Microscopic structure of the olfactory organ of the clearnose skate, Raja eglanteria

The olfactory system of sharks and rays in numbers

Cartilaginous fishes have large and elaborate olfactory organs, but only a small repertoire of olfactory receptor genes. Here, we quantitatively analyze the olfactory system of 21 species of sharks and rays, assessing many features of the olfactory organ (OOR) (number of primary lamellae, branches of the secondary folds, sensory surface area, and density and number of sensory neurons) and the olfactory bulb (OB) (number of neurons and non-neuronal cells), and estimate the ratio between the number of neurons in the two structures. We show that the number of lamellae in the OOR does not correlate with the sensory surface area, while the complexity of the lamellar shape does. The total number of olfactory receptor neurons ranges from 30.5 million to 4.3 billion and the total number of OB neurons from 1.5 to 90 million. The number of neurons in the olfactory epithelium is 16 to 158 times higher (median ratio is 46) than the number of neurons in the OB. These ratios considerably exceed those reported in mammals. High convergence from receptor neurons to neurons processing olfactory information, together with the remarkably small olfactory receptor repertoire, strongly suggests that the olfactory system of sharks and rays is well adapted to detect a limited number of odorants with high sensitivity.

Microstructure of the Bonnethead Shark (Sphyrna tiburo) Olfactory Rosette

Synopsis

The unusual shape of sphyrnid (hammerhead shark) heads has led to many functional hypotheses of potential sensory advantages and enhanced olfactory performance. Recent investigations into the flow of water within the sphyrnid olfactory chamber demonstrate that this complex structure exhibits a differential pressure system between the 2 nares that induces flow through the chamber. This leads to differential fluid velocities in different parts of the olfactory chamber. Particularly, lamellae at the medial end of the olfactory chamber experience a near-stagnant recirculation of water. The objectives of this study were to (1) describe the microstructure of the olfactory rosette of bonnethead sharks (Sphyrna tiburo) and (2) given the variability of water flow within the sphyrnid olfactory rosette, investigate differences of individual lamellae based on their positioning within the rosette. Specifically, we investigated degree of secondary folding, percent sensory area, and relative surface along the lateral-to-medial gradient. Both degree of secondary folding and percent sensory area may serve as proxies for olfactory sensitivity, providing connectivity between area devoted to sensitivity and water flow within the olfactory organ. We found that bonnethead sharks exhibited similar lamellar morphology to other shark species. We also described the projection of the olfactory nerve layer through an individual lamella. Additionally, we found that lamellae within the medial portion of the organ, which experience slower water velocities, had less secondary lamellar folds and less sensory area. These findings imply that these areas may be less sensitive. Future work should test for sensitivity differences within the rosette along the lateral-to-medial gradient.

Spanish

La forma inusual de las cabezas de los esfírnidos (tiburones martillo) ha llevado a muchas hipótesis funcionales de posibles ventajas sensoriales y unas mejores capacidades olfativas. Las investigaciones recientes sobre el flujo de agua dentro del órgano olfativo de los esfírnidos, demuestran que esta estructura compleja exhibe un sistema de presión diferente entre las dos fosas nasales que induce el flujo en el órgano. Esto conduce a velocidades de fluido diferentes en distintas partes del órgano olfativo. En particular, las láminas en el extremo medial del órgano olfativo experimentan una recirculación de agua casi estancada. Los objetivos de este estudio fueron 1) describir la microestructura de la roseta olfativa de los tiburones cabeza de pala (Sphyrna tiburo) y 2) considerando la variabilidad del flujo de agua dentro de la roseta olfativa de los esfírnidos, investigar las diferencias de las laminillas individuales, basadas en su posición dentro de la roseta. Específicamente, hemos investigado el grado de plegamiento secundario, el porcentaje del área sensorial y el área relativa de superficie a lo largo del gradiente de lateral a medial. El grado de plegamiento secundario y el porcentaje del área sensorial pueden servir como indicadores de la sensibilidad olfativa, proporcionando conectividad entre el área dedicada a la sensibilidad y el flujo de agua dentro del órgano olfativo. Descubrimos que los tiburones cabeza de pala exhibían una morfología laminar similar a la de otras especies de tiburones. También hemos descrito la proyección del estrato del nervio olfativo dentro de una lámina individual. Además, encontramos que las laminillas dentro de la porción medial del órgano que experimentan velocidades de agua más lentas, tenían menos pliegues laminares secundarios y una menor área sensorial. Estos hallazgos implican que estas áreas pueden ser menos sensitivas. El trabajo futuro debería evaluar las diferencias de sensibilidad dentro de la roseta a lo largo del gradiente de lateral a medial.

German

Die ungewöhnliche Kopfform der Sphyrniden (Hammerhaie) hat schon zu vielen funktionellen Hypothesen bezüglich möglicher sensorischer Vorteile und verbesserter olfaktorischer Leistung geführt. Kürzlich veröffentlichte Studien zur Wasserströmung innerhalb der olfaktorischen Kammern von Sphyrniden zeigen, dass diese komplexe Struktur unterschiedliche Drucksysteme zwischen den beiden Nasenlöchern erzeugt, welches eine Strömung durch die Nasenkammer erzeugt. Dies wiederum führt zu unterschiedlichen Flüssigkeitsströmungen in verschiedenen Abschnitten der olfaktorischen Kammer. Besonders bei den Lamellen am medialen Ende der olfaktorischen Kammer gibt es eine fast schon stillstehende Rezirkulation von Wasser. Die Ziele dieser Studie waren 1) das Beschreiben der Mikrostruktur der olfaktorischen Rosette des Schaufelnasen-Hammerhais (Sphyrna tiburo) und 2) wollten wir, aufgrund der Variabilität der Wasserströmung innerhalb der olfaktorischen Rosette der Sphyrniden, die Unterschiede von individuellen Lamellen basierend auf ihrer unterschiedlichen Position innerhalb der Rosette untersuchen. Wir untersuchten den Grad an sekundären Falten, den Prozentsatz an sensorischer Fläche und die relative Oberfläche entlang dem lateral-zu-medialem Gradienten. Sowohl der Grad an sekundären Falten wie auch der Prozentsatz an sensorischer Fläche mögen als Annäherung für die olfaktorische Sensibilität dienen, weil sie für eine Verbindung zwischen der Fläche, die dem Geruchssinn und der Strömung zwischen den olfaktorischen Organen sorgt. Wir fanden, dass die Schaufelnasen-Hammerhaie eine ähnliche lamellare Morphologie zeigen wie andere Hai-Arten. Wir beschreiben auch wie der Geruchsnerv durch eine individuelle Lamelle verläuft. Weiter fanden wir, dass die Lamellen innerhalb des mittleren Teils des Organs, welches geringe Strömungsgeschwindigkeiten erfährt, weniger sekundäre lamellare Falten enthält und weniger sensorische Fläche. Diese Entdeckungen implizieren, dass diese Bereiche weniger sensibel sind auf Gerüche. Zukünftige Arbeiten sollten die unterschiedlichen Sensibilitäten innerhalb der Rosette entlang des lateral-medialem Gradienten testen.

Multimodal Imaging and Analysis of the Neuroanatomical Organization of the Primary Olfactory Inputs in the Brownbanded Bamboo Shark, Chiloscyllium punctatum

There is currently a limited understanding of the morphological and functional organization of the olfactory system in cartilaginous fishes, particularly when compared to bony fishes and terrestrial vertebrates. In this fish group, there is a clear paucity of information on the characterization, density, and distribution of olfactory receptor neurons (ORNs) within the sensory olfactory epithelium lining the paired olfactory rosettes, and their functional implications with respect to the hydrodynamics of incurrent water flow into the nares. This imaging study examines the brownbanded bamboo shark Chiloscyllium punctatum (Elasmobranchii) and combines immunohistochemical labeling using antisera raised against five G-protein α-subunits (Gα_s/olf, Gα_q/11/14, Gα_i–1/2/3, Gα_i–3, Gα_o) with light and electron microscopy, to characterize the morphological ORN types present. Three main ORNs (“long”, “microvillous” and “crypt-like”) are confirmed and up to three additional microvilli-bearing types are also described; “Kappe-like” (potential or homologous “Kappe” as in teleosts), “pear-shaped” and “teardrop-shaped” cells. These morphotypes will need to be confirmed molecularly in the future. Using X-ray diffusible iodine-based contrast-enhanced computed tomography (diceCT), high-resolution scans of the olfactory rosettes, olfactory bulbs (OBs), peduncles, and telencephalon reveal a lateral segregation of primary olfactory inputs within the OBs, with distinct medial and lateral clusters of glomeruli, suggesting a potential somatotopic organization. However, most ORN morphotypes are found to be ubiquitously distributed within the medial and lateral regions of the olfactory rosette, with at least three microvilli-bearing ORNs labeled with anti-Gα_o found in significantly higher densities in lateral lamellae [in lateral lamellae] and on the anterior portion of lamellae (facing the olfactory cavity). These microvilli-bearing ORN morphotypes (microvillous, “Kappe-like,” “pear-shaped,” and “teardrop-shaped”) are the most abundant across the olfactory rosette of this species, while ciliated ORNs are less common and crypt cells are rare. Spatial simulations of the fluid dynamics of the incurrent water flow into the nares and within the olfactory cavities indicate that the high densities of microvilli-bearing ORNs located within the lateral region of the rosette are important for sampling incoming odorants during swimming and may determine subsequent tracking behavior.

Secondary Folds Contribute Significantly to the Total Surface Area in the Olfactory Organ of Chondrichthyes

The olfactory organ of Chondrichthyes is characterized by a central support with several lamellae covered by a sensory olfactory epithelium. Although secondary folds are present on the lamellae in all the chondrichthyan species analyzed to date, their shape and size have not been described. We here analyze the olfactory organ of 13 elasmobranch and 1 holocephalan species, describe the shape of the secondary folds and evaluate how they contribute to the epithelial surface area. The secondary folds vary in shape and size, and they should always be considered when comparing the epithelial surface area among species; in fact, the increase of the area, due to the presence of the secondary folds, spans from 70 to 495% in the considered species. Because of the complexity of the shapes, we approach the description of the secondary folds by analyzing histological sections of the olfactory lamellae to obtain illustrative silhouettes. We introduce two indexes in order to describe a 2D-sectioned shape of the secondary folds. Considering the different numerical parameters which describe the morphology of the olfactory organ (secondary folds included), a principal component analysis elucidates the possible ecological role and phylogenetic relationship of the chondrichthyan olfactory organ.

Clarification of the Terminology of the Olfactory Lamellae in Chondrichthyes

Several papers regard the anatomy of the peripheral olfactory organ, the olfactory rosette, in the class of Chondrichtyes. The complex shape of this organ and the differences among species give clues to functional, evolutionary, and ecological observations; data on a larger number of species are needed in order to have a more complete insight. The rosette is made up of a central support and of numerous lamellae, which are lined by the sensory epithelium. The size, shape and number of these lamellae, which are highly variable among species, are noteworthy because they affect the sensory surface area, the water dynamic within the olfactory chamber, and the organization of the olfactory bulb. In the literature about Chondrichthyes, the definition of olfactory lamella is confused, because authors use the same words for different structures. The average number of lamellae is distinctive for each species, and the meaning of this difference is not completely understood and, in this frame, a not unambiguous definition of lamella leads to difficulties in comparing data from different publications and analyzing them together. We aim to give here an unambiguous definition of olfactory lamella, that should be each single fold of tissue extending from the raphe, and of lamellar number of a given species, that should be the average number of lamellae in one olfactory organ. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 300:2039-2045, 2017. © 2017 Wiley Periodicals, Inc.

Gross anatomy and histology of the olfactory rosette of the shark Heptranchias perlo

Sharks belonging to the family Hexanchidae have six or seven gill slits, unlike all other elasmobranchs, which have five gill slits. Their olfactory organs have a round shape, which is common for holocephalans, but not for elasmobranchs. Thus, the shape of the olfactory organ represents a further, less striking, peculiarity of this family among elasmobranchs. Despite that, the microscopic anatomy and histology of the olfactory organ have not yet been studied in any species of this family. Here, an anatomical and histological description of the olfactory organ of the sharpnose sevengill shark Heptranchias perlo is given. The organ is a rosette, with a central raphe and 31-34 primary lamellae, which bear secondary lamellae with a more or less branched shape. The elastic connective capsule which envelops the olfactory rosette possibly changes its shape along with water influx. In the olfactory epithelium, the supporting cells also have a secretory function, while no specialized mucous cells are visible; regarding this feature the olfactory epithelium of H. perlo differs from that of other chondrichthyan species. The immunohistochemical investigation of the sensory epithelium shows the absence of immunoreactivity for Gαolf in receptor neurons, which confirms previous observations in Chondrichthyes.

Presence of Sex Steroid-Metabolizing Enzymes in the Olfactory Mucosa of Rats

Although several lines of evidence have suggested that sex steroids influence olfaction, little is known about the cellular basis of steroid-metabolizing enzymes in the olfactory system. Thus, we aimed to examine gene expression and immunolocalization of four sex steroid-metabolizing enzymes in the olfactory mucosa (OM) of albino rats; steroid side chain-cleaving enzyme (P450scc), 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD-1), 17β-HSD type 2 (17β-HSD-2), and aromatase. P450scc is known to catalyze conversion from cholesterol to pregnenolone. 17β-HSD-1 catalyzes conversion from estrone to estradiol, and 17β-HSD-2 does the reverse. Aromatase catalyzes the conversion from testosterone to estradiol-17β. Messenger (m) RNAs of all four enzymes mentioned above were detected in the OM. Western blot analysis demonstrated that P450scc, 17β-HSD-1, and 17β-HSD-2 were detected in the OM. Immunoreactivity for these three enzymes was observed in sustentacular cells of the olfactory epithelium and acinar cells of Bowman's glands. Immunoelectron microscopy analysis demonstrated immunoreactivity for P450scc in mitochondria, and for 17β-HSD-1 and 17β-HSD-2 in the well-developed smooth endoplasmic reticulum and myeloid bodies of the sustentacular cells. The present study suggests that sustentacular cells and acinar cells of the Bowman's glands in the rat OM express at least three of the steroid-metabolizing enzymes, that is, P450scc 17β-HSD-1, and 17β-HSD-2, and de novo synthesis of estradiol takes place in the OM. Anat Rec, 300:402-414, 2017. © 2016 Wiley Periodicals, Inc.

Effects of urea on the molecules involved in the olfactory signal transduction: a preliminary study on Danio rerio

Among vertebrates, the physiologically uremic Chondrichthyes are the only class which are not presenting the ciliated olfactory receptor neurons in the olfactory neuroepithelium. The only sequenced genome for this class revealed only three olfactory receptor genes and the immunohistochemical detection of G protein alpha subunit typically coupled to the olfactory receptors (Gα(olf)) failed in different species. Chronic renal disease can represent a cause of olfactory impairment in human. In this context, our present study focused on investigating potential effects of high urea concentration on the olfactory epithelium of vertebrates. Larvae of the teleost fish Danio rerio were exposed to urea in order to assess the effects on the olfactory signal transduction; in particular on both the olfactory receptors and the Gα(olf). The endocytosis of neutral red dye in the olfactory mucosa was detected in control and urea-exposed larvae. The amount of neutral red dye uptake was used as a marker of binding and internalization of the Gα(olf). The neutral red dye endocytosis was not affected by urea exposure, hence suggesting that the presence of the Gα(olf) and their binding to the odorants are not affected by urea treatment, either. The presence and distribution of Gα(olf) were investigated in the olfactory epithelium of control and urea-exposed larvae, using a commercial antibody. The immunoreactivity was increased after urea treatment, suggesting an effect of urea on the expression or degradation of this G protein alpha subunit.

Olfactory Sensory Neuron Morphotypes in the Featherback Fish, Notopterus notopterus (Osteoglossiformes: Notopteridae)

BACKGROUND

As in other vertebrates, olfactory sensory neurons (OSNs) in fishes are the main components of sensory part of olfactory epithelium that relay olfactory information (smell and taste) to the brain.

PURPOSE

Objective of the present study was to analyze if any polymorphism occurs in the OSNs in a featherback fish, Notopterus notopterus as far as the teleost lineage is concerned.

METHODS

With the help of neuronal staining technique, polymorphism of OSNs in N. notopterus was studied.

RESULTS

Three polymorphic forms of OSNs were identified which are ciliated OSNs, microvillus OSNs and crypt OSNs. These morphotypes were identified on the basis of location of their somata within the depth of olfactory epithelium and resulting length of their dendrites. The ciliated OSNs have basally situated somata and long, thin dendrites with a few apically arranged cilia while microvillous OSNs have somata located midway in the epithelium and thick moderate-length dendrites with microvilli. Third cell type is crypt OSNs which are spherical or pear-shaped, located apically just close to the epithelial surface having cilia and microvilli in an invagination and devoid of any dendrite.

CONCLUSION

N. notopterus belongs to order Osteoglossiformes which is a representative of an early evolutionary lineage of teleost fishes. OSN polymorphism reported in the present work indicates that it is a fairly conserved trait throughout the evolution of teleosts. To our knowledge, we are the first ones to report OSN polymorphism in a member of the order Osteoglossiformes.

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.

Molecular dynamics simulations of water/mucus partition coefficients for feeding stimulants in fish and the implications for olfaction

The odorant partition coefficient is a physicochemical property that has been shown to dramatically influence odorant deposition patterns in the mammalian nose, leading to a chromatographic separation of odorants along the sensory epithelium. It is unknown whether a similar phenomenon occurs in fish. Here we utilize molecular dynamics simulations, based on a simplified molecular model of olfactory mucus, to calculate water/mucus partition coefficients for amino acid odorants (alanine, glycine, cysteine, and valine) that are known to elicit feeding behavior in fish. Both fresh water and salt water environments are considered. In fresh water, all four amino acids prefer the olfactory mucus phase to water, and the partition coefficient is shown to correlate with amino acid hydrophobicity. In salt water, a reversal in odorant partitioning is found, where each of the feeding stimulants (except glycine) prefer the water phase to olfactory mucus. This is due to the interactions between the salt ions and the odorant molecules (in the water phase), and between the salt and simplified mucin (in the olfactory mucus phase). Thus, slightly different odorant deposition patterns may occur in the fish olfactory organ in fresh and salt water environments. However, in both underwater environments we found that the variation of the water/mucus odorant partition coefficient is approximately one order of magnitude, in stark contrast to air/mucus odorant partition coefficients that can span up to six orders of magnitude. We therefore anticipate relatively similar deposition patterns for most amino acid odorants in the fish olfactory chamber. Thus, in contrast to terrestrial species, living in an underwater environment may preclude appreciable chromatographic odorant separation that may be used for spatial coding of odor identity across the olfactory epithelium. This is consistent with the reported lack of spatial organization of olfactory receptor neurons in the fish olfactory epithelium.

The somatotopic organization of the olfactory bulb in elasmobranchs

The olfactory bulbs (OBs) are bilaterally paired structures in the vertebrate forebrain that receive and process odor information from the olfactory receptor neurons (ORNs) in the periphery. Virtually all vertebrate OBs are arranged chemotopically, with different regions of the OB processing different types of odorants. However, there is some evidence that elasmobranch fishes (sharks, rays, and skates) may possess a gross somatotopic organization instead. To test this hypothesis, we used histological staining and retrograde tracing techniques to examine the morphology and organization of ORN projections from the olfactory epithelium (OE) to the OB in three elasmobranch species with varying OB morphologies. In all three species, glomeruli in the OB received projections from ORNs located on only the three to five lamellae situated immediately anterior within the OE. These results support that the gross arrangement of the elasmobranch OB is somatotopic, an organization unique among fishes and most other vertebrates. In addition, certain elasmobranch species possess a unique OB morphology in which each OB is physically subdivided into two or more "hemi-olfactory bulbs." Somatotopy could provide a preadaptation which facilitated the evolution of olfactory hemibulbs in these species.

Sensitivity and specificity of the olfactory epithelia of two elasmobranch species to bile salts

Odor detection in vertebrates occurs when odorants enter the nose and bind to molecular olfactory receptors on the cilia or microvilli of olfactory receptor neurons (ORNs). Several vertebrate groups possess multiple, morphologically distinct types of ORNs. In teleost fishes, these different ORN types detect specific classes of biologically relevant odorants, such as amino acids, nucleotides and bile salts. For example, bile salts are reported to be detected exclusively by ciliated ORNs. The olfactory epithelium of elasmobranch fishes (sharks, rays and skates) is comprised of microvillous and crypt ORNs, but lacks ciliated ORNs; thus, it was questioned whether the olfactory system of this group of fishes is capable of detecting bile salts. The present investigation clearly indicates that the olfactory system of representative shark and stingray species does detect and respond to bile salts. Additionally, these species detect glycine-conjugated, taurine-conjugated and non-conjugated bile salts, as do teleosts. These elasmobranchs are less sensitive to the tested bile salts than reported for both agnathans and teleosts, but this may be due to the particular bile salts selected in this study, as elasmobranch-produced bile salts are commercially unavailable. Cross-adaptation experiments indicate further that the responses to bile salts are independent of those to amino acids, a major class of odorant molecules for all tested fishes.

Retrograde labeling and fine structure of olfactory receptor neurons in cat sharks

Ciliated and microvillar olfactory receptor cells have been reported in many fish species, including teleosts and elasmobranchs. Morphological studies have suggested that microvillar cells are the only olfactory receptor cells in the elasmobranchs; however, there is no direct evidence for this hypothesis. Here we used a cat shark (Scyliorhinus torazame) to determine the cell type of the olfactory receptor cells in elasmobranchs. Retrograde labeling with a fluorescent dye, Dil, labeled only cells in the second layer from the surface of the olfactory epithelium, suggesting that ciliated cells located in the surface layer are not olfactory receptor cells. In addition, electron microscopic observation revealed that the labeled cells in the second layer have a thin dendritic knob extending from the cell body to the free surface of the epithelium. A part of the dendritic knob facing the mucous layer did not have ciliary structures. These results provide evidence that the aciliate cells are the only olfactory receptor cells in the cat shark olfactory organ.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

Neuronal nitric oxide synthase (nNOS) immunoreactivity in the olfactory system of a cartilaginous fish

Nitric oxide is a regulative molecule with important roles in the olfactory system of vertebrates. Chondrichtyans have a key position in vertebrate evolution and nothing is known about nitric oxide in their olfactory system. Aim of this work was to investigate the neuronal nitric oxide synthase (nNOS) immunoreactivity in the olfactory system of the shark Scyliorhinus canicula. Because nitric oxide is often related to GABA in the olfactory system, also the distribution of GABA and its synthesis enzyme GAD has been investigated. In the olfactory epithelium scattered cells in the basal and medial zone of the epithelium thickness presented nNOS-like immunoreactivity. In the olfactory bulb the nNOS-like immunoreactivity has been highlighted in nerve fibers around some blood vessels and in scattered GABAergic granule cells. The presence of nNOS in the olfactory system of S. canicula is overall lesser than that described in other vertebrates, even if nitric oxide probably keeps some essential functions.

Immunolocalization of G-protein alpha subunits in the olfactory system of the cartilaginous fish Scyliorhinus canicula

In the olfactory and vomeronasal systems of vertebrates, the morphology of the receptor neurons, the receptor gene family they express, the G-protein coupled with the receptor (in particular the G-protein alpha subunit), and their projection to the olfactory bulb are correlated. Much information about this complicated system have been collected in different groups, but nothing is known about Chondrichthyes. In this work, the presence and distribution of immunoreactivity for different types of G-protein alpha subunit (Galpha(o), Galpha(q) and Galpha(s/olf)) were investigated in the olfactory mucosa and olfactory bulb of the shark Scyliorhinus canicula. Only Galpha(o)-like immunoreactivity was detected in the olfactory mucosa and bulb, both in tissues and homogenates. Its distribution was partially similar to that found in other vertebrates: it was localized in the microvillous receptor neurons, in numerous axon bundles of the fila olfactoria, in the stratum nervosum and in the most of glomeruli in the stratum glomerulosum. No immunoreactivity was instead observed in the crypt neurons, the second type of olfactory neurons present in cartilaginous fish. The projections of crypt neurons to olfactory bulb probably correspond to the few ventrally-located glomeruli which were negative to the antiserum against Galpha(o). These data suggest, in S. canicula, different olfactory neuron types send projections to the olfactory bulb with a segregated distribution, as observed in other vertebrates.

G protein alpha subunits in the olfactory epithelium of the holocephalan fish Chimaera monstrosa

Receptor neurons in the olfactory and vomeronasal epithelia of vertebrates have dendritic specialization that is correlated to the receptor gene family they express and the G protein coupled with that receptor (in particular the G protein alpha subunit). There are not very many data in the literature about the morphological and molecular features of the olfactory epithelium of Chondrichthyes. In this work, the presence and distribution of different types of G protein alpha subunits (Galpha(o), Galpha(q) and Galpha(olf)) were investigated in the olfactory epithelium of the holocephalan Chimaera monstrosa using immunohistochemistry. Only Galpha(o)-like immunoreactivity was detected in the microvillous receptor neurons and in numerous axon bundles of the fila olfactoria. These preliminary data are in agreement with studies showing G protein alpha subunits in elasmobranchs and support the data present in the literature about putative odorant receptor families in the sequenced genome of the holocephalan Callorhinchus milii.

Morphometric and ultrastructural comparison of the olfactory system in elasmobranchs: the significance of structure-function relationships based on phylogeny and ecology

This study investigated the relationship between olfactory morphology, habitat occupancy, and lifestyle in 21 elasmobranch species in a phylogenetic context. Four measures of olfactory capability, that is, the number of olfactory lamellae, the surface area of the olfactory epithelium, the mass of the olfactory bulb, and the mass of the olfactory rosette were compared between individual species and groups, comprised of species with similar habitat and/or lifestyle. Statistical analyses using generalized least squares phylogenetic regression revealed that bentho-pelagic sharks and rays possess significantly more olfactory lamellae and larger sensory epithelial surface areas than benthic species. There was no significant correlation between either olfactory bulb or rosette mass and habitat type. There was also no significant difference between the number of lamellae or the size of the sensory surface area in groups comprised of species with similar diets, that is, groups preying predominantly on crustaceans, cephalopods, echinoderms, polychaetes, molluscs, or teleosts. However, some groups had significantly larger olfactory bulb or rosette masses than others. There was little evidence to support a correlation between phylogeny and morphology, indicating that differences in olfactory capabilities are the result of functional rather than phylogenetic adaptations. All olfactory epithelia exhibited microvilli and cilia, with microvilli in both nonsensory and sensory areas, and cilia only in sensory areas. Cilia over the sensory epithelia originated from supporting cells. In contrast to teleosts, which possess ciliated and microvillous olfactory receptor types, no ciliated olfactory receptor cells were observed. This is the first comprehensive study comparing olfactory morphology to several aspects of elasmobranch ecology in a phylogenetic context.

Localization of G protein alpha subunits and morphology of receptor neurons in olfactory and vomeronasal epithelia in Reeve's turtle, Geoclemys reevesii

Most vertebrates have two nasal epithelia: the olfactory epithelium (OE) and the vomeronasal epithelium (VNE). The apical surfaces of OE and VNE are covered with cilia and microvilli, respectively. In rodents, signal transduction pathways involve G alpha olf and G alpha i2/G alpha o in OE and VNE, respectively. Reeve's turtles (Geoclemys reevesii) live in a semiaquatic environment. The aim of this study was to investigate the localization of G proteins and the morphological characteristics of OE and VNE in Reeve's turtle. In-situ hybridization analysis revealed that both G alpha olf and G alpha o are expressed in olfactory receptor neurons (ORNs) and vomeronasal receptor neurons (VRNs). Immunocytochemistry of G alpha olf/s and G alpha o revealed that these two G proteins were located at the apical surface, cell bodies, and axon bundles in ORNs and VRNs. Electron microscopic analysis revealed that ORNs had both cilia and microvilli on the apical surface of the same neuron, whereas VRNs had only microvilli. Moreover G alpha olf/s was located on only the cilia of OE, whereas G alpha o was not located on cilia but on microvilli. Both G alpha olf/s and G alpha o were located on microvilli of VNE. These results imply that, in Reeve's turtle, both G alpha olf/s and G alpha o function as signal transduction molecules for chemoreception in ORNs and VRNs.

First detection of olfactory marker protein (OMP) immunoreactivity in the olfactory epithelium of a cartilaginous fish

Olfactory marker protein (OMP) is a protein expressed in the mature olfactory and vomeronasal neurons of many vertebrates, such as mammals, amphibians and bony fishes. Aim of this work was to investigate the OMP expression in the olfactory epithelium of the shark Scyliorhinus canicula (Linnaeus, 1758), by immunohistochemistry (IHC). Immunoreactivity was detected in the olfactory receptor neurons, in the crypt neurons and in the nerve fibers below the epithelium. Although very little is known about the OMP's function, its involvement in synaptogenesis, transduction cascade, neurogenesis and development of olfactory system has been suggested. The present work shows for the first time OMP's presence in a cartilaginous fish.

Observations of crypt neuron-like cells in the olfactory epithelium of a cartilaginous fish

A new receptor neuron (RN) type was recently described in bony fish olfactory epithelium (OE): the crypt receptor neuron. This name is due to its main feature: the presence, at the apical part, of a deep invagination into which cilia protrude. The presence of this receptor neuron type is well documented in different species of bony fishes but it has never been described in cartilaginous fishes. In this study we demonstrate that crypt neuron-like cells are present in the olfactory epithelium of the elasmobranch Scyliorhinus canicula (Linnaeus, 1758). Histological observations allowed us to detect the presence of a few egg-shaped cells, characterized by a crypt like zone; alpha-tubulin immunoreactivity suggested the presence of cilia in the same area; fluorocrome conjugated lectin bindings suggested a distinctive mucus composition inside the presumptive crypt. The possible presence of crypt neuron-like cells in chondrichthyes would represent an interesting common feature between bony and cartilaginous fishes.

Ultrastructural study of the olfactory organ in adult and developing European sea bass,Dicentrarchus labrax

The olfactory organ of the European sea bass (Dicentrarchus labrax) in adults and during development has been studied by light microscopy and by transmission and scanning electron microscopy. This organ includes two cavities, each extended by an accessory sac and opening to the outside through two nostrils. It contains a rosette consisting of about forty lamellae. The olfactory epithelium is characterized by the presence of two types of receptor cells, ciliated or with microvilli, and numerous ciliated nonsensory cells. Rod cells, essentially found in the altered epithelia of farmed bass, and rodlet cells are also observed. The olfactory organ forms very early in the developmental process. Two olfactory pits holding both types of sensory receptors appear 24 h before hatching. The ciliated nonsensory cells only appear at the end of the endotrophic period, shortly before the mouth opens. Although it is rather unspectacular during the larval stage, the development of the olfactory organ is characterized at the start of the juvenile stage by three important events: the formation of the nostrils, the hollowing of the accessory sacs, and the development of the rosette. This is created by raising the floor of the cavity and forming successive folds, which are the lamellae where the sensory epithelium is found.