The vomeronasal organ and incisive duct of harbor seals are modified to secrete acidic mucus into the nasal cavity

Most terrestrial mammals have a vomeronasal system to detect specific chemicals. The peripheral organ of this system is a vomeronasal organ (VNO) opening to the incisive duct, and its primary integrative center is an accessory olfactory bulb (AOB). The VNO in seals is thought to be degenerated like whales and manatees, unlike otariids, because of the absence of the AOB. However, olfaction plays pivotal roles in seals, and thus we conducted a detailed morphological evaluation of the vomeronasal system of three harbor seals (Phoca vitulina). The VNO lumen was not found, and the incisive duct did not open into the oral cavity but was recognized as a fossa on the anteroventral side of the nasal cavity. This fossa is rich in mucous glands that secrete acidic mucopolysaccharides, which might originate from the vomeronasal glands. The olfactory bulb consisted only of a main olfactory bulb that received projections from the olfactory mucosa, but an AOB region was not evident. These findings clarified that harbor seals do not have a VNO to detect some chemicals, but the corresponding region is a specialized secretory organ.

Macro- and micro morphology of the olfactory organ of African bonytongue, Heterotis niloticus (Cuvier 1829), compared with other species of the family Osteoglossidae (Teleostei)

Osteoglossiformes (bonytongue fishes) possess many morphological specializations associated with functions such as airbreathing, feeding, and electroreception. The olfactory organ also varies among species, notably in the family Osteoglossidae. Herein, we describe the olfactory organ of an osteoglossid, Heterotis niloticus, to compare it with the olfactory organs of other osteoglossiforms. We demonstrate the presence of an olfactory rosette within the olfactory chamber. This structure consists of a short median raphe surrounded by olfactory lamellae, which possess dorsal lamellar processes. On the surface of the olfactory lamellae, there are secondary lamellae formed by the olfactory epithelium. Within the olfactory epithelium, two zones can be distinguished: parallel brands of sensory cells located in the cavities between the secondary lamellae and a nonsensory area covering the remaining part of the olfactory lamellae. The olfactory epithelium is formed by ciliated and microvillus olfactory sensory neurons, supporting cells, goblet cells, basal cells and ciliated nonsensory cells. Additionally, rodlet cells were observed. The results confirm large variability in terms of the olfactory organ of Osteoglossiformes, particularly of Osteoglossidae, and support the secondary lamellae evolution hypothesis within this family.

[Olfactory dysfunction in COVID-19, a review of the evidence and implications for pandemic management]

There is debate as to whether olfactory dysfunction should be considered a symptom of COVID-19 infection. We undertook a systematic literature review of the articles indexed in PubMed on olfactory disorders in viral respiratory tract conditions, with special emphasis on COVID-19. The main objective was to find evidence of clinical interest to support the relationship between anosmia and COVID-19. Olfactory disorders in upper respiratory tract infections are frequent, most caused by obstruction due to oedema of the nasal mucosa. Occasionally, post-viral sensorineural olfactory dysfunction occurs, with a variable prognosis. The evidence on anosmia in COVID-19 patients is extremely limited, corresponding to a level 5 or D of the Centre for Evidence-Based Medicine. According to the available evidence, it seems reasonable to apply isolation, hygiene and social distancing measures in patients with recent olfactory disorders as the only symptom, although the usefulness of diagnostic tests for this type of patient should be studied.

Comparative growth in the olfactory system of the developing chick with considerations for evolutionary studies

Despite the long-held assumption that olfaction plays a relatively minor role in the behavioral ecology of birds, crown-group avians exhibit marked phylogenetic variation in the size and form of the olfactory apparatus. As part of a larger effort to better understand the role of olfaction and olfactory tissues in the evolution and development of the avian skull, we present the first quantitative analysis of ontogenetic scaling between olfactory features [olfactory bulbs (OBs) and olfactory turbinates] and neighboring structures (cerebrum, total brain, respiratory turbinates) based on the model organism Gallus gallus. The OB develops under the predictions of a concerted evolutionary model with rapid early growth that is quickly overcome by the longer, sustained growth of the larger cerebrum. A similar pattern is found in the nasal cavity where the morphologically simple (non-scrolled) olfactory turbinates appear and mature early, with extended growth characterizing the larger and scrolled respiratory turbinates. Pairwise regressions largely recover allometric relationships among the examined structures, with a notable exception being the isometric trajectory of the OB and olfactory turbinate. Their parallel growth suggests a unique regulatory pathway that is likely driven by the morphogenesis of the olfactory nerve, which serves as a structural bridge between the two features. Still, isometry was not necessarily expected given that the olfactory epithelium covers more than just the turbinate. These data illuminate a number of evolutionary hypotheses that, moving forward, should inform tradeoffs and constraints between the olfactory and neighboring systems in the avian head.

Potential roles of smell and taste in the orientation behaviour of coral-reef fish larvae: insights from morphology

An ontogenetic analysis of the olfactory organ and the number and distribution of internal taste buds was carried out in two neon gobies (Elacatinus lori and Elacatinus colini) with the goal of revealing morphological trends that might inform an understanding of the roles of olfaction and taste in larval orientation behaviour. The pattern of development of the olfactory organ is unremarkable and enclosure of the olfactory epithelium occurs concurrently with metamorphosis and settlement in both species. Like other gobies, juvenile and adult E. lori and E. colini lack complex olfactory lamellae, and lack the accessory nasal sacs present in some adult gobies that could facilitate active olfactory ventilation (i.e., sniffing). A small number of internal taste buds are present at hatch with most found in the caudal region of the buccal cavity (on gill arches, roof of buccal cavity). As taste bud number increases, they demonstrate an anterior spread to the lips, buccal valves and tongue (i.e., tissue covering the basihyal). In the absence of an active ventilatory mechanism for the olfactory organs, the water that moves through the buccal cavity with cyclic gill ventilation may provide chemical cues allowing the internal taste buds to play a role in chemical-mediated orientation and reef-seeking behavior in pelagic larval fishes.

Anatomy of the olfactory mucosa

The classic notion that humans are microsmatic animals was born from comparative anatomy studies showing the reduction in the size of both the olfactory bulbs and the limbic brain relative to the whole brain. However, the human olfactory system contains a number of neurons comparable to that of most other mammals, and humans have exquisite olfactory abilities. Major advances in molecular and genetic research have resulted in the identification of extremely large gene families that express receptors for sensing odors. Such advances have led to a renaissance of studies focused on both human and nonhuman aspects of olfactory physiology and function. Evidence that olfactory dysfunction is among the earliest signs of a number of neurodegenerative and neuropsychiatric disorders has led to considerable interest in the use of olfactory epithelial biopsies for potentially identifying such disorders. Moreover, the unique features of the olfactory ensheathing cells have made the olfactory mucosa a promising and unexpected source of cells for treating spinal cord injuries and other neural injuries in which cell guidance is critical. The olfactory system of humans and other primates differs in many ways from that of other species. In this chapter we provide an overview of the anatomy of not only the human olfactory mucosa but of mucosae from a range of mammals from which more detailed information is available. Basic information regarding the general organization of the olfactory mucosa, including its receptor cells and the large number of other cell types critical for their maintenance and function, is provided. Cross-species comparisons are made when appropriate. The polemic issue of the human vomeronasal organ in both the adult and fetus is discussed, along with recent findings regarding olfactory subsystems within the nose of a number of mammals (e.g., the septal organ and Grüneberg ganglion).

[Localization and distribution of human olfactory mucosa in the nasal cavities]

INTRODUCTION

The exact distribution of the human olfactory mucosa can only be determined in studies that evaluate the entire olfactory region. The purpose of this study is to determine the distribution of human olfactory mucosa in the nasal cavities, by performing the histological analysis, by light microscopy, of anatomical specimens of the olfactory region obtained from cadavers.

MATERIAL AND METHODS

The specimens were taken during the autopsy of fresh cadavers. In each of the specimens, the distance between the cribiform plate and the lower limit of the olfactory region was determined in three different locations of the septal and lateral walls.

RESULTS

Of the 230 anatomical specimens available, 217 were excluded for medical or technical reasons. Morphometric studies were performed on 13 specimens (total 156 measurements). The lower limit of the olfactory mucosa in the nasal septum was 15.9 ± 3.2 mm, 15.3 mm ± 3 and 16 ± 2.8 mm in the anterior, middle and posterior olfactory regions. The lower limit of the olfactory mucosa in the turbinate wall was 15.3 ± 2.4 mm, 14.8 ± 2.3 mm and 12.3 ± 1.9 mm in the equivalent regions. The minimum value observed was 12 mm.

CONCLUSIONS

The olfactory mucosa extends through the upper and middle turbinates and the confronting nasal septum in a minimum distance of 12 mm and that may exceed 16 mm. Knowing the exact distribution of the olfactory mucosa can guide the collection of this tissues in humans, for diagnostic or therapeutic purposes.

The development of the olfactory organs in newly hatched monotremes and neonate marsupials

Olfactory cues are thought to play a crucial role in the detection of the milk source at birth in mammals. It has been shown that a marsupial, the tammar wallaby, can detect olfactory cues from its mother's pouch at birth. This study investigates whether the main olfactory and accessory olfactory system are similarly well developed in other marsupials and monotremes at birth/hatching as in the tammar. Sections of the head of various marsupial and two monotreme species were investigated by light microscopy. Both olfactory systems were less well developed in the kowari and Eastern quoll. No olfactory or vomeronasal or terminal nerves could be observed; the main olfactory bulb (MOB) had only two layers while no accessory olfactory bulb or ganglion terminale were visible. All other investigated marsupials and monotremes showed further developed olfactory systems with olfactory, vomeronasal and terminal nerves, a three-layered MOB, and in the marsupials a prominent ganglion terminale. The main olfactory system was further developed than the accessory olfactory system in all species investigated. The olfactory systems were the least developed in species in which the mother's birth position removed most of the difficulty in reaching the teat, placing the neonate directly in the pouch. In monotremes they were the furthest developed as Bowman glands were found underlying the main olfactory epithelium. This may reflect the need to locate the milk field each time they drink as they cannot permanently attach to it, unlike therian mammals. While it still needs to be determined how an odour signal could be further processed in the brain, this study suggests that marsupials and monotremes possess well enough developed olfactory systems to be able to detect an odour cue from the mammary area at birth/hatching. It is therefore likely that neonate marsupials and newly hatched monotremes find their way to the milk source using olfactory cues, as has been previously suggested for the marsupial tammar wallaby, rabbits, rats and other eutherians.

[Sense of smell, physiological ageing and neurodegenerative diseases: I. Anatomy and clinical examination]

INTRODUCTION

Different studies on normal and pathological ageing have shown structural and functional impairment by means of psychophysical measures, electrophysiological studies and brain imaging studies. Lower sensory perception from peripheral olfactory structures, together with alterations in the medulla oblongata and cortex, are the most frequent causes of olfactory impairment in ageing.

AIM

To outline the most important tests that can be applied in clinical evaluation.

DEVELOPMENT

The article begins by reviewing concepts involved in olfactory anatomy and physiology and then goes on to describe the different methods of examination and their applicability to medical diagnosis.

CONCLUSIONS

Incorporating olfactory tests into clinical practice makes it possible to study the presence of sensory and cognitive impairment in greater depth, especially in ageing and in neurodegenerative diseases; this will allow a possible diagnosis to be expanded and completed.

Light and transmission electron microscopy study of the peripheral olfactory organ of the guppy, Poecilia reticulata (Teleostei, Poecilidae)

A study of the peripheral olfactory organ, with special attention to the olfactory epithelium, has been carried out in the guppy (Poecilia reticulata). Guppy is well known to have a vision-based sexual behavior. The olfactory chamber caudally opens directly in an accessory nasal sac, which is bent medially and gives rise to two recesses that can be considered secondary accessory nasal sacs, antero-medial and postero-medial, respectively. The sensory epithelium, which lines only the medial wall of the nasal cavity, is basically flat rising in a very low lamella only in the posterior part. The olfactory receptors are not evenly distributed in the olfactory mucosa, but aggregate in shallow folds separated by epithelial cells with evident microridges. Ciliated olfactory sensory neurons and microvillous olfactory sensory neurons are clearly identified by transmission electron microscopy (TEM). Scarce crypt olfactory neurons are found throughout the sensory folds. The nasal sacs indicates the capacity to regulate the flow of odorant molecules over the sensory epithelium, possibly through a pump-like mechanism associated with gill ventilation. The organization of the olfactory organ in guppy is simple and reminds what is found in early posthatching stages of fish which at the adult state have a well developed olfactory organ. This simple organization supports the idea that the guppy rely on olfaction less than other fish species provided with more extended olfactory receptorial surface.

The general architecture of sensory neuroepithelia

All neuroepithelia are sheets of cells lining an internal or external surface of the body and resting on a basement membrane. They consist of at least two kinds of cell, receptor cells and sustentacular (supporting) cells. Some contain undifferentiated precursor cells and senescent or degenerating cells. The potential for plasticity and regeneration in different sensory neuroepithelia varies widely according to their origins and structure in any individual animal and according to the species in which they occur. Four sensory neuroepithelia are described as examples of the range of construction, complexity, and life history.

Identification of human olfactory cleft mucus proteins using proteomic analysis

In humans, the olfactory epithelium is located in two narrow passages, the olfactory clefts, at the upper part of the nasal cavities. The olfactory epithelium is covered by a mucus layer which is essential for the function of the olfactory neurons that are directly connected with the brain through the cribriform plate. This anatomical weakness of the brain protection may be the source of infection. Little is known about the composition of this mucus in humans. Previous proteomic analyses have been performed on washes of the entire nasal cavities and therefore might better correspond to the mucus over the respiratory epithelium than to the mucus covering the olfactory epithelium. In the present study, we sampled the olfactory mucus directly from the clefts of 16 healthy adult volunteers, and 83 proteins were identified in the samples using two-dimensional gel electrophoresis, MALDI-TOF, RPLC, and Edman sequencing. Forty-three proteins were not previously observed either in nasal mucus sampled through washings, saliva, tear, or cerebrospinal fluid. In Accordance with the data in the protein databases, the most abundant proteins are secreted, whereas some others correspond to intracellular proteins covering a large range of functions: anti-inflammatory, antimicrobial, protease inhibition, antioxidant, transport, transcription, transduction, cytoskeletal, regulation, binding, and metabolism of odorant molecules. This study clearly demonstrates the complexity of the mucus covering the human olfactory epithelium, which might comprise potential markers for characterizing pathophysiological states.

Sex steroid hormones and sexual dimorphism of chemosensory structures in a terrestrial salamander (Plethodon shermani)

The volume of the vomeronasal organ (VNO) in the terrestrial salamander Plethodon shermani was approximately 1.7 times larger in adult males compared to adult females, even though male body size was, on average, slightly smaller than female body size. VNO cell density, however, was the same in adult males and females. The sex difference in VNO volume was found in sexually immature animals as well, indicating that the increase of plasma androgens that occurs at sexual maturity does not produce the sex difference in VNO volume. There was no difference in VNO volume between reproductive and non reproductive adult females, despite differences in plasma estradiol (E2) levels. The volumes of the main olfactory epithelium and muscles regulating diameter of the external nares were similar between males and females, indicating that the VNO per se, and not other aspects of the nasal cavity, was sexually dimorphic. To conclude, the sex difference in VNO volume appears to be a permanent sex difference that develops before sexual maturity. Future studies will examine the functional consequences of this structural sexual dimorphism in a peripheral sensory organ, the VNO.

Growth-deficient vomeronasal organs in the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Heterocephalus glaber) is unusual in numerous life history characteristics as well as its eusocial organization. This species demonstrates widespread sexual suppression and prominent scent marking, behaviors that have been associated with pheromonal communication involving the vomeronasal organ in other rodents. Yet, previous studies indicate that urinary signals do not mediate sexual suppression in Heterocephalus. Surprisingly, no previous studies have examined the vomeronasal organ in this species. Here, we show that Heterocephalus is unique among rodents in showing no evidence of postnatal volumetric growth in the vomeronasal neuroepithelium. Subadults from birth to weaning fell within the same volume range as adults regardless of breeding/non-breeding status of the latter. A comparison of existing ontogenetic data on other mammals suggests that the proportionally small VNOs of Heterocephalus may be explained by a deficiency in VNNE growth. Growth deficiency of the vomeronasal organ in Heterocephalus may relate to a diminished role that pheromones play in certain social interactions for this species, such as breeding suppression. In light of the unique aspects of the vomeronasal organ in Heterocephalus, comparative studies of rodents may provide a model for understanding variation of this sensory system in other mammalian orders including primates, an order which shows a range from vestigial to demonstrably functional vomeronasal organs.

Olfactory receptor gene repertoires in mammals

In mammals, olfaction is mediated by two distinct organs that are located in the nasal cavity: the main olfactory epithelium (MOE) that binds volatile odorants is responsible for the conscious perception of odors, and the vomeronasal organ (VNO) that binds pheromones is responsible for various behavioral and neuroendocrine responses between individuals of a same species. Odorants and pheromones bind to seven transmembrane domain G-protein-coupled receptors that permit signal transduction. These receptors are encoded by large multigene families that evolved in mammal species in function of specific olfactory needs.

Subarachnoid injection of Microfil reveals connections between cerebrospinal fluid and nasal lymphatics in the non-human primate

Based on quantitative and qualitative studies in a variety of mammalian species, it would appear that a significant portion of cerebrospinal fluid (CSF) drainage is associated with transport along cranial and spinal nerves with absorption taking place into lymphatic vessels external to the central nervous system. CSF appears to convect primarily through the cribriform plate into lymphatics associated with the submucosa of the olfactory and respiratory epithelium. However, the significance of this pathway for CSF absorption in primates has never been established unequivocally. In past studies, we infused Microfil into the subarachnoid compartment of numerous species to visualize CSF transport pathways. The success of this method encouraged us to use a similar approach in the non-human primate. Yellow Microfil was injected post mortem into the cisterna magna of 6 years old Barbados green monkeys (Cercopithecus aethiops sabeus, n = 6). Macroscopic and microscopic examination revealed that Microfil was (1) distributed throughout the subarachnoid compartment, (2) located in the perineurial spaces associated with the fila olfactoria, (3) present within the olfactory submucosa, and (4) situated within an extensive network of lymphatic vessels in the nasal submucosa, nasal septum and turbinate tissues. We conclude that the Microfil distribution patterns in the monkey were very similar to those observed in many other species suggesting that significant nasal lymphatic uptake of CSF occurs in the non-human primate.

Scaling of mammalian ethmoid bones can predict olfactory organ size and performance

The relation between size and performance is central for understanding the evolution of sensory systems, and much interest has been focused on mammalian eyes and ears. However, we know very little about olfactory organ size (OOS), as data for a representative set of mammals are lacking. Here, we present a cranial endocast method for estimating OOS by measuring an easily accessible part of the system, the perforated part of the ethmoid bone, through which the primary olfactory axons reach the olfactory bulb. In 16 species, for which relevant data are available, the area of the perforated ethmoid bone is directly proportional to the area of the olfactory epithelium. Thus, the ethmoid bone is a useful indicator enabling us to analyse 150 species, and describe the distribution of OOS within the class Mammalia. In the future, a method using skull material may be applied to fossil skulls. In relation to skull size, humans, apes and monkeys have small olfactory organs, while prosimians have OOSs typical for mammals of their size. Large ungulates have impressive olfactory organs. Relating anatomy to published thresholds, we find that sensitivity increases with increasing absolute organ size.

Evidence for functional asymmetry in the olfactory system of the Senegalese sole (Solea senegalensis)

The two olfactory epithelia of flatfish of the family Soleidae are essentially in contact with two distinct environments; the upper (right) side samples open water while the lower (left) side samples interstitial water. This study assessed whether there are differences in the responsiveness of the two epithelia by use of the electro-olfactogram in the Senegalese sole (Solea senegalensis). The upper epithelium was significantly more responsive to the basic amino acids (L-lysine and L-arginine), glycine, and L-threonine than the lower epithelium. The lower epithelium was significantly more responsive to aromatic amino acids (L-tryptophan, L-tyrosine, L-DOPA, and L-phenylalanine), L-leucine, and L-asparagine than the upper. Both epithelia had similar responsiveness to the sulphur-containing amino acids (L-cysteine and L-methionine), L-alanine, L-serine, and L-glutamine. Neither side was responsive to the acidic amino acids (L-aspartate and L-glutamate) or the D-isomers of any amino acid tested. The upper olfactory organ was much more responsive to conspecific-derived stimuli (bile and intestinal fluid) than the lower organ. We suggest that these differences in responsiveness may be related to different functional roles of the upper and lower epithelia in feeding and chemical communication.

Expression of neuron-specific markers by the vomeronasal neuroepithelium in six species of primates

Vomeronasal organ (VNO) morphology varies markedly across primate taxa. Old World monkeys display no postnatal VNO. Humans and at least some apes retain a vestigial VNO during postnatal life, whereas the strepsirrhines and New World Monkeys present a morphologically well-defined VNO that, in many species, is presumed to function as an olfactory organ. Available microanatomical and behavioral studies suggest that VNO function in these species does not precisely duplicate that described in other mammalian taxa. The questions of which species retain a functional VNO and what functions they serve require inquiry along diverse lines but, to be functional, the vomeronasal epithelium must be neuronal and olfactory. We used immunohistochemistry to establish these criteria in six primate species. We compared the expression of two neuronal markers, neuron-specific beta-tubulin (BT) and protein gene product 9.5, and olfactory marker protein (OMP), a marker of mature olfactory sensory neurons, in paraffin-embedded VNO sections from two strepsirrhine and four haplorhine species, all of which retain morphologically well-defined VNOs during postnatal life. The infant Eulemur mongoz, adult Otolemur crassicaudatus, neonatal Leontopithicus rosalia, and adult Callithrix jacchus express all three proteins in their well-defined vomeronasal neuroepithelia. The infant Tarsius syrichta showed some BT and OMP immunoreactivity. We establish that two strepsirrhine species and at least some New World haplorhines have mature sensory neurons in the VNO. In contrast, at all ages examined, Saguinus geoffroyi VNO expresses these markers in only a few cells.

Distribution of olfactory epithelium in the primate nasal cavity: are microsmia and macrosmia valid morphological concepts?

The terms "microsmatic" and "macrosmatic" are used to compare species with greater versus lesser olfactory capabilities, such as carnivores compared to certain primates. These categories have been morphologically defined based on the size of olfactory bulb and surface area of olfactory epithelium in the nasal fossa. The present study examines assumptions regarding the morphological relationship of bony elements to the olfactory mucosa, the utility of olfactory epithelial surface area as a comparative measurement, and the utility of the microsmatic concept. We examined the distribution of olfactory neuroepithelium (OE) across the anteroposterior length of the nasal fossa (from the first completely enclosed cross-section of the nasal fossa to the choanae) in the microsmatic marmoset (Callithrix jacchus) compared to four species of nocturnal strepsirrhines (Otolemur crassicaudatus, O. garnetti, Microcebus murinus, and Cheirogaleus medius). Adults of all species were examined and infant C. jacchus, O. crassicaudatus, M. murinus, and C. medius were also examined. All specimens were serially sectioned in the coronal plane and prepared for light microscopic study. Distribution of OE across all the turbinals, nasal septal surfaces, and accessory spaces of the nasal chamber was recorded for each specimen. The right nasal fossae of one adult C. jacchus and one neonatal M. murinus were also three-dimensionally reconstructed using Scion Image software to reveal OE distribution. Findings showed OE to be distributed relatively more anteriorly in adult C. jacchus compared to strepsirrhines. It was also distributed more anteriorly along the nasal septal walls and recesses in neonates than adults. Our findings also showed that OE surface area was not a reliable proxy for receptor neuron numbers due to differing OE thickness among species. Such results indicate that nasal cavity morphology must be carefully reconsidered regarding traditional functional roles (olfaction versus air conditioning) assigned to various nasal cavity structures. At present, the microsmatic concept itself lacks a basis in nasal chamber morphology, since OE may have varying patterns of distribution among different primates.

Differential expression of histochemical characteristics in the developing olfactory receptor cells in a flatfish, barfin flounder (Verasper moseri)

Differentiation of the histochemical characteristics of the olfactory receptor cells (ORC) was examined by immunohistochemistry for protein gene product 9.5 (PGP 9.5) and calretinin (CR) and lectin histochemistry for Phaseolus vulgaris agglutinin-L (PHA-L) in the developing olfactory epithelium (OE) of the barfin flounder. PGP 9.5 immunoreactivity was diffuse and CR immunoreactivity was restricted at day 7, but these immunoreactivities became intense in the OE toward day 91. Crypt cells were first identified at day 56. PHA-L staining was faint at day 28, but became intense toward day 91. These findings suggest that PGP 9.5-immunopositive cells, CR-immunopositive cells, crypt cells and PHA-L-reactive cells differentiate independently in the developing OE and constitute subsets of the ORC in the OE.

Microsmatic primates: reconsidering how and when size matters

The terms "microsmatic" and "macrosmatic" refer to species with lesser or greater levels, respectively, of olfactory function. Historically, primates are considered microsmats (olfactory sense reduced) with a concomitant increased emphasis on vision. The olfactory bulbs (forebrain centers that receive peripheral olfactory input) are proportionately smaller in primates compared to most other mammals. Similarly, the regions of the nasal cavity that are covered with olfactory epithelium (containing receptor cells) have proportionately less surface area in primates than other mammals. Thus, the generalization that primates are microsmatic is most frequently stated in terms of the proportional rather than absolute size of olfactory structures. Yet the importance of scaling to body size is unclear in regard to the chemical senses such as the olfactory or vomeronasal systems-do chemosensory structures such as olfactory bulbs and olfactory epithelium exhibit the same neural relationship to body mass that is seen for neural tissues that supply innervation to musculature or the skin? Previous studies examining neuronal density, volume, and/or surface area of the olfactory epithelium illustrate that different conclusions may be supported based on the parameter used. Plots of olfactory bulb volume versus body mass that generated for large-scale taxonomic studies or growth studies benefit from body mass (or total brain volume) with a comparative perspective. However, our examination of proportional versus absolute measurements implies that in comparisons within taxa, body size adjustments needlessly distort the data. As a final consideration, another embryonic derivative of the nasal placode, the vomeronasal organ, may warrant consideration regarding a definition of microsomia versus macrosomia.

Histological and histochemical studies on the olfactory rosette of Mugil parsia (Hamilton)

The structure and functions of the olfactory organs in Mugil parsia (Ham.) has been described. Histologically each lamella consists of supporting, olfactory receptor, basal, labyrinth and mast cells. The distribution and localization of acid and neutral mucins in the various cells of olfactory epithelium in M. parsia has been studied histochemically. Variations in the localization of glycogen in the different cells of the olfactory epithelium have been correlated with the functional significance of the region concerned in the fish studied.

The septal organ of the rat during postnatal development

The septal organ of Masera (SO) is a small, isolated patch of olfactory epithelium, located in the ventral part of the nasal septum. We investigated in this systematic study the postnatal development of the SO in histological sections of rats at various ages from the day of birth (P1) to P666. The SO-area increases to a maximum at P66-P105, just as the animals reach sexual maturity, and decreases thereafter, significantly however only in males, indicating a limited neurogenetic capacity for regeneration. In contrast, the main olfactory epithelium area continues to expand beyond P300. The modified respiratory epithelium ('zwischen epithelium') separating the SO and the main olfactory epithelium contains a few olfactory neurons up to age P66. Its length increases postnatally so that the SO becomes more ventral to the OE. Although the position of the SO relative to other anatomical landmarks changes with development it is consistently located just posterior to the opening of the nasopalatine duct (NPAL). Thus, a possible function of the SO is in sensing chemicals in fluids entering the mouth by licking and then delivered to the nasal cavity via the NPAL; therefore the SO may be involved in social/sexual behavior as is the vomeronasal organ (VNO). We suggest that the SO is a separate accessory olfactory organ with properties somewhat different from both OE and VNO and may exist only in species where the NPAL does not open into the VNO.

[Protocortex versus protomap: a perspective from the olfactory bulb]

INTRODUCTION AND AIM

The olfactory sensory system is a unique model for the research of guidance and connectivity of growing axons. During development, the olfactory epithelium, the olfactory bulb and the olfactory cortex differentiate several cell types and extend projection axons. Because there is a close relationship between these three structures, we ask the question as to whether establishment of the olfactory bulb central projections can proceed independently of the arrival of the olfactory sensory afferents. This raises another more general question: is establishment of afferent connections necessary to awake a developmental program in target cells?.

DEVELOPMENT

The initial establishment of the olfactory bulb central projections occurs independently of the arrival of the olfactory axons from the olfactory epithelium, which reinforces the idea that cortical regions are already patterned before migration of newborn neurons, at least for the olfactory bulb and maybe for the entire brain. This implies a strict intrinsic molecular control of the distinct olfactory structures, independent one of each other.

CONCLUSIONS

How then, do axonal projections find their correct way within the brain? Contact-mediated mechanisms and chemotropic molecules cooperate to fix their position in the telencephalon, prevent bulbar axons from invading structures other than the olfactory cortex and, at the same time, stimulate axonal branching in an orchestra of both, attractive/promoting and repulsive/inhibiting signals. At later stages, the mature appearance of the olfactory bulb will be completed and refined.

Odorant receptor expression patterns are restored in lesion-recovered rat olfactory epithelium

Lesions of the olfactory periphery provide a means for examining the reconstitution of a diverse and highly regulated population of sensory neurons and the growth, en masse, of nascent axons to the bulb. The olfactory epithelium and its projection onto the bulb are reconstituted after ablation by methyl bromide gas, and some measure of olfactory function is restored. The extent to which the system regenerates the full repertoire of odorant receptor-expressing neurons, particularly their spatially restricted distribution across the epithelial sheet, is unknown, however, and altered odorant receptor expression might contribute to the persistent distortion of odorant quality that is observed in the lesioned-recovered animals. To address the question of receptor expression in the recovered epithelium, we performed in situ hybridization with digoxigenin-labeled riboprobes for eight odorant receptors on the olfactory epithelium from unilaterally methyl bromide-lesioned and control rats. The data demonstrate that the distribution of sensory neuron types, as identified and defined by odorant receptor expression, is restored to normal or nearly so by 3 months after lesion. Likewise, the numbers of probe-labeled neurons in the lesioned-recovered epithelium are nearly equivalent to the unlesioned side at this time. Finally, our evidence suggests that odorant receptors are distributed in multiple overlapping bands in the normal, unlesioned, and lesioned-recovered epithelium rather than in the conventionally accepted three or four zones. Thus, the primary sensory elements required for functional recovery of the olfactory system after damage are restored, and altered function implies the persistence of a more central failure in regeneration.

Mucosal activity patterns as a basis for olfactory discrimination: comparing behavior and optical recordings

In over half a century numerous studies have demonstrated that different odorants produce individually different spatial patterns of neural receptor activity on the olfactory mucosa. However, the thought that these differential activity patterns could be the neural code underlying olfactory perception has not been tested directly. In the present study using operant techniques, rats were trained to differentially identify five odors from a homologous series of iso-intensive straight-chain aldehydes that differed serially by only one carbon atom, viz. hexaldehyde to decaldehyde. The rats identified each of the five odorants with greater than 90% correct identification. The degree of perceptual similarity between any pair of the five odorants was determined. Using multidimensional scaling techniques (MDS) the similarity measures yielded a two-dimensional perceptual odorant space. Optical techniques were used to record the olfactory mucosal activity patterns in response to these same five iso-intensive aldehydes. The mucosal activity elicited by each odorant revealed individually distinct band-like patterns that varied both within and across these bands. More importantly, the relative differential responsivity of the bands was related to chain length. An MDS analysis of the dissimilarity measure between all possible pairs of odorant induced activity patterns yielded a two-dimensional neurophysiologic odorant space. Further analysis indicated that the neurophysiologic and psychophysically determined odorant spaces were highly correlated (F(1,39)=23.9, P=nil). These results give additional credence to the concept that the odorant-induced mucosal activity patterns may serve as the substrate for the perception of odorant quality.

Surface glycoconjugates in the olfactory system of Ambystoma mexicanum

Lectin binding histochemistry was performed on the olfactory system of neotenic and metamorphosed Ambystoma mexicanum to investigate the distribution and density of defined carbohydrate residues on the cell surface glycoproteins of the olfactory and vomeronasal receptor cells and their terminals in the olfactory bulbs. The lectin binding patterns indicate that the main olfactory system possesses a high density of N-acetyl-galactosamine and N-acetyl-glucosamine residues, while the vomeronasal system contains a high density of N-acetyl-galactosamine and galactose moieties and a moderate density of N-acetyl-glucosamine. The presence of specific glycoproteins, whose terminal sugars are detected by lectin binding, might be related to the chemoreception and transduction of the odorous message into a nervous signal or to the histogenesis and development of the olfactory system. In fact, the olfactory and vomeronasal receptor cells are neurons that undergo a continuous cycle of proliferation not only during development but also in mature animals.

[Remarks on the olfactory perception mechanism]

After presenting the morphology of the olfactory epithelium, the author describe the smell stimulation mechanism of the olfactory receptor. The importance of the connection between the receptor and protein G is emphasised. The author also presents the theories on the possibility of smell identification in the peripheral olfactory system.

Immunohistochemical studies on the differential maturation of three types of olfactory organs in the rats

Differential maturation of three types of olfactory organs, the olfactory epithelium (OE), the vomeronasal organ (VNO) and the septal olfactory organ of Masera (MO), was examined immunohistochemically in embryonic and newborn rats by the use of antiprotein gene product 9.5 (PGP 9.5) serum. These olfactory organs were derived in common from the olfactory placode as neuroepithelia. In the OE, PGP 9.5-immunopositive olfactory cells first appeared at 13 days of gestation. The OE maturated completely, and showed the same cytological features as in the adult at 20 days of gestation. The MO first appeared as a dense mass of PGP 9.5-immunopositive sensory cells on the most ventrocaudal part of the nasal septum at 15 days of gestation and was evidently isolated from the OE by the decrease of immunopositive cells in the intercalated epithelium between the OE and the MO at 20 days of gestation. However, even at 7 days after birth, the MO did not complete its development and contained sensory cells aggregating in the mass. The VNO was separated from the nasal cavity at 13 days of gestation as a tubular structure of a neuroepithelium including PGP 9.5-immunopositive sensory cells. These cells gradually increased in number in the sensory epithelium of the VNO and extended their dendritic processes to the free surface at 7 days after birth. These findings clarified the differential maturation of these olfactory organs. That is, the OE completes its development before birth, while the MO and VNO after birth.

Zonal organization of the mammalian main and accessory olfactory systems

Zonal organization is one of the characteristic features observed in both main and accessory olfactory systems. In the main olfactory system, most of the odorant receptors are classified into four groups according to their zonal expression patterns in the olfactory epithelium. Each group of odorant receptors is expressed by sensory neurons distributed within one of four circumscribed zones. Olfactory sensory neurons in a given zone of the epithelium project their axons to the glomeruli in a corresponding zone of the main olfactory bulb. Glomeruli in the same zone tend to represent similar odorant receptors having similar tuning specificity to odorants. Vomeronasal receptors (or pheromone receptors) are classified into two groups in the accessory olfactory system. Each group of receptors is expressed by vomeronasal sensory neurons in either the apical or basal zone of the vomeronasal epithelium. Sensory neurons in the apical zone project their axons to the rostral zone of the accessory olfactory bulb and form synaptic connections with mitral tufted cells belonging to the rostral zone. Signals originated from basal zone sensory neurons are sent to mitral tufted cells in the caudal zone of the accessory olfactory bulb. We discuss functional implications of the zonal organization in both main and accessory olfactory systems.

The structure of the nasal chemosensory system in squamate reptiles. 1. The olfactory organ, with special reference to olfaction in geckos

The luminal surface of the chemosensory epithelia of the main olfactory organ of terrestrial vertebrates is covered by a layer of fluid. The source of this fluid layer varies among vertebrates. Little is known regarding the relative development of the sources of fluid (sustentacular cells and Bowman's glands) in reptiles, especially in gekkotan lizards (despite recent assertions of olfactory speciality). This study examined the extent and morphology of the main olfactory organ in several Australian squamate reptiles, including three species of gekkotans, two species of skinks and one snake species. The olfactory mucosa of two gekkotan species (Christinus marmoratus and Strophurus intermedius) is spread over a large area of the nasal cavity. Additionally, the sustentacular cells of all three gekkotan species contained a comparatively reduced number of secretory granules, in relation to the skinks or snake examined. These observations imply that the gekkotan olfactory system may function differently from that of either skinks or snakes. Similar variation in secretory granule abundance was previously noted between mammalian and non-mammalian olfactory sustentacular cells. The observations in gekkotans suggests that the secretory capacity of the non-mammalian olfactory sustentacular cells show far more variation than initially thought.

The structure of the nasal chemosensory system in squamate reptiles. 2. Lubricatory capacity of the vomeronasal organ

The vomeronasal organ is a poorly understood accessory olfactory organ, present in many tetrapods. In mammals, amphibians and lepidosaurian reptiles, it is an encapsulated structure with a central, fluid-filled lumen. The morphology of the lubricatory system of the vomeronasal organ (the source of this fluid) varies among classes, being either intrinsic (mammalian and caecilian amphibian vomeronasal glands) or extrinsic (anuran and urodele nasal glands). In the few squamate reptiles thus far examined, there are no submucosal vomeronasal glands. In this study, we examined the vomeronasal organs of several species of Australian squamates using histological, histochemical and ultrastructural techniques, with the goal of determining the morphology of the lubricatory system in the vomeronasal organ. Histochemically, the fluid within the vomeronasal organ of all squamates is mucoserous, though it is uncertain whether mucous and serous constituents constitute separate components. The vomeronasal organ produces few secretory granules intrinsically, implying an extrinsic source for the luminal fluid. Of three possible candidates, the Harderian gland is the most likely extrinsic source of this secretion.

Anterior distribution of human olfactory epithelium

OBJECTIVES/HYPOTHESIS

To functionally investigate the distribution of the olfactory epithelium in humans by means of the electro-olfactogram (EOG) and anatomically located biopsy specimens.

STUDY DESIGN

Prospective, nonrandomized, investigational.

METHODS

Supra-threshold EOG recordings were made on 12 healthy, trained volunteers (6 women, 6 men; age range, 21-48 y). Vanillin was used as the stimulus, since it exclusively excites olfactory receptor neurons. The EOG was recorded with tubular electrodes that were placed using thin-fiber endoscopic guidance. Biopsy specimens were obtained of anterosuperior nasal cavity mucosa in the same regions as the positive EOGs in 15 smell-tested patients (7 women, 8 men; age range, 22-60 y) during routine nasal and sinus surgery. This biopsied tissue was histologically processed and stained for olfactory and neural proteins.

RESULTS

Viable responses to EOG testing were obtained in 7 of 12 subjects. In these seven subjects it was possible to identify nine sites above or below the anterior middle turbinate insertion where EOGs were obtained. The biopsy results showed mature olfactory receptor neurons in this same area.

CONCLUSIONS

Human olfactory epithelium appears to be distributed more anteriorly than previously assumed.

[A numerical simulation of the aerodynamics of the nasal cavity]

We present the results of the numerical simulation of the flow in the nasal cavity, going from the tip of the nostril to the nasopharyngeal region. The volume of the nasal cavity, obtained from axial and coronal scans, takes into account the geometries of the nasal valve and turbinates. The simulation is carried out with the FLUENT code which solves the equations of fluid mechanics. The obtained results for the inspiratory phase are analyzed from the velocities and pressures, paying special attention to the separation of the streamlines in the region located between the middle meatus and the olfactory area. The presented results show the potential of simulation when used in parallel with the clinical approach.

Olfaction and peripheral olfactory connections in methimazole-treated rats

Methimazole has been reported to produce extensive degenerative changes in olfactory epithelium and a severe deficit in odor detection [Genter BM, Owens DM, Carlone HB, Crofton KM. Fundam. Appl. Toxicol. 1996;29:71-77; Genter BM, Owens DM, Deamer NJ, Blake BL, Wesley DS, Levi PE. Toxicol. Pathol. 1995;23:477-486.]. To examine this further, rats were tested on olfactory detection and discrimination problems before and after intraperitoneal injection of 300 mg/kg methimazole. In the first 2 days after treatment, experimental rats had nasal congestion and a modest decrement on odor detection and odor mixture discrimination tasks. They performed almost as well as control rats on the third post injection day. In a separate group of rats, anterograde transport of horseradish peroxidase from olfactory epithelium to the bulb was examined 1, 2, 3, and 5 days after administration of methimazole. The treatment produced a modest but progressive disruption of bulbar input: 2 days after administration only approximately 10% of bulbar glomeruli had reduced levels of reaction product while 30-40% of glomeruli had little or no reaction product in 3-5 day survival rats. These results indicate that methimazole is not a particularly effective olfactotoxin and does not produce anosmia or even a severe hyposmia.

Analysis of the possible altering function of the septal organ in rats: a lesional and behavioral study

The septal olfactory organ is a small patch of sensory epithelium located on the septal wall at the entry of the nasopharynx. There is a general consensus that by sampling olfactory stimuli during periods of rest, this organ may have an alerting function. To verify this hypothesis, we have lesioned by electrocoagulation the septal organ of male rats and recorded by polygraphy their awakening reaction in response to biologically meaningful (trimethyl-thiazoline, dimethyl sulfite, food) and meaningless (geraniol, eucalyptol) odorants. The awakening reactions of both lesioned and intact rats in response to these odorants were studied according to 3 parameters, frequency, latency and duration of awakening, and were analyzed using three-way analyses of variance. Data show that no significant difference in the awakening reactions was observed between control and lesioned animals. In all cases, the biologically meaningful odors presented the highest awakening influence. In addition, two of these odors (trimethyl-thiazoline and dimethyl sulfite) elicited a later habituation in comparison to biologically meaningless odors. From our results, it could be inferred that the hypothesis regarding an alerting function that would be specific to the septal organ, appears no longer current.

Age- and gender-related trends in the expression of glutathione S-transferases in human nasal mucosa

The cellular expression of alpha, mu, and pi classes of glutathione S-transferases (GSTs) was investigated in human nasal mucosa by means of immunocytochemical techniques. In the olfactory mucosa, immunoreactivity for GST-alpha was most intense in the acinar cells of the Bowman's glands, with weak immunoreactivity in the supranuclear region of sustentacular cells. Whereas GST-pi was localized only in the sustentacular cells, no GST-mu was detected. In the respiratory mucosa, GST-alpha and GST-pi were detected at the brush borders of ciliated columnar epithelial cells. There were age- and gender-related trends in the expression of GST-alpha, but not GST-pi, in the olfactory mucosa. The intensity of immunoreactivity in the olfactory mucosa was decreased in older subjects. The expression of GST-alpha in the olfactory mucosa of females consistently exhibited greater intensity than that of males at all the ages studied. These differences were not observed in the respiratory mucosa. These results indicate that acinar cells of the Bowman's glands and sustentacular cells are the major sites of phase II biotransformation in the human nasal mucosa.

GnRH-like immunoreactivity in the peripheral olfactory system and forebrain of Atlantic salmon (Salmo salar): a reassessment at multiple life history stages

Neurons with gonadotropin releasing hormone-like immunoreactivity (GnRH-ir) were identified within the peripheral olfactory system of Atlantic salmon (Salmo salar) at multiple life history stages. Within the forebrain, GnRH-ir somata were found in the preoptic area and in the caudomedial olfactory bulb in a position comparable to the ganglion of the nervus terminalis of other teleosts. Somata positive for GnRH were also found throughout the rostro-caudal extent of the olfactory nerve, and clustered within the medial component of the olfactory nerve as it arises from the olfactory epithelium. Results from tract tracing experiments with horseradish peroxidase indicate that at least some cells in this cluster project to the retina, suggesting that they too are part of a terminal nerve ganglion as anatomically defined in other vertebrates. We suggest that the presence of a distinct cluster of terminal nerve ganglion cells in the immediate vicinity of the olfactory epithelium may point to a peripheral site of neuromodulatory control in the olfactory system in salmon and perhaps in other teleosts as well.

Olfaction and multiple chemical sensitivity

In this paper, a description of olfactory anatomy is presented, followed by a brief review of modern procedures for testing olfactory function. Information from the sole study which has quantitatively examined olfactory function in patients with apparent multiple chemical sensitivity (MCS) is presented. In essence, this study suggests that MCS is associated with increased nasal airflow resistance, respiration rate, heart rate, and scores on the Beck Depression Inventory, but not with significant changes in odor detection threshold sensitivity to phenyl ethyl alcohol and methyl ethyl ketone, the two target stimuli evaluated. Whether MCS patients evidence hypersensitivity to other chemicals is unknown.

Topographic patterns of responsiveness to odorants in the rat olfactory epithelium

1. Regional differences in odorant-induced responsiveness of the rat olfactory epithelium were measured via electrophysiological recordings [negative component of electro-olfactogram (Veog(-)) made from the surface of the olfactory epithelium on the nasal septum]. The nasal septum provided a flat surface from which multiple recordings could be made. 2. Veog(-)s were recorded from a standardized grid of 16 sites. This grid of recording sites extended over most of the surface of the olfactory epithelium on the nasal septum. 3. Twenty-one animals were tested for their responses to seven odorants. The animals were divided into three groups, each of which was tested with two different odorants plus amyl acetate, which provided a comparison between the groups. 4. For each odorant in each animal, topographic maps of relative responsiveness were derived to test whether odorants elicited different patterns of responses in the same individual. Topographic maps of responsiveness were derived also for the animal groups to test for the generality of the form of the maps for different odorants. Response latencies were also measured for each odorant at each recording site. 5. All individuals showed different topographic patterns of responses to the three test odorants. For most odorants, the location of the most responsive site was similar in all animals. In different animals the topographic maps for the same odorant were remarkably similar. Topographic maps for the odorants were all different from one another. 6. These results are consistent with the hypothesis that odorant quality is encoded in the differential spatial distribution of receptor cells whose differences in responsiveness appear to be distributed as a continuum across the epithelium. The results establish for a mammalian species what was previously reported in amphibians. These differences are presumed to be due to differential expression of odorant receptor proteins. 7. The mean response latency was 32 ms. This period was similar for all odorants, all animals, and all recording sites and was independent of Veog(-) amplitude. It is concluded that diffusion through the mucus contributed approximately 6 ms to the latency of onset of the responses to these odorants.

Localization of neurotrophin receptors in olfactory epithelium and bulb

We used in situ hybridization to localize trk, trkB and trkC mRNA, in rat and cat olfactory bulb. Expression of mRNA encoding truncated trkB receptors was seen in all layers, while only very modest full-length trkB expression could be detected. trkC hybridization was seen in all layers, most dense in the mitral cell layer. The localization of full-length tyrosine kinase trkB receptor in olfactory bulb and epithelium was examined with immunohistochemistry. trkB-like immunoreactivity was seen in the fila olfactoria, epithelium and in vitro, in olfactory sensory neurones. Since BDNF is expressed by olfactory sensory neurone target cells in the olfactory bulb, these data suggest that BDNF may act as a target derived neurotrophic factor in the primary olfactory system.

Spatial segregation of odorant receptor expression in the mammalian olfactory epithelium

The signal elicited by the interaction of odorous ligands with receptors on olfactory sensory neurons must be decoded by the brain to determine which of the numerous receptors have been activated. We have examined the patterns of odorant receptor expression in the rat olfactory epithelium to determine whether the mammalian olfactory system employs spatial segregation of sensory input to encode the identity of an odorant stimulus. In situ hybridization experiments with probes for 11 different odorant receptors demonstrate that sensory neurons expressing distinct receptors are topologically segregated into a small number of broad, yet circumscribed, zones within the olfactory epithelium. Within a given zone, however, olfactory neurons expressing a specific receptor appear to be randomly distributed, rather than spatially localized. The complex mammalian olfactory system may therefore compartmentalize the epithelium into anatomically and functionally discrete units, such that each zone expresses only a subset of the entire receptor repertoire.

Fine structure of the septal olfactory organ of Masera and its associated gland in the golden hamster

Fine structures of the septal olfactory organ of Masera (MO) and its associated gland, a kind of olfactory glands, were examined in the golden hamster in comparison with those of the olfactory epithelium (OE) and vomeronasal organ (VNO) and their associated olfactory glands. Bipolar neurons of the MO were divided into two types according to their apical morphology, one similar to the receptors of the OE and the other to those of the vomeronasal sensory epithelium (VSE). The former was dominant and interpreted as main receptors of the MO. The other was less in number, but also regarded as a kind of olfactory receptors. The ultrastructural features of supporting cells of the MO were similar to those of the OE rather than to those of the VSE, while those of basal cells were almost in common in the OE, VSE and MO. The associated gland of the MO was positive to both PAS and alcian blue as the Bowman's gland of the OE. The fine structure of the associated gland of the MO was also similar to that of Bowman's gland. The present findings on the fine structure of the MO and its associated gland strongly suggest that the MO fulfills an olfactory function similar to that of the OE.

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

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

Phylogeny of the vomeronasal system and of receptor cell types in the olfactory and vomeronasal epithelia of vertebrates

In this paper, the evolutionary origin of the vomeronasal system as a discrete sensory system separate from olfaction is examined. The presence of a discrete vomeronasal system appears to be a derived character in tetrapods, and its presence in larval amphibians indicates that the system did not arise as a terrestrial adaptation. The vomeronasal system has been lost independently in several taxa, including crocodilians, some bats, cetaceans, and some primates. The presence of microvillar receptor cells in the vomeronasal epithelium appears to be the ancestral condition for tetrapods, and alternative hypotheses concerning the ancestral condition for receptor cell types in the vertebrate olfactory epithelium are discussed. Finally, the possibility that the vomeronasal system is present in some fishes in a form that has not been recognized is discussed in relation to the phylogenetic distribution of receptor cell types in vertebrates.

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

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