Sensory structures at the surface of fish skin I. Putative chemoreceptors

Fish skin contains solitary epidermal sensory cells which, on evidence from their cytology, are believed to be chemosensory. The external appearance of the apical sensory processes of these cells, as seen by scanning electron microscopy, is shown in four species of ostariophysan teleosts, and is compared with the morphology of the pores of external taste buds. The apical processes of the gustatory cells are simple in form in all cases so far investigated in gnathostome fishes, but in some cases the solitary sensory cells have apical processes divided distally into a number of smaller processes. In the dipnoan fish Protopterus amphibius , external taste buds have simple blunt gustatory processes protruding through a cap of mucus that covers the taste bud pore. Solitary sensory cells in this species have a bulbous undivided apical process. In the lampreys, the 'end buds' have an apical morphology different from the taste bud pores of teleost fish. Lamprey epidermis has numerous solitary sensory cells each bearing a number of microvilli.     

Gills of the medaka (Oryzias latipes): A scanning electron microscopy study

The general morphology and surface ultrastructure of the gills of adult and larvae medaka (Oryzias latipes) were studied in freshwater and seawater using scanning electron microscopy. The gills of all examined fish were structurally similar to those of other teleosts and consisted of four pairs of arches supporting (i) filaments bearing lamellae and (ii) rakers containing taste buds. Three cell types, specifically pavement cells, mitochondria‐rich cells (MRCs), and mucous cells, constituted the surface layer of the gill epithelium. Several distinctive characteristics of medaka gills were noted, including the presence of regularly distributed outgrowth on the lamellae, enlarged filament tips, the absence of microridges in most pavement cells in the filament and lamellae and the presence of MRCs in the arch at the filament base. A rapid mode of development was recorded in the gills of larval fish. At hatching, the larvae already had four arches with rudimentary filaments, rakers, and taste buds. The rudimentary lamellae appeared within 2 days after hatching. These results suggest the early involvement of larval gills in respiratory and osmoregulation activities. The responses of the macrostructures and microstructures of gills to seawater acclimation were similar in larvae and adult fish and included modification of the apical surface of MRCs, confirming the importance of these cells in osmoregulation. The potential roles of these peculiarities of the macrostructures and microstructures of medaka gills in the major functions of this organ, such as respiration and osmoregulation, are discussed.     

伊乐藻和黑藻断枝根和芽的发生及生长研究

为了解外来种伊乐藻的无性繁殖力、评价其生态安全性,采用插植方式比较研究了伊乐藻(Elodea nuttallii)和本土种黑藻(Hydrilla verticillata)两种沉水植物不同节数(1至4节)和不同节位断枝的不定根和新芽的发生及生长情况。通过室内4周的3次平行实验,结果表明:两者顶芽段均有形成不定根继而形成新植株的能力,而顶芽以下茎段只有本身具有腋芽的断枝才有形成新芽和不定根的能力。两者具相同节数的断枝形成不定根的百分率及根、芽长度,以具顶芽断枝的均明显高于不具顶芽断枝的,具顶芽四节断枝的不定根生成率最高达到90%以上。不具顶芽断枝形成新芽和不定根的百分率及长度随着断枝节数的增加均呈显著递增趋势,每类断枝的发芽率显著大于其生根率;伊乐藻和黑藻枝条一般分别每7节和5节具有一个腋芽,只有具腋芽断枝才能存活,因此,对不具顶芽断枝,7节和5节分别是其形成新苗所需的最短断枝长度。根和芽的长度随节位的下降大致呈递增的趋势。但是节数对形成根、芽的影响显著大于节位的影响。具顶芽断枝的顶芽的增长量和具顶芽4节断枝的生物量增量伊乐藻的高于黑藻,其余指标伊乐藻均显著低于黑藻。伊乐藻断枝的繁殖力总体上低于黑藻。       

The barbel‐like specialization of the pelvic fins in Ophidion rochei (Ophidiidae)

Pelvic fins in Ophidion rochei are reduced to four rod‐like structures situated at the ventral jaws. While the fish is swimming, they make continuous sweeping movements on the bottom. This paper examines and describes the anatomy of the pelvic fins to determine the possible functions of these appendages in relation to the mode of life of this fish species. The pelvic fins of O. rochei show strong similarities with barbels because they have identical sensory cell types, (taste buds, solitary chemosensory cells, and goblet cells), innervations and sensory function. Having nocturnal habits, specialization of pelvic fins in O. rochei corresponds to a supporting role to the life in dark environment. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Gustatory apparatus of the oropharyngeal cavity in juvenile rainbow trout <Emphasis Type="Italic">Parassalmo mykiss</Emphasis>

Study of the structural organization of gustatory apparatus in rainbow trout Parasalmo mykiss performed using electron scanning microscopy demonstrated that external taste buds are absent in this species in skin covers of the head and in the circumoral region. In the oropharyngeal cavity (oral and gill cavities and pharynx) of the rainbow trout, a well-developed gustatory receptor apparatus was found. In correspondence with specific features of morphology and anatomy of the skull, taste buds form seven gustatory zones. Morphometric analysis demonstrated differences between gustatory zones in the pattern and density of distribution of taste buds, as well as in average sizes of their sensory field. Zones of similar innervation have many common features in morphology. Morphologically similar zones form three regions in the oropharyngeal cavity: rostral, central, and caudal. A tendency for a decrease in the concentration of taste buds in the rostrocaudal direction common for all sensory zones was revealed. The highest concentration of taste buds was recorded at papillae of rostral regions near big teeth. A typical feature of taste buds in rainbow trout is irregular shape of the taste pore. Analysis of ultrastructural specific features of apical processes of taste cells allows us to distinguish five cell shapes in the composition of taste buds. The numeric ratio of cell shapes varies in buds of different localization. The quantitative distribution of taste buds over sensory zones, specific features of morphology and sizes of their sensory field are discussed in relation to the feeding pattern of the species.     

Calcium-binding proteins in chemoreceptors of Xenopus laevis.

Calcium-binding proteins were investigated immunohistochemically in chemo-receptors of the olfactory epithelium and taste buds of the clawed frog, Xenopus laevis. Calmodulin-, S-100- and calbindin-immunoreactive material were found in sensory cells of the olfactory epithelium; however, parvalbumin-like material was absent in these cells. Taste buds of the palate showed calmodulin-, S-100- and parvalbumin-immunoreactive material in sensory cells, while calbindin-immunoreactive material in supporting cells. Merkel cells, surrounding the base of the taste buds in a ring-like manner, exhibited calmodulin- and S-100-immunoreactive material.     

Fine structure of the nasal barbel of the channel catfish,Ictalurus punctatus

Ultrathin sections of the nasal barbel of the channel catfish, Ictalurus punctatus, were studied in the electron microscope and the fine structure was compared to that of barbels of other teleosts and to the mandibular (dentary) barbels of I. punctatus. While the overall histology of the nasal barbel is similar to that of barbels described previously, this study revealed far greater cellular complexity and variability than was previously reported. A layer of stratified epidermal cells rests upon a connective tissue dermis containing a cartilage rod, a large number of nerve fibers and numerous blood vessels, fibroblasts and pigment cells. Taste buds are present in the epidermal layer. This layer was found to contain probably 16 kinds of cell types, several of which may represent transitional stages, in addition to taste bud cells. Observations were made pertaining to innervation and cell types in the taste buds. A new terminology for designating the barbels of I. punctatus is suggested.     

Histochemical observations on the taste buds of the marmosets (Callithrix jacchus and Callithrix penicillata).

The epithelial cells in the taste buds of C. jacchus and C. penicillata show a moderate amount of ribonucleic acid an a concentration of a PAS-positive diastase-resistant material at their apical part. These cells are devoid of UDPG-GT, phosphorylases, G-6-PA, alanyl aminopeptidase, leucine aminopeptidase, cholinesterase and MAO; they present a weak reaction of F-1, 6-P Ald, LDH, SDH, MDH, cytochrome oxidase, beta-OHBDH, nonspecific esterase and acid phosphatase and a stronger reaction to ADH, NADPH2-TR, ATPases, alpha-GPDH, alkaline phosphatase, 5-nucleotidase and GDH. Although some enzymes (alkaline phosphatase, 5-nucleotidase and ATPases) have an almost uniform reactivity by the several taste buds, the other ones react with a lesser intensity in the smaller uniform reactivity by the several taste buds, the other ones react with a lesser intensity in the smaller taste buds of the fungiform papillae. As a rule the apical part of the cells shows a stronger enzymatic reactivity. The taste buds of the marmosets are penetrated by acetylcholinesterase positive nerve fibers whereas the autonomic ganglia in the connective tissue contain both-acetyl and butyrylcholinesterase.     

Comparison of the oropharyngeal cavity in the Starksiini (Teleostei: Blenniiformes: Labrisomidae): Taste buds and teeth,including a comparison with closely‐related genera

The present study describes the distribution of taste buds and teeth in the oropharyngeal cavity of 13 species of adult (18–60 mm SL) Starksiini fishes inhabiting subtidal waters of the Neotropical region. Four types of taste buds described previously in other fish groups were observed within the oropharyngeal cavity, of which type I, situated on prominent protruding papillae, is the most common. The number of taste buds in this cavity varies considerably, ranging from ca. 202 in Starksia lepicoelia to ca. 770 in S. sluiteri. In all the studied species, taste buds are more numerous on the posterior (160–396) than on the anterior (42–294) part of the oropharyngeal cavity. The presence of different numbers of taste buds in different Starksiini species of the same standard length suggests that numbers of taste buds are not directly correlated with size and may be species‐specific. Teeth are found on the premaxilla, dentary, vomer, palatine (in some species) and the upper and lower pharyngeal jaws (third pharyngobranchials and fifth ceratobranchials, respectively); the form and number of teeth and taste buds on each of these sites differs among the various species of Starksiini and between them and closely related species of the labrisomid tribes Labrisomini, Mnierpini, and Paraclinini. The results thus suggest potential systematic value in certain features of the oropharyngeal cavity for blenniiform fishes. It is also shown that benthic‐feeding omnivorous fishes have higher densities of taste buds than piscivorous fishes. A possible correlation among numbers of taste buds, their positions in the oropharyngeal cavity, and other parameters is discussed. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Are there efferent synapses in fish taste buds?

In fish, nerve fibers of taste buds are organized within the bud's nerve fiber plexus. It is located between the sensory epithelium consisting of light and dark elongated cells and the basal cells. It comprises the basal parts and processes of light and dark cells that intermingle with nerve fibers, which are the dendritic endings of the taste sensory neurons belonging to the cranial nerves VII, IX or X. Most of the synapses at the plexus are afferent; they have synaptic vesicles on the light (or dark) cells side, which is presynaptic. In contrast, the presumed efferent synapses may be rich in synaptic vesicles on the nerve fibers (presynaptic) side, whereas the cells (postsynaptic) side may contain a subsynaptic cistern; a flat compartment of the smooth endoplasmic reticulum. This structure is regarded as a prerequisite of a typical efferent synapse, as occurring in cochlear and vestibular hair cells. In fish taste buds, efferent synapses are rare and were found only in a few species that belong to different taxa. The significance of efferent synapses in fish taste buds is not well understood, because efferent connections between the gustatory nuclei of the medulla with taste buds are not yet proved.     

Espin cytoskeletal proteins in the sensory cells of rodent taste buds

Espins are multifunctional actin-bundling proteins that are highly enriched in the microvilli of certain chemosensory and mechanosensory cells, where they are believed to regulate the integrity and/or dimensions of the parallel-actin-bundle cytoskeletal scaffold. We have determined that, in rats and mice, affinity purified espin antibody intensely labels the lingual and palatal taste buds of the oral cavity and taste buds in the pharyngo-laryngeal region. Intense immunolabeling was observed in the apical, microvillar region of taste buds, while the level of cytoplasmic labeling in taste bud cells was considerably lower. Taste buds contain tightly packed collections of sensory cells (light, or type II plus type III) and supporting cells (dark, or type I), which can be distinguished by microscopic features and cell type-specific markers. On the basis of results obtained using an antigen-retrieval method in conjunction with double immunofluorescence for espin and sensory taste cell-specific markers, we propose that espins are expressed predominantly in the sensory cells of taste buds. In confocal images of rat circumvallate taste buds, we counted 21.5 ± 0.3 espin-positive cells/taste bud, in agreement with a previous report showing 20.7 ± 1.3 light cells/taste bud when counted at the ultrastructural level. The espin antibody labeled spindle-shaped cells with round nuclei and showed 100% colocalization with cell-specific markers recognizing all type II [inositol 1,4,5-trisphosphate receptor type III (IP₃R₃)_, α-gustducin, protein-specific gene product 9.5 (PGP9.5)] and a subpopulation of type III (IP₃R₃, PGP9.5) taste cells. On average, 72%, 50%, and 32% of the espin-positive taste cells were labeled with antibodies to IP₃R₃, α-gustducin, and PGP9.5, respectively. Upon sectional analysis, the taste buds of rat circumvallate papillae commonly revealed a multi-tiered, espin-positive apical cytoskeletal apparatus. One espin-positive zone, a collection of ～3 μm-long microvilli occupying the taste pore, was separated by an espin-depleted zone from a second espin-positive zone situated lower within the taste pit. This latter zone included espin-positive rod-like structures that occasionally extended basally to a depth of 10–12 μm into the cytoplasm of taste cells. We propose that the espin-positive zone in the taste pit coincides with actin bundles in association with the microvilli of type II taste cells, whereas the espin-positive microvilli in the taste pore are the single microvilli of type III taste cells.     

Investigation of the budding process in Tethya citrina and Tethya aurantium (Porifera, Demospongiae)

The budding process has been studied in two congeneric Mediterranean species belonging to Tethya from different sampling sites: Marsala and Venice Lagoons (Tethya citrina); Marsala Lagoon and Porto Cesareo Basin (Tethya aurantium). Buds, connected to the adult by a spiculated stalk, differ between the two species in morphology and size, since those of T. citrina are small with elongated bodies, showing only a few spicules protruding from the apical region, whereas those of T. aurantium are round, larger, and show spicules radiating from the peripheral border. In T. citrina, cells with inclusions, varying in electron density and size, represent the main cell types of the buds. In T. aurantium, the cell component shows a major diversification, resulting from spherulous cells, grey cells, vacuolar cells and peculiar micro-vesicle cells. Neither canals nor choanocyte chambers were observed in the buds of the two species. In T. citrina, bud production is similar in both sampling sites. In T. aurantium, budding occurs more rarely in Porto Cesareo Basin, probably in relation with environmental factors, such as the covering of the cortex by sediment and micro-algae. Finally, in the buds of both species, the spicule size does not differ from that of the cortex of the adult sponges, further supporting the main involvement of the cortex in organizing the skeletal architecture of the buds.     

Cytokeratin 14 is expressed in immature cells in rat taste buds

We analyzed the differentiation of taste bud cells, by precisely describing expression profiles of cytokeratins (CKs) 8 and 14 in relation to those of marker molecules including label of 5-bromo-2′-deoxy uridine (BrdU) injected. In rat circumvallate papillae, cell division was observed at the basal layer of the epithelium expressing CK14 and located outside taste buds. The progenitor cells began to migrate toward the apical surface and maintained CK14 expression at 1 day after BrdU injection (day 1). On the other hand, a minor population of newly divided cells was infrequently incorporated into taste buds and also maintained CK14 expression at day 1. In taste buds, the conversion of CK subtypes occurred from CK14 to cytokeratin 8 (CK8) at day 2–3, showing the differentiation from immature cells expressing CK14 into mature or maturing cells expressing CK8. Functionally matured cells such as taste receptor cells expressing inositol triphospate receptor type 3 (IP₃R3) never expressed CK14, suggesting that CK14 would be expressed only in immature cells. On the other hand, a small but distinct population of BrdU-positive cells still showed CK14 immunoreactivity in taste buds even at day 12, which might correspond to the cells that remain undifferentiated for a long period within taste buds.     

Gustatory receptors and neighbouring cells in the surface layer of an amphibian taste disc: in situ relationships and response to cell isolation

Summary The outermost epithelial cell layer of frog taste buds consists of (1) mucus-secreting cells; (2) thin, sheet-like processes of wing cells, which enclose the basal and lateral aspects of the mucus cells; (3) slim apical processes (sensory dendrites) of taste receptor cells; the latter ascend within wing-cell processes and each process terminates apically in one villus-like rod, a dense, highly ordered bundle of 500–1500 actin filaments supporting the chemoreceptive membrane. Staining with fluorescent phalloidin indicates that there are about 760 receptor terminals in a taste disc of 200 m in diameter, or 200000 per tongue. Viable spindle-shaped taste receptor cells were isolated using a lowcalcium / collagenase / mechanical agitation protocol. Following fluorescent labelling of apical membranes prior to cell separation, the label remained at the dendritic tip of these cells during isolation. The tip was still stainable with phalloidin. Thus the sensory dendrite, including its apical pole, had been completely removed from the encasing wing-cell process. However, scanning electron microscopy and staining by lectin and phalloidin indicated that many taste receptor cells had undergone cytoskeletal reorganization in response to cell separation; they became flask-shaped following the shortening and thickening of their sensory dendrite. The cells also began to disassemble the actin filament bundle at the chemoreceptive apical cell pole.     

Acid stimulation (sour taste) elicits GABA and serotonin release from mouse taste cells

Several transmitter candidates including serotonin (5-HT), ATP, and norepinephrine (NE) have been identified in taste buds. Recently, γ-aminobutyric acid (GABA) as well as the associated synthetic enzymes and receptors have also been identified in taste cells. GABA reduces taste-evoked ATP secretion from Receptor cells and is considered to be an inhibitory transmitter in taste buds. However, to date, the identity of GABAergic taste cells and the specific stimulus for GABA release are not well understood. In the present study, we used genetically-engineered Chinese hamster ovary (CHO) cells stably co-expressing GABA(B) receptors and Gαqo5 proteins to measure GABA release from isolated taste buds. We recorded robust responses from GABA biosensors when they were positioned against taste buds isolated from mouse circumvallate papillae and the buds were depolarized with KCl or a stimulated with an acid (sour) taste. In contrast, a mixture of sweet and bitter taste stimuli did not trigger GABA release. KCl- or acid-evoked GABA secretion from taste buds was Ca(2+)-dependent; removing Ca(2+) from the bathing medium eliminated GABA secretion. Finally, we isolated individual taste cells to identify the origin of GABA secretion. GABA was released only from Presynaptic (Type III) cells and not from Receptor (Type II) cells. Previously, we reported that 5-HT released from Presynaptic cells inhibits taste-evoked ATP secretion. Combined with the recent findings that GABA depresses taste-evoked ATP secretion, the present results indicate that GABA and 5-HT are inhibitory transmitters in mouse taste buds and both likely play an important role in modulating taste responses.     

Structural Organization of the Taste Apparatus in Characins (Characidae,Teleostei)

The morphology and distribution of taste buds in the outer integument of the body and in the oral cavity of two forms (blind cave and sighted terrestrial ones) of the astyanax Astyanax fasciatus and in intact and blinded individuals of the Buenos Aires tetra Hyphessobrycon anisitsi have been studied using electronic scanning and light microscopy. In sighted individuals of both species, the morphometric parameters of the taste apparatus and the distribution of taste receptors are similar; the taste apparatus in the oral cavity is more developed than in the outer covers. Morphologically different taste zones were found in the oral cavity of characins. In blind fish, the taste apparatus of the maxillary zones is distinguished by smaller taste buds and a greater density of their distribution. The sensory field of taste buds in blind and sighted individuals of astyanax and tetra has a similar ultrastructure; it is formed by taste cells of three types. In blind astyanaxes and blinded individuals of tetra, numerous modified epidermal cells were found for the first time in the epithelium of the taste zones and in contact with taste buds, which are regarded as tactile receptors and a constituent element of polysensory taste-tactile complexes localized in blind fish in mainly ventral sensory zones.     

Polyp morphogenesis in a scyphozoan: Evidence for a head inhibitor from the presumptive foot end in vegetative buds ofCassiopeia andromeda

Summary Buds ofCassiopeia andromeda have been transected into fragments of various sizes. Depending on their original position in the organism, on their size and on the age of the dissected buds, the fragments either regenerated or developed to a solitary polyp's head without stalk and peduncle. Generally, basal fragments tended to regenerate complete buds, young apical parts mostly differenciated polyp heads whereas apical and middle parts of progressively older buds regenerated buds with increasing frequency. To explain the alteration of the developmental capacities a head inhibitor is postulated which originates from the basal end of the buds and which expands towards the apical pole with increasing age of the buds.     

Characterization of different microenvironments at the surface of the frog's taste organ

We used a panel of histochemical techniques to identify and characterize the cell-associated extracellular material at the surface of the frog's taste organ. We employed morphological and histochemical techniques using both the light microscope and the electron microscope. Results show that the apical, external aspect of cells reaching the surface of the taste organ is in close contact with a layer of amorphous material. The histochemical characteristics of this material vary according to the cell type with which it is in contact. Three different microenvironments can be identified at the surface of the frog's taste organ: type 1 microenvironment is associated with the superficial layer of mucus (secretory) cells; type 2 microenvironment characterizes the surface of the so-called wing cells, which reach the surface of the taste organ as thin laminae running among mucus cells; and type 3 microenvironment shrouds the free endings of putative taste cells and is rich in calcium and lipids. Type 2 and type 3 microenvironments fix peroxidase (a sapid macromolecule) with increasing affinity. We conclude that highly differentiated microenvironments exist at the surface of the frog's taste organ, and these could play a role in the chain of biological events leading to the taste sensation. Furthermore, characterization of the cell-associated, specific microenvironments can help clarify the role of the different cell types in the frog's taste organ.