Fine structure of the epidermis of the optic tentacle in a slug, Limax flavus L.

The epidermis at the tip of the optic tentacle in Limax flavus is constructed of columnar epithelial cells, distal processes of nerve cells, and scattered processes of the collar cells. The epithelial cells extend stout microvilli called plasmatic processes by Wright perpendicularly from the free surface. Each plasmic process branches into a few terminal twigs embedded in a fuzzy filamentous substance. Most nerve cells have their nuclei under the basal lamina. The distal processes of these nerve cells reach the free surface and send long microvilli to form the spongy layer under a filamentous covering. At the side surface of the tentacle the epithelial cells are cuboidal or squamous and the neural elements are fewer. Here, no spongy layer is formed; and the collar cell processes are replaced by the lateral cell processes. Peculiar secretion granules are contained in the lateral and collar cell processes as well as in their cell bodies situated beneath the basal lamina.     

An ultrastructural and immunohistological study of the rat olfactory epithelium: Unique properties of olfactory sensory cells

The olfactory epithelium contains three cell types: basal cells, supporting cells and sensory neurons. Electron microscopy as well as immunofluorescence microscopy with intermediate-filament antibodies were used to study the rat olfactory epithelium in order to obtain more information about these different cell types and to try to investigate their histogenetic origins. We found mitoses in the basal-cell layer, as well as multiple centrioles and tonofilaments in some basal cells. As revealed by electron microscopy, the supporting cells contained tonofilaments and reacted strongly with antibodies to keratin, in line with their known epithelial nature. When antibodies to other intermediate-filament types were used, i.e. glial fibrillary acidic protein, vimentin, desmin and neurofilaments, no reaction was seen in the cells of the olfactory epithelium, with the exception of occasional staining of a few axons in the subepithelial layer by neurofilament antibodies. In particular, the cell bodies, dendrites and most axons of the sensory neurons were negative for a variety of antibodies against neurofilaments. Olfactory sensory neurons therefore belong to the very few cells in adult animals which seem to lack intermediate filaments. We discuss whether this finding is related to the fact that these cells are also unique among neurons in that they are not permanent cells but constantly turn over.     

Observations on the olfactory organ of adult and juvenile Octopus joubini.

The olfactory organ has an epithelium containing many sense cells and a large subepithelial mass of receptor cells. The epithelium includes cells with cup-shaped, ciliated endings, and hollow, flask-shaped sense cells with ciliated cavities that open to the surface, through a small pore. Below the epithelium are large hollow cells with ciliated cavities and distal processes that either form patent connections between the ciliated cavity and the surface or have a ciliated ending at the surface. There are many synapses between processes in the olfactory nerve. The possible chemosensory function of the olfactory organ is discussed.     

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

Summary 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.     

Topographical features of the substratum for growth of pioneering neurons in the Manduca wing disc

The sensory neurons of the Manduca wing form a planar network nestled between the wing's upper and lower monolayers. The pioneering axons of this network grow in a distal-to-proximal direction over the basal surface of the upper epithelial monolayer. The basal surface of this monolayer has been examined ultrastructurally during the period of axonal outgrowth. The cellular terrain traversed by axons shows a graded distribution of epithelial processes, with the number of processes increasing in a proximal direction. Growth cones of axons, therefore, encounter increasing surface areas for contact with their substratum as they move toward the base of the wing. Because a basal lamina is laid down over these epithelial processes after axons have pioneered the neural pathways of the wing, axonal guidance cues apparently lie on surfaces of these basal processes. At branch points of the neural pathway examined in this study, axons avoid pathways in which the basal surfaces of cells in the upper wing monolayer interdigitate with basal surfaces of underlying tracheal cells. This interaction between wing epithelial cells and tracheal epithelial cells could act as a physical barrier to axonal outgrowth.     

Structure of isolated sensilla and sensory neurons in vitro: Observations on developing silkmoth antennae

Summary By combined enzymatic and mechanical treatment, it was possible to dissociate the sensory epithelium of developing antennae of male Antheraea polyphemus and A. pernyi silkmoths from the stage of separation of the antennal branches up to the early stages of cuticle deposition. Large numbers of entire developing trichoid sensilla were isolated. These are characterized by a large trichogen cell with a long apical, hair-forming process and a large nucleus. A cluster of 2–3 sensory neurons, enclosed by the thecogen cell, is situated in the basal region. The dendrites run past the nucleus of the trichogen cell into the apical process from which they protrude laterally. The nuclei of the tormogen and a 4^th enveloping cell can be distinguished near the base of the prospective hair. After further dissociation, only the neuron clusters remain, still enclosed by their thecogen cell and often attached to the antennal branch nerve via their axons. It is finally possible to disrupt the thecogen cells and the axons, leaving the sensory neurons with inner dendritic segments and axon stumps. The majority of these neurons can be expected to be olfactory.     

The system of cerebrospinal fluid-contacting neurons. Its supposed role in the nonsynaptic signal transmission of the brain

Recent investigations confirm the importance of nonsynaptic signal transmission in several functions of the nervous tissue. Present in various periventricular brain regions of vertebrates, the system of cerebrospinal fluid (CSF)-contacting neurons seems to have a special role in taking up, transforming and emitting nonsynaptic signals mediated by the internal and external CSF and intercellular fluid of the brain. Most of the CSF-contacting nerve cells send dendritic processes into the internal CSF of the brain ventricles or central canal where they form terminals bearing stereocilia and a 9+0-, or 9+2-type cilium. Some of these neurons resemble known sensory cells of chemoreceptor-type, others may be sensitive to the pressure or flow of the CSF, or to the illumination of the brain tissue. The axons of the CSF-contacting neurons transmit information taken up by dendrites and perikarya to synaptic zones of various brain areas. By forming neurohormonal terminals, axons also contact the external CSF space and release various bioactive substances there. Some perikarya send their axons into the internal CSF, and form free endings there, or synapses on intraventricular dendrites, perikarya and/or on the ventricular surface of ependymal cells. Contacting the intercellular space, sensory-type cilia were also demonstrated on nerve cells situated in the brain tissue subependymally or farther away from the ventricles. Among neuronal elements entering the internal CSF-space, the hypothalamic CSF-contacting neurons are present in the magnocellular and parvicellular nuclei and in some circumventricular organs like the paraventricular organ and the vascular sac. The CSF-contacting dendrites of all these areas bear a solitary 9 x 2+0-type cilium and resemble chemoreceptors cytologically. In electrophysiological experiments, the neurons of the paraventricular organ are highly sensitive to the composition of the ventricular CSF. The axons of the CSF-contacting neurons terminate not only in the hypothalamic synaptic zones but also in tel-, mes- and rhombencephalic nuclei and reach the spinal cord as well. The supposed chemical information taken up by the CSF-contacting neurons from the ventricular CSF may influence the function of these areas of the central nervous system. Some nerve cells of the photoreceptor areas form sensory terminals similar to those of the hypothalamic CSF-contacting neurons. Special secondary neurons of the retina and pineal organ contact the retinal photoreceptor space and pineal recess respectively, both cavities being embryologically derived from the 3rd ventricle. The composition of these photoreceptor spaces is important in the photochemical transduction and may modify the activity of the secondary neurons. Septal and preoptic CSF-contacting neurons contain various opsins and other compounds of the phototransduction cascade and represent deep encephalic photoreceptors detecting the illumination of the brain tissue and play a role in the regulation of circadian and reproductive responses to light. The medullo-spinal CSF-contacting neurons present in the oblongate medulla, spinal cord and terminal filum, send their dendrites into the fourth ventricle and central canal. Resembling mechanoreceptors of the lateral line organ, the spinal CSF-contacting neurons may be sensitive to the pressure or flow of the CSF. The axons of these neurons terminate at the external CSF-space of the oblongate medulla and spinal cord and form neurohormonal nerve endings. Based on information taken up from the CSF, a regulatory effect on the production or composition of CSF was supposed for bioactive materials released by these terminals. Most of the axons of the medullospinal CSF-contacting neurons and the magno- and parvicellular neurosecretory nuclei running to neurohemal areas (neurohypophysis, median eminence, terminal lamina, vascular sac and urophysis) do not terminate directly on vessels, instead they form neurohormonal nerve terminals attached by half-desmosomes on the basal lamina of the external and vascular surface of the brain tissue. Therefore, the bioactive materials released from these terminals primarily enter the external CSF and secondarily, by diffusion into vessels and the composition of the external CSF, may have a modulatory effect on the bioactive substances released by the neurohormonal terminals. Contacting the intercellular space, sensory-type cilia were also demonstrated on nerve cells situated subependymally or farther away from the ventricles, among others in the neurosecretory nuclei. Since tight-junctions are lacking between ependymal cells of the ventricular wall, not only CSF-contacting but also subependymal ciliated neurons may be influenced by the actual composition of the CSF besides that of the intercellular fluid of the brain tissue. According to the comparative histological data summarised in this review, the ventricular CSF-contacting neurons represent the phylogenetically oldest component detecting the internal fluid milieu of the brain. The neurohormonal terminals on the external surface of the brain equally represent an ancient form of nonsynaptic signal transmission.     

Identification of cytoskeletal markers for the different microvilli and cell types of the rat vomeronasal sensory epithelium

The vomeronasal organ (VNO) of the mammal nose is specialized to detect pheromones. The presumed site of the chemosensory signal transduction of pheromones is the vomeronasal brush border of the VNO sensory epithelium, which has been shown to contain two different sets of microvilli: (i) the tall microvilli of supporting cells and (ii) the short microvilli of the chemoreceptive VNO neurons that branch and intermingle with the basal portions of the longer supporting cell microvilli. A key problem when studying the subcellular distribution of possible VNO signal transduction molecules at the light microscope level is the clear discrimination of immunosignals derived from dendritic microvilli of the VNO neurons and surrounding supporting cell structures. In the present study we therefore looked for cytoskeletal marker proteins, that might help to distinguish at the light microscope level between the two sets of microvilli. By immunostaining we found that the VNO dendritic microvilli can be selectively labelled with antibodies to the calcium-sensitive actin filament-bundling protein villin, whereas supporting cell microvilli contain the actin filament cross-linking protein fimbrin, but not villin. Useful cytoplasmic marker molecules for cellular discrimination were cytokeratin 18 for supporting cells and β-tubulin for dendrites of VNO neurons. A further finding was that the non-sensory epithelium of the rat VNO contains brush cells, a cell type that appears to be involved in certain aspects of chemoreception in the gut. Brush cells or other structures of the vomeronasal brush border did not contain α-gustducin.     

Projection pattern of nerve fibers from the septal organ: DiI-tracing studies with transgenic OMP mice

The septal organ represents one of the three chemosensory subsystems found in most vertebrate species. Analyzing the projection pattern of septal organ neurons using the OMP-GFP transgenic mouse line revealed that axons navigate in highly variable fiber tracks across the main olfactory epithelium toward the main olfactory bulb. All septal organ axons cross through the cribriform plate at a spatially defined site and terminate exclusively in the posterior, ventromedial aspect of the bulb. Here, one portion of axons forms a dense network on the medial side where they apparently enter glomeruli which are mainly innervated by axons of olfactory sensory neurons from the main olfactory epithelium. Another significant portion of the axons targets a few glomeruli which appear to receive input exclusively from the septal organ neurons.     

The Olfactory System as a Model to Study Axonal Growth Patterns and Morphology In Vivo

The olfactory system has the unusual capacity to generate new neurons throughout the lifetime of an organism. Olfactory stem cells in the basal portion of the olfactory epithelium continuously give rise to new sensory neurons that extend their axons into the olfactory bulb, where they face the challenge to integrate into existing circuitry. Because of this particular feature, the olfactory system represents a unique opportunity to monitor axonal wiring and guidance, and to investigate synapse formation. Here we describe a procedure for in vivo labeling of sensory neurons and subsequent visualization of axons in the olfactory system of larvae of the amphibian Xenopus laevis. To stain sensory neurons in the olfactory organ we adopt the electroporation technique. In vivo electroporation is an established technique for delivering fluorophore-coupled dextrans or other macromolecules into living cells. Stained sensory neurons and their axonal processes can then be monitored in the living animal either using confocal laser-scanning or multiphoton microscopy. By reducing the number of labeled cells to few or single cells per animal, single axons can be tracked into the olfactory bulb and their morphological changes can be monitored over weeks by conducting series of in vivo time lapse imaging experiments. While the described protocol exemplifies the labeling and monitoring of olfactory sensory neurons, it can also be adopted to other cell types within the olfactory and other systems.     

Immunolabeled neuroactive substances in the osphradium of the pond snail Lymnaea stagnalis

The osphradium of molluscs is assumed to be a sensory organ. The present investigation in Lymnaea stagnalis has established two ultrastructurally different types of dendrites in the sensory epithelium. Cells immunoreactive to leucine-enkephalin and FMRFamide send processes to the sensory epithelium. These neurons of the osphradial ganglion are thus considered to be part of the sensory system, as are methionine-enkephalin-immunoreactive cells in the mantle wall in the vicinity of the osphradium. The complexity of the osphradial ganglion is further demonstrated by serotonin-immunoreactive neurons innervating the muscular coat around the osphradial canal and methionine-enkephalin-immunoreactive cells sending projections to the central nervous system.     

The Neuroanatomical Organization of Projection Neurons Associated with Different Olfactory Bulb Pathways in the Sea Lamprey,Petromyzon marinus

Although there is abundant evidence for segregated processing in the olfactory system across vertebrate taxa, the spatial relationship between the second order projection neurons (PNs) of olfactory subsystems connecting sensory input to higher brain structures is less clear. In the sea lamprey, there is tight coupling between olfaction and locomotion via PNs extending to the posterior tuberculum from the medial region of the olfactory bulb. This medial region receives peripheral input predominantly from the accessory olfactory organ. However, the axons from olfactory sensory neurons residing in the main olfactory epithelium extend to non-medial regions of the olfactory bulb, and the non-medial bulbar PNs extend their axons to the lateral pallium. It is not known if the receptive fields of the PNs in the two output pathways overlap; nor has the morphology of these PNs been investigated. In this study, retrograde labelling was utilized to investigate the PNs belonging to medial and non-medial projections. The dendrites and somata of the medial PNs were confined to medial glomerular neuropil, and dendrites of non-medial PNs did not enter this territory. The cell bodies and dendrites of the non-medial PNs were predominantly located below the glomeruli (frequently deeper in the olfactory bulb). While PNs in both locations contained single or multiple primary dendrites, the somal size was greater for medial than for non-medial PNs. When considered with the evidence-to-date, this study shows different neuroanatomical organization for medial olfactory bulb PNs extending to locomotor control centers and non-medial PNs extending to the lateral pallium in this vertebrate.     

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 alarm reaction in crucian carp is mediated by olfactory neurons with long dendrites

In the present study, we applied a lipophilic tracer, Dil (1,1-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), to the synaptic region of the medial olfactory bulb in formaldehyde-fixed preparations from the crucian carp. We observed staining both in the axons of secondary neurons leading to the brain and in the olfactory receptor neurons (ORNs) of the olfactory epithelium. In those preparations, where staining of the tract was restricted to axons of the medial part of the medial olfactory tract, the majority (86-98%) of the somata of the sensory neurons were found in the deep layers of olfactory epithelium. Since the medial bundle of the medial olfactory tract mediates alarm behaviour in the crucian carp, we conclude that the sensory neurons with long dendrites participate in the reception of alarm pheromones.     

Fine structure of the lateral-line organ of the common eel,Anguilla japonica

Summary The sensory epithelium of the lateral line organ of the common eel consists of two types of cells, (sensory and supporting). The sensory cell bears a kinocilium together with about 40 to 60 stereocilia on its surface. The kinocilium is situated either at rostral or at caudal margin of this cilial group. Such polarity of the cilial group of one cell is inverse to that of an adjacent cell.Two types of crystal-like inclusions exist in the sensory cells, consisting of granules 100 Å in diameter. Granules in one type are arranged regularly whereas those in the other rather irregularly.Two types of nerve endings exist at the base of sensory cells: one is predominant in number and contains few vesicles, accompanied by a dense spherical body surrounded by small vesicles in the sensory cell and the other is rare in number and contains many vesicles, accompanied by a small flat sac just beneath the plasma membrane of the sensory cell.The supporting cells contain numerous mitochondria, a well developed Golgi apparatus and rough-surfaced endoplasmic reticulum, and surround a sensory cell completely. Physiologic significance of some of these components is discussed.     

Surface specializations of the epithelial cells at the tip of the optic tentacle,dorsal surface of the head and ventral surface of the foot in Helix aspersa

Summary Morphologically the surface specializations of the epithelium covering the dorsal head and ventral foot regions in Helix aspersa consists either of cilia or microvilli respectively. The epithelium at the tip of the optic tentacle is a simple one. Each epithelial cell has a number of cilia-like projections from their free surfaces. These projections usually branch at their tips into two or three slender, microvilli-like structures. From the bases of the cilia-like projections arise numerous, tubular processes which form a thick, spongy layer interspersed between these projections. The microvilli-like structures are immersed in a fine, fibrous mat; unlike the fibrous mats on the dorsal head and ventral foot epithelia this material does not autofluoresce. It is suggested that it arises from the collar cells and not from typical mucocytes. The functional relationship between these surface specializations of the optic tentacle epithelium and the abundance of sensory axons in this region is discussed. These epithelial cell projections on the tentacle probably function not only as a protective covering but also to create a fluid trap for odours in the ambient air. The various contacts between epithelial cells serve to maintain the integrity of the epithelium while allowing for stretching due to protrusion of the tentacle.This work has been supported by the Australian Research Grants Committee.     

Qualitative and quantitative freeze-fracture studies on olfactory and nasal respiratory structures of frog,ox, rat,and dog

Summary A comparative study using freeze-fracturing has been made of surface structures of olfactory and nasal respiratory epithelia of frog, ox, rat and dog. Special attention has been paid to cilia and microvilli present at these surfaces, although the observations include various other structures such as small intracellular vacuoles present in the olfactory receptor endings and infrequent brush cells. Within the mucus overlying the olfactory epithelium membranous vesicles, often attached to olfactory cilia, are seen. Some of these show intramembranous particle distributions similar to those of the rest of the cilia, whereas others are devoid of particles. Smooth vesicles are also found in the mucus of other types of epithelium (respiratory epithelium and Bowman's glands). The freeze-fracture morphology of intracellular secretory vacuoles present in olfactory supporting, Bowman's and respiratory glandular cells of the frog is similar in all these epithelia. Quantitative comparisons are made of the different structures of interest. When corrected for cilia which were not observed, mammalian receptor endings bear 17 cilia on average, whereas frog receptor endings have 6 cilia. The relative magnitudes of the diameters of the cilia and microvilli are, except for frog, the same for all species studied. Dimensions of other structures e.g., axons, dendrites and dendritic endings are compared in the various species. Freeze-fracture diameters are usually larger than those seen by techniques using dehydration. Dendritic ending densities range from 4.5 × 10⁶ (frog) to 8.3 × 10⁶ (dog) endings per cm². Possible sex-dependent differences are only found for these densities and dendritic ending diameters.     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.     

Sensory cells in the head skin of pond snails

Summary Several types of receptor endings were identified with scanning electron microscopy and silver-impregnation techniques in the skin of the tentacles, lips, dorsal surface of the head and mouth region of the pond snails Lymnaea stagnalis and Vivipara viviparus. Sensory endings at the tips of dendrites of primary receptor neurones, scattered below the epithelium, differ in structure, i.e., the endings exposed to the surface of the skin possess different proportions of cilia and microvilli, which vary in number, length, and packing. Type-I endings have microvilli and a few (1–5) cilia, 5–12 m in length. Type-2 endings have abundant (20–40), interwoven long (9–12 m) cilia and random microvilli. Type-3 endings show typical packing of 10–25 cilia in the form of bundles or brushes. They may be composed either of long (9–18 m) or short (2–7 m) cilia, or of both long and short ones. Microvilli here are absent. Type-4 endings have only microvilli. Two other types of skin receptors do not extend their sensory endings to the surface and can be indentified only in silver-stained preparations. Type-5 endings are branching dendrites of skin receptors cells that terminate among epithelial cells. In type-6, the sensory endings also terminate among epithelial cells but their cell bodies are located outside of the skin. In both species all skin regions examined possess the receptors of all six types differing only in their relative proportion. Possible functional roles of different receptors are discussed.     

Morphology and histochemistry of the peripheral olfactory organ in the round goby,Neogobius melanostomus (Teleostei: Gobiidae)

This first comprehensive study of the peripheral olfactory organ from a representative of the large and economically important order of teleost fishes, the Perciformes, shows a compact structure with olfactory sensory neurons distributed widely throughout the olfactory chamber. The spatial organization of the nasal cavity in the bottom-dwelling round goby (Gobiidae, Neogobius melanostomus) was examined using impression material injection, immunocytochemistry, and transmission electron microscopy. The olfactory chamber contains a single olfactory lamella; prominent dorsocaudal lachrymal and ethmoidal accessory nasal sacs are situated ventrocaudal to the chamber. The location of the olfactory mucosa within the olfactory chamber is novel for teleost fish, as it extends beyond the ventral surface to the lateral and dorsal regions. Microvillar olfactory sensory neurons and ciliated olfactory sensory neurons were identified by transmission electron microscopy and the spatial distribution of these two cell types was assessed through immunocytochemistry against olfactory receptor coupled G-proteins. Both G(alphaolf)-immunoreactive ciliated olfactory sensory neurons and the G(alphao)-immunoreactive microvillar form were located throughout the olfactory epithelium. Ciliated crypt cells were G(alphao) immunoreactive and were found throughout the olfactory epithelium of some specimens. The widespread occurrence of olfactory sensory neurons in the olfactory chamber supports the idea that olfactory signaling is important to the survival of the round goby. The prominence of the lachrymal and ethmoidal accessory nasal sacs indicates the capacity to regulate the flow of odorant molecules over the sensory surface of the olfactory sensory neurons, possibly through a pump-like mechanism driven by opercular activity associated with gill ventilation.