrapods. In many ways, they have to deal with exposure to both terrestrial and aquatic environments: (i) phylogenetically, as derivatives of the first tetrapod group that conquered the terrestrial environment in evolution; (ii) ontogenetically, with a development that includes aquatic and terrestrial stages connected via metamorphic remodeling; and (iii) individually, with common changes in habitat during the life cycle. Our knowledge about the structural organization and function of the amphibian olfactory system and its relevance still lags behind findings on mammals. It is a formidable challenge to reveal underlying general principles of circuity-related, cellular, and molecular properties that are beneficial for an optimized sense of smell in water and air. Recent findings in structural organization coupled with behavioral observations could help to understand the importance of the sense of smell in this evolutionarily important animal group. We describe the structure of the peripheral olfactory organ, the olfactory bulb, and higher olfactory centers on a tissue, cellular, and molecular levels. Differences and similarities between the olfactory systems of anurans and other vertebrates are reviewed. Special emphasis lies on adaptations that are connected to the distinct demands of olfaction in water and air environment. These particular adaptations are discussed in light of evolutionary trends, ontogenetic development, and ecological demands.

Expression, Distribution and Role of Aquaporins in Various Rhinologic Conditions

Aquaporins (AQPs) are water-specific membrane channel proteins that regulate cellular and organismal water homeostasis. The nose, an organ with important respiratory and olfactory functions, is the first organ exposed to external stimuli. Nose-related topics such as allergic rhinitis (AR) and chronic rhinosinusitis (CRS) have been the subject of extensive research. These studies have reported that mechanisms that drive the development of multiple inflammatory diseases that occur in the nose and contribute to the process of olfactory recognition of compounds entering the nasal cavity involve the action of water channels such as AQPs. In this review, we provide a comprehensive overview of the relationship between AQPs and rhinologic conditions, focusing on the current state of knowledge and mechanisms that link AQPs and rhinologic conditions. Key conclusions include the following: (1) Various AQPs are expressed in both nasal mucosa and olfactory mucosa; (2) the expression of AQPs in these tissues is different in inflammatory diseases such as AR or CRS, as compared with that in normal tissues; (3) the expression of AQPs in CRS differs depending on the presence or absence of nasal polyps; and (4) the expression of AQPs in tissues associated with olfaction is different from that in the respiratory epithelium.

Sustentacular Cell Enwrapment of Olfactory Receptor Neuronal Dendrites: An Update

The pseudostratified olfactory epithelium (OE) may histologically appear relatively simple, but the cytological relations among its cell types, especially those between olfactory receptor neurons (ORNs) and olfactory sustentacular cells (OSCs), prove more complex and variable than previously believed. Adding to the complexity is the short lifespan, persistent neurogenesis, and continuous rewiring of the ORNs. Contrary to the common belief that ORN dendrites are mostly positioned between OSCs, recent findings indicate a sustentacular cell enwrapped configuration for a majority of mature ORN dendrites at the superficial layer of the OE. After vertically sprouting out from the borderlines between OSCs, most of the immature ORN dendrites undergo a process of sideways migration and terminal maturation to become completely invaginated into and enwrapped by OSCs. Trailing the course of the dendritic sideways migration is the mesodendrite (mesentery of the enwrapped dendrite) made of closely apposed, cell junction connected plasma membrane layers of neighboring folds of the host sustentacular cell. Only a minority of the mature ORN dendrites at the OE apical surface are found at the borderlines between OSCs (unwrapped). Below I give a brief update on the cytoarchitectonic relations between the ORNs and OSCs of the OE. Emphasis is placed on the enwrapment of ORN dendrites by OSCs, on the sideways migration of immature ORN dendrites after emerging from the OE surface, and on the terminal maturation of the ORNs. Functional implications of ORN dendrite enwrapment and a comparison with myelination or Remak’s bundling of axons or axodendrites in the central and peripheral nervous system are also discussed.

Olfactory cell cultures to investigate health effects of air pollution exposure: Implications for neurodegeneration

Air pollution is a major, global public health concern. A growing body of evidence shows that exposure to air pollutants may impair the brain. Living in highly polluted areas has been linked to several neurodegenerative diseases, where exposure to complex mixtures of air pollutants in urban environments may have harmful effects on brain function. These harmful effects are thought to originate from elevated inflammation and oxidative stress. The olfactory epithelium is a key entry site of air pollutants into the brain as the particles are deposited in the upper airways and the nasal region. A potential source of patient-derived cells for study of air pollutant effects is the olfactory mucosa, which constitutes a central part of the olfactory epithelium. This review first summarizes the current literature on the available in vitro models of the olfactory epithelium. It then describes how alterations of the olfactory mucosa are linked to neurodegeneration and discusses potential therapeutic applications of these cells for neurodegenerative diseases. Finally, it reviews the research performed on the effects of air pollutant exposure in cells of the olfactory epithelium. Patient-derived olfactory epithelial models hold great promise for not only elucidating the molecular and cellular pathophysiology of neurodegenerative disorders, but for providing key understanding about air pollutant particle entry and effects at this key brain entry site.