A scanning electron microscopic study of secondary lamellae and chloride cells of rainbow trout (Salmo gairdneri)

Cortical contraction drives the 3D patterning of epithelial cell surfaces

Cellular protrusions create complex cell surface topographies, but biomechanical mechanisms regulating their formation and arrangement are largely unknown. To study how protrusions form, we focused on the morphogenesis of microridges, elongated actin-based structures that are arranged in maze-like patterns on the apical surfaces of zebrafish skin cells. Microridges form by accreting simple finger-like precursors. Live imaging demonstrated that microridge morphogenesis is linked to apical constriction. A nonmuscle myosin II (NMII) reporter revealed pulsatile contractions of the actomyosin cortex, and inhibiting NMII blocked apical constriction and microridge formation. A biomechanical model suggested that contraction reduces surface tension to permit the fusion of precursors into microridges. Indeed, reducing surface tension with hyperosmolar media promoted microridge formation. In anisotropically stretched cells, microridges formed by precursor fusion along the stretch axis, which computational modeling explained as a consequence of stretch-induced cortical flow. Collectively, our results demonstrate how contraction within the 2D plane of the cortex can pattern 3D cell surfaces.

Morphological and histopathological changes in seahorse (Hippocampus reidi) gills after exposure to the water-accommodated fraction of diesel oil

Industrial activities and urbanization are the main sources of pollutants in estuarine environments. Diesel, which is widely used in urban and port activities, is an important source of hydrocarbons in the aquatic environment, and its water-accommodated fraction (WAF) is toxic to the local biota. This study was performed to analyze the effects of diesel oil WAF on fish. Specifically, we characterized the gill morphology of the seahorse Hippocampus reidi and analyzed the histopathological changes in the gills after exposure to 50% diesel oil WAF. Acute (12, 24, 48, and 96 h) and subchronic (168 and 336 h) toxicity tests were performed. Furthermore, a recovery protocol was conducted: after exposure to 50% WAF for 168 h, the fish were transferred and kept in seawater without contaminants for 336 h, for a total experimental period of 504 h. The seahorse branchial apparatus was found to be tufted with short filaments; the apical surfaces of the pavement cells in the filament and lamellar epithelia formed "crests" and had microridges. Mitochondria-rich cells were distributed exclusively in the lamellar epithelium, while mucous cells were distributed in the filament epithelium. All pathologies observed after acute and subchronic exposure featured progressive time-dependent alterations of lamellar structure that might disrupt gill physiological and metabolic functions. During the recovery period, the gill alterations were gradually repaired.

Morpho-histological and ultra architectural changes during early development of endangered golden mahseer Tor putitora

Ultrastructural and histological changes in the embryonic and larval surface during ontogenesis of the endangered golden mahseer Tor putitora is studied here for the first time. Embryonic development was completed 91-92 h after fertilization at an ambient temperature of 23° ± 1° C (mean ± s.d.). The gastrula stage was characterized by presence of the Kupffer's vesicle, notochord, ectoderm and endoderm cells. Primordial germ cells were clearly identifiable from c. 55 h post-fertilization at the organogenesis stage. Mean total length of newly hatched larvae was 7·0 ± 0·5 mm. Scanning electron microscopy of newly hatched larvae demonstrated vitelline arteries, microridged epithelial cells and mucous gland openings over much of the body surface. Eye, oral cavity, pharyngeal arches, heart, intestinal loop, prosencephalon, cephalic vesicle and nasal epithelium were clearly distinguished in 3 day old hatched individuals. In 6 day old individuals, caudal-fin rays and internal organs were evident. The dorsal fin became prominent at this stage and larvae began swimming at the surface. The reserved yolk material was totally absorbed 8-11 days after hatching and larvae began feeding exogenously. Tor putitora exhibited a longer early developmental period than other cyprinids reared at similar temperatures.

The Camel Rhinarium: A Study Revealing the Presence of the Nasal Plane in Dromedary Camel (Camelus dromedarius), with Special Reference to Its Epidermal Structure

The aim of this study was to prove that the nasal plane (Planum nasale) present in camel (Camelus dromedarius). Furthermore, it was the first description of the gross and primary microscopic anatomy as well as the epidermal ultrastructure of the camel nasal plane. Grossly, the camel rhinarium was formed of the glabrous grayish-black skin that extended to cover the philtrum and the medial nasal angles. It was composed of two perinasal parts and an inter-labial part. A shallow groove was passed across its middle. A dermatoglyphic pattern of epidermal ridges with primary and secondary fissures in between was revealed by scanning electron microscopy. The nasal plane was very small in relation to the camel head size. In general, the morphological appearance of the camel rhinarium was greatly similar to that of the proboscis-bearing mammals. The basic histological structure as well as the epidermal ultrastructure of the camel nasal plane was observed to resemble very closely the common type. Few differences were found, including indistinct or absent hypodermal layer, thinner stratum corneum and some basal cells have unusual sinuous bases.

Effects of low environmental salinity on the cellular profiles and expression of Na+, K+-ATPase and Na+, K+, 2Cl- cotransporter 1 of branchial mitochondrion-rich cells in the juvenile marine fish Monodactylus argenteus

The goal of this study was to determine the osmoregulatory ability of a juvenile marine fish, silver moony (Monodactylus argenteus), for the purpose of developing a new experimental species for ecophysiological research. In this study, M. argenteus was acclimated to freshwater (FW), brackish water (BW), or seawater (SW). The salinity tolerance of this euryhaline species was effective, and the fish survived well upon osmotic challenges. The largest apical surface of mitochondrion-rich cells was found in the FW individuals. Immunohistochemical staining revealed that Na(+), K(+)-ATPase immunoreactive (NKA-IR) cells were distributed in the interlamellar region of the gill filaments of the silver moony in all experimental groups. In addition to the filaments, NKA-IR cells were also found in the lamellae of the FW individuals. The number of NKA-IR cells in the gills of the FW individuals exceeded that of the BW and SW individuals. The NKA-IR cells of FW and SW individuals exhibited bigger size than that of BW fish. The NKA activities and protein expression of the NKA α-subunit in the gills of the FW individuals were significantly higher than in the BW and SW groups. Additionally, the relative amounts of Na(+), K(+), 2Cl(-) cotransporter 1 (NKCC1) were salinity-dependent in the gills. Immunofluorescent signals of NKCC1 were localized to the basolateral membrane of NKA-IR cells in all groups. In the gills of the FW individuals, however, some NKA-IR cells did not exhibit a basolateral NKCC1 signal. In conclusion, the present study illustrated the osmoregulatory mechanisms of this easy- and economic-to-rear marine teleost with euryhaline capacity and proved the silver moony to be a good experimental animal.

Structural diversity of nuptial pads in Phyllomedusinae (Amphibia: Anura: Hylidae)

We studied the morphological variation of the nuptial pads using light microscopy and scanning electron microscopy (SEM) in 26 species of phyllomedusines (Anura: Hylidae), representing the five currently recognized genera. All phyllomedusines have single nuptial pads with dark colored epidermal projections (EPs). Spine-shaped EPs occur in Cruziohyla calcarifer, Phrynomedusa appendiculata and in one species of Phasmahyla. The other species have roundish EPs. The density of the EPs on the pad is variable. Species in the Phyllomedusa hypochondrialis Group have EPs with a density that varies between 764 ± 58/mm(2) and 923 ± 160/mm(2). In all other studied species (including the Phyllomedusa burmeisteri and Phyllomedusa perinesos groups, Phyllomedusa camba, Phyllomedusa boliviana, Phyllomedusa sauvagii, Phyllomedusa bicolor, and Phyllomedusa tomopterna) the density of EPs varies between 108 ± 20/mm(2) and 552 ± 97/mm(2). Pores were observed with SEM in C. calcarifer, Agalychnis lemur, Agalychnis moreletii, but its presence is confirmed through histological sections on several other species. Its visibility using SEM seems to be related with the level of separation between adjacent EPs. The pores in the four studied species of Agalychnis are shown with SEM and histological sections to have a characteristic epidermal rim, that is absent in the otherphyllomedusines. Unlike most previous reports on breeding glands, those of phyllomedusines are alcian blue positive, indicating the presence of acidic mucosubstances on its secretions.

Gill morphology and acute hypoxia: responses of mitochondria-rich, pavement, and mucous cells in the Amazonian oscar (Astronotus ocellatus) and the rainbow trout (Oncorhynchus mykiss), two species with very different approaches to the osmo-respiratory compromise

The hypoxia-intolerant rainbow trout ( Oncorhynchus mykiss (Walbaum, 1792)) exhibits increased branchial ion permeability and Na+ influx during acute exposure to moderate hypoxia (Po2 = 80 torr; 1 torr = 133.3224 Pa), manifesting the usual trade-off between gas exchange and electrolyte conservation. In contrast, the hypoxia-tolerant oscar ( Astronotus ocellatus (Agassiz, 1831)) is unusual in exhibiting decreased branchial ion permeability to ions and Na+ influx during acute exposure to severe hypoxia (Po2 = 10–20 torr). These different physiological approaches to the osmo-respiratory compromise correlate with rapid, oppositely directed changes in gill morphology. In oscar, pavement cells (PVCs) expanded, partially covering neighboring mitochondria-rich cells (MRCs), which were recessed and reduced in size. Those remaining open were transformed from “shallow-basin” to “deep-hole” forms with smaller openings, deeper apical crypts, and smaller numbers of subapical microvesicles, changes that were largely reversed during normoxic recovery. In contrast, moderate hypoxia caused outward bulging of MRCs in rainbow trout with increases in size, surface exposure, and number of subapical microvesicles, accompanied by PVC retraction. These changes were partially reversed during normoxic recovery. In both rainbow trout and oscar, hypoxia caused discharge of mucus from enlarged mucous cells (MCs). Rapid, divergent morphological changes play an important role in explaining two very different physiological approaches to the osmo-respiratory compromise.

Gills of the snow trout, Schizothorax curvifrons heckel: a SEM study

Using the scanning electron microscope, the gills of Schizothorax curvifrons have been studied after carrying out their primary fixation in 2.5% glutaraldehyde in 0.1 M sodiumcacodylate buffer (pH 7.3) for 24 h at 0-4 degrees C and secondary fixation in 1-4% osmiumtetraoxide in 0.1 M sodiumcacodylate buffer (pH 7.3) for 1-2 h at 0-4 degrees C. The gills are observed to comprise of gill arch, gill rakers having minute projections on their inner sides and gill filaments. The filaments originating from the gill arch are in a linear fashion and equidistant from each other, bearing secondary lamellae on both sides of the filament axis and each lamellae running parallel to the adjacent lamellae. The filament epithelium is found to be equipped with pavement cells, mucous cells and chloride cells. The pavement cells have smooth surface with sparse and irregular microridges defining their cell limits. The mucous cells are mainly found on the leading and trailing edges of the filament and the chloride cells are located close to the onset of secondary lamellae. The possible roles of these structures and cells in relation to respiration, water flow and feeding are discussed, setting thereby a platform for further studies in relation to pathology, pollution and stress conditions in aquaculture, natural and polluted environments.

Swelling-activated transport of taurine in cultured gill cells of sea bass: physiological adaptation and pavement cell plasticity

We have investigated volume-activated taurine transport and ultrastructural swelling response of sea bass gill cells in culture, assuming that euryhaline fish may have developed particularly efficient mechanisms of salinity adaptation. In vivo, when sea basses were progressively transferred from seawater to freshwater, we noticed a decrease in blood osmotic pressure. When gill cells in culture were subjected to 30% hypotonic shock, we observed a five-fold stimulation of [3H]taurine efflux. This transport was reduced by various anion channel inhibitors with the following efficiency: 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid > DIDS = diphenylamine-2-carboxylic acid. With polarized gill cells in culture, the hypotonic shock produced a five-fold stimulation of apical taurine transport, whereas basolateral exit was 25 times higher. Experiments using ionomycin, thapsigargin, BAPTA-AM, or removal of extracellular calcium suggested that taurine transport was regulated by external calcium. The inhibitory effects of lanthanum and streptomycin support Ca2+ entry through mechanosensitive Ca2+ channels. Branchial cells also showed hypotonically activated anionic currents sensitive to DIDS and NPPB. Similar pharmacology and time course suggested the potential existence of a common pathway for osmosensitive taurine and Cl− efflux through volume-sensitive organic osmolyte and anion channels. A three-dimensional structure study revealed that respiratory gill cells began to swell only 15 s after hypoosmotic shock. Apical microridges showed membrane outfoldings: the cell surface became smoother with a progressive disappearance of ridges. Therefore, osmotic swelling may not actually induce membrane stretch per se, inasmuch as the microridges may provide a reserve of surface area. This work demonstrates mechanisms of functional and morphological plasticity of branchial cells during osmotic stress.

Genotoxic and histopathological effects of water pollution on two fish species, Barbus capito pectoralis and Chondrostoma nasus in the Büyük Menderes River, Turkey

The genotoxic and histopathological effects of water pollution were investigated on two fish species caught from the Buyuk Menderes River and from its tributary, the Cine Stream. The Buyuk Menderes basin is an important agricultural area in Turkey. The levels of copper, zinc, cadmium, cobalt, and lead were measured at the surface of the water and in gills, liver, and muscle tissue of Chondrostoma nasus and Barbus capito pectoralis. In some tissues, the concentrations of some of these metals exceeded acceptable levels for human consumption. Zinc was found to be the most abundant metal in water and tissues. Maximal metal accumulation was observed in the liver. To detect the genotoxic potential of contaminants, the formation of micronucleus in erythrocytes was used as indicator of chromosomal damage. The frequency of micronucleus formation did not show significant differences between locations and controls in B. capito pectoralis caught from three locations and C. nasus from two locations. The histological changes included significant decreases of the mean lengths of primary and secondary lamellae. In gills epithelia, we observed cellular proliferation that developed Because of secondary lamellae fusion, ballooning degenerations, or club deformation of secondary lamellae and cystic structures in secondary lamellae. In the liver, the changes included swollen and ruptured parenchymal cells, loss of cord structure, vacuoles filled with cellular debris, focal necrosis, and a significant increase in Kupffer cells.

Investigation of histopathological and cytogenetic effects on Lepomis gibbosus (Pisces: Perciformes) in the Cine stream (Aydin/Turkey) with determination of water pollution

Water quality and the distribution of some heavy metals in three different organs of Lepomis gibbosus from the Cine Stream were studied. Also, histopathological changes in gill, liver, and muscle tissue were examined at light microscopical level. Micronucleus (MN) formation in fish erytrocytes, as an indicator of chromosomal damage, has been increasingly used to detect the genotoxic potential of environmental contaminants. The frequency of MN was examined from samples of fish from the Cine Stream and a control group. MN frequency was higher in fish samples caught from the Cine Stream than that in the control group. The chemicals ammonia, nitrite, nitrate, orthophosphate, and sulphate were determined as parameters that possibly affect the gill, liver, and muscle morphology. Zn was the most accumulated metal in tissues as well as in water. Maximum metal accumulation occurred in both liver and gills. For histopathological examinations, samples of gills, liver, and muscle tissues of L. gibbosus were studied by using light microscopy. In this study, a significant decrease in mean length of primary and secondary lamellae were observed. Moreover, cellular proliferation developed with secondary lamellae fusion, ballooning degenerations or club deformation of secondary lamellae, as well as distribution of necrotic, hyperplastic and clavate secondary lamellae. In the liver, altered staining, swollen and ruptured parenchymal cells, loss of cord structure, reduce of glycogen in hepatocytes, and vacuolar structure filled with cellular debris and many dark particles were seen. In muscle tissue, focal necrosis, cellular dissolution, and a decline or loss of striation in muscle fibres were found.

Surface ultrastructure of the gills of the mullets Mugil curema, M. liza and M. platanus (Mugilidae, Pisces)

Mugil curema, M. liza, and M. platanus were collected from the southeastern and southern coast of Brazil. The second gill arches were analyzed by scanning electron microscopy and histology. The highest density of chloride and mucus-secreting cells was observed in the gill filaments of M. liza and M. platanus. Spines are scarce and were found only in the pharyngeal region of M. curema. The dorsal angle of curvature of the simple projections is most reduced in the rakers of M. liza and M. platanus. The raker borderline on the internal side of the arches of M. curema has grooves that do not occur in the other two species. On the external side of the branchial arches, the borders of the rakers of M. liza and M. platanus are smooth. The shape of the rakers is characteristic for each species: in M. curema, it resembles the letter "D"; in M. liza, it is trapezoidal, and in M. platanus, it is triangular. Thus there is a morphologic similarity between M. liza and M. platanus, and both differ from M. curema. All three species show elongated and extremely elaborated rakers that are placed next to each other and turned toward the opercular cavity. There are few taste buds and only several mucus-secreting cells along the whole pharyngeal region. These characteristics suggest that these species do not select food chemically but obtain it mechanically with the rakers and aggregate it with mucus.

Gill morphology and acid-base regulation in freshwater fishes

This review examines the recent advances in our understanding of the mechanisms of ion transport and acid-base regulation in the freshwater fish gill. The application of a combination of morphological, immunocytochemical and biochemical techniques has yielded considerable insight into the field. An important mechanism for regulation of Cl- uptake/base excretion is by morphological modification of the gill epithelium. During acidosis, the chloride cell associated Cl-/HCO3- exchanger is effectively removed from the apical epithelium because of a covering by adjacent pavement cells; this mechanism reduces base excretion and contributes to the compensation of the acidosis. In addition, acidosis induces changes in both the surface structure and ultrastructure of pavement cells. Evidence is accumulating to support the hypothesis that Na+ uptake/H+ excretion is accomplished by the pavement cell. Further, specific localization of a V-type H+-ATPase on the pavement cell epithelium and an increased expression during acidosis provides support for the model originally proposed, that this exchange is accomplished by an electrochemically coupled H+-ATPase/Na+ channel mechanism.

The chloride cell: structure and function in the gills of freshwater fishes

This review focuses on the structure and function of the branchial chloride cell in freshwater fishes. The mitochondria-rich chloride cell is believed to be the principal site of trans-epithelial Ca2+ and Cl- influxes. Though currently debated, there is accruing evidence that the pavement cell is the site of Na+ uptake via channels linked electrically to an apical membrane vacuolar H(+)-ATPase (proton pump). Chloride cells perform an integral role in acid-base regulation. During conditions of alkalosis, the surface area of exposed chloride cells is increased, which serves to enhance base equivalent excretion as the rate of Cl-/HCO3- exchange is increased. Conversely, during acidosis, the chloride cell surface area is diminished by an expansion of the adjacent pavement cells. This response reduces the number of functional Cl-/HCO3- exchangers. Under certain conditions that challenge ion regulation, chloride cells proliferate on the lamellae. This response, while optimizing the Ca2+ and Cl- transport capacity of the gill, causes a thickening of the blood-to-water diffusion barrier and thus impedes respiratory gas transfer.

Calcium balance in embryos and larvae of the freshwater-adapted teleost, Oreochromis mossambicus

Changes in Ca(2+) content and flux, and the development of skin chloride cells in embryos and larvae of tilapia, Oreochromis mossambicus, were studied. Tilapia embryos hatched within 96h at an ambient temperature of 26-28°C. Total body Ca(2+) content was maintained at a constant level, about 4-8 nmol per individual, during embryonic development. However, a rapid increase in body Ca(2+) level was observed after hatching, 12.8 to 575.3 nmol per individual from day 1 to day 10 after hatching. A significant influx and efflux of Ca(2+) occurred during development, with the average influx rate for Ca(2+) increasing from 5.9 pmol mg(-1) h(-1) at 48h postfertilization to 47.8 pmol mg(-1) h(-1) at 1 day posthatching. The skin was proposed as the main site for Ca(2+) influx before the development of gills, and the increased Ca(2+) influx may be ascribed to gradual differentiation of skin surface and chloride cells during embryonic development. Ca(2+) efflux was 16-56 pmol mg(-1) h(-1) in 1-day-old larvae. The resulting net influx of Ca(2+), 10-12 pmol mg(-1) h(-1), accounted for the increased Ca(2+) content after hatching. When comparing the measured and estimated ratios of efflux and influx, active transport was suggested to be involved in the uptake of Ca(2+). Chloride cells, which may be responsible for the active uptake of Ca(2+), started to differentiate in the skin of embryos 48h after fertilization, and the density of chloride cells increased following the development. A possibility of active transport for Ca(2+) in early developmental stages of tilapia is suggested.

Primary culture of gill epithelial cells from the sea bass Dicentrarchus labrax

We have developed the first explant technique that allows the in vitro study of gill physiology and biochemistry in marine species. Gill fragments were cultured at 17 degrees C, in atmospheric PCO2, with nutrient medium (Leibovitz L15), pH 7.8, supplemented with 10% fetal bovine serum and adjusted to the osmolarity of fish plasma (350 mOsm/liter). Coating plates with collagen, gelatin, or polylysin did not improve our results. Decrease in osmotic pressure, removal of bovine serum, or its replacement by fish serum inhibited growth from the explants. Approximately 50% of the explants produced cell growth, and after 4 days of culture a monolayer of contiguous cells was formed. This technique is rapid and does not require the use of enzymes. The cells appeared flat and thin with an epitheloid shape. They looked polygonal with a maximum length of 10 to 50 microm. Evidence that they are unique gill cells is the presence of polymorphic surface crenelations (microplicae), prominent Golgi apparatus, tight junctions and desmosomes. Comparison with in vivo tissue showed them to be epithelial cells having differentiated in a homogeneous population of respiratory-like (pavement) cells. They are polarized with their apical surface facing the culture medium. The development of this culture system represents a new tool for cellular approaches to determine precisely the functions and transport mechanism of gill cells.

Ultrastructure of the taste disc in the red-bellied toad Bombina orientalis (Discoglossidae, Salientia)

The taste disc of the red-bellied toad Bombina orientalis (Discoglossidae) has been investigated by light and electron microscopy and compared with that of Rana pipiens (Ranidae). Unlike the frog, B. orientalis possesses a disc-shaped tongue that cannot be ejected for capture of prey. The taste discs are located on the top of fungiform papillae. They are smaller than those in Ranidae, and are not surrounded by a ring of ciliated cells. Ultrastructurally, five types of cells can be identified (mucus cells, wing cells, sensory cells, and both Merkel cell-like basal cells and undifferentiated basal cells). Mucus cells are the main secretory cells of the taste disc and occupy most of the surface area. Their basal processes do not synapse on nerve fibers. Wing cells have sheet-like apical processes and envelop the mucus cells. They contain lysosomes and multivesicular bodies. Two types of sensory cells reach the surface of the taste disc; apically, they are distinguished by either a brush-like arrangement of microvilli or a rod-like protrusion. They are invaginated into lateral folds of mucus cells and wing cells. In contrast to the situation in R. pipiens, sensory cells of B. orientalis do not contain dark secretory granules in the perinuclear region. Synaptic connections occur between sensory cells (presynaptic sites) and nerve fibers. Merkel cell-like basal cells do not synapse onto sensory cells, but synapse-like connections exist between Merkel cell-like basal cells (presynaptic site) and nerve fibers.

Evidence for a morphological component in acid-base regulation during environmental hypercapnia in the brown bullhead (Ictalurus nebulosus)

Exposure of adult brown bullheads Ictalurus nebulosus (120-450 g) to environmental hypercapnia (2% carbon dioxide in air) and subsequent recovery caused transient changes in whole body net sodium flux (JnetNa+) and net chloride flux (JnetCl-) resulting largely from changes in whole body sodium influx (JNa+in) and chloride influx (JinCl-). Scanning electron microscopy (SEM) revealed that the fractional area of chloride cells (CCs) on the interlamellar regions was reduced by 95% during environmental hypercapnia. During post-hypercapnic recovery, gill filament CC fractional area increased. The changes in JinCl- during and after environmental hypercapnia were closely associated with the changes in CC fractional area while the changes in JinNa+ did not correspond to the changes in CC fractional area. Transmission electron microscopy (TEM) supported the SEM observations of CC surface area changes and demonstrated that these changes were caused by covering/uncovering by adjacent pavement cells (PVCs). Lamellar and filament PVC microvilli density increased during hypercapnia while there was a subsequent reduction in the posthypercapnic period. These data suggest that an important mechanism of acid-base regulation during hypercapnic acidosis is modification of the chloride cell-associated Cl-/HCO3- exchange mechanism. We suggest that bullheads vary availability, and thus functional activity, of this transporter via reversible morphological alterations of the gill epithelium. The increase in density of PVC microvilli may be associated with sodium uptake and/or acidic equivalent excretion during acidosis.

Accessory cells in the gill epithelium of the freshwater rainbow trout Salmo gairdneri

Two types of mitochondria-rich cells were identified in the gill epithelium of the freshwater-adapted rainbow trout, Salmo gairdneri, after selective impregnation of their tubular system with reduced osmium. A first type consisted of large cells with a poorly developed and loosely anastomosed tubular system; thus, that resembled the chloride cells commonly encountered in the gill epithelium of freshwater-adapted euryhaline fishes. A second type comprised smaller cells with an extensively developed and tightly anastomosed tubular system. These never reached the basal lamina of the gill epithelium and were adjacent to chloride cells, to which they were linked by shallow apical junctions (100-200 nm); thus, they resembled accessory cells, which are currently found in the gill epithelium of seawater-adapted fishes but are usually lacking in freshwater living fishes. Transfer of the freshwater-adapted trout into seawater induced the proliferation of the tubular system in the chloride cells and the formation of lateral plasma membrane interdigitations between accessory cells and the apical portion of the chloride cells. The length of the apical junction sealing off this extended intercellular space was reduced to 20-50 nm. The tubular system of the accessory cells was not modified. The extension of the tubular system in the chloride cells of the seawater-adapted fishes indicated that, as in most euryhaline fishes, these cells have a role in the adaptation of the rainbow trout to seawater. In contrast, the function of the presumptive accessory cells in freshwater trout remains to be established.

Localization of cytokeratins in tissues of the rainbow trout: fundamental differences in expression pattern between fish and higher vertebrates

Using a panel of antibodies against different cytokeratins in immunofluorescence microscopy on frozen tissue sections and two-dimensional gel electrophoresis of cytoskeletal proteins from these tissues, we have studied the tissue distribution of cytokeratins in a fish, the rainbow trout Salmo gairdneri. We have distinguished at least 14 different cytokeratin polypeptides in only a limited number of tissues, thus demonstrating the great complexity of the cytokeratin pattern in a fish species. The simplest cytokeratin pattern was that present in hepatocytes, comprising one type-II (L1) and two type-I (L2, L3) polypeptides that appear to be related to mammalian cytokeratins 8 and 18, respectively. Two or all three cytokeratins of this group were also identified in several other epithelial tissues, such as kidney. Epithelia associated with the digestive tract contained, in addition, other major tissue-specific cytokeratins, such as components D1-D3 (stomach, intestine and swim bladder) and B1 and B2 (biliary tract). With the exception of D1, all these polypeptides were also found in a cultured cell line (RTG-2). Epidermal keratinocytes contained D1 and six other major cytokeratins, termed E1-E6. The most complex cytokeratin pattern was that found in the gill epithelium. Surprisingly, antibodies specific for cytokeratins of the L1-L3 group also reacted with certain cell-sheet-forming tissues that are not considered typical epithelia and in higher vertebrates express primarily, if not exclusively, vimentin. Such tissues were (a) endothelia, including the pillar cells of the "gill filaments", (b) scale-associated cells, and (c) the ocular lens epithelium, and also several nonepithelial cell types, such as (d) fibroblasts and other mesenchymal cells, (e) chondrocytes, (f) certain vascular smooth muscle cells, and (g) astroglial cells of the optic nerve. The differences between the patterns of cytokeratin expression in this fish species and those of higher vertebrates are discussed. It is concluded that the diversity of cytokeratins has already been established in lower vertebrates such as fish, but that the tissue-expression pattern of certain cytokeratins has been restricted during vertebrate evolution. We discuss the value of antibodies specific for individual cytokeratin polypeptides as marker molecules indicating cell and tissue differentiation in fish histology, embryology, and pathology.

Microridges of oral mucosal epithelium in carp, Cyprinus carpio

The surface of carp oral mucosa is characterized by various patterns of microridges about 0.3 micron wide, 0.1 micron high, and of various lengths. To elucidate the derivation and function of these microridges, the oral epithelium was examined by light- and electron microscopy. Microridges were present only on the surfaces of the superficial cells. Therefore, microridges on renewed superficial cells have been discarded, and the various patterns of microridges found on the cell surface appear to indicate the progress of their development. In thin sections, the outer leaflet of the plasma membranes of microridges stained strongly with ruthenium red, and the underlying cytoplasm was packed with many fine filaments. The superficial cells contained many secretory vesicles that were PAS-positive but Alcian blue-negative at pH 2.5 and pH 1.0. However, after sulfation the vesicles gave a positive reaction with toluidine blue. These vesicles are secreted by exocytosis at the free surface of the cells. After release, the membranes of the vesicles are thought to be utilized for formation of microridges. On the basis of these observations, the possible function of microridges is discussed.

Surface ultrastructure of the gill arch of the killifish, Fundulus heteroclitus, from seawater and freshwater, with special reference to the morphology of apical crypts of chloride cells

The surface ultrastructure of the gill arches of the killifish, Fundulus heteroclitus, adapted to seawater or freshwater, was found to be similar to that reported for other euryhaline teleosts. Two rows of gill filaments (about 42 filaments per row) extended posterolaterally, and two rows of gill rakers (about 10 rakers per row) extended anteromedially from each arch. Leaf-like respiratory lamellae protruded along both sides of each filament, from its base to its apex. The distributions, sizes, and numbers of various surface cells and structures were also determined. All surfaces were covered by a mosaic of pavement cells, which measured about 7 X 4 microns and exhibited concentrically arranged surface ridges. Taste buds were especially prominent on the rakers and the pharyngeal surfaces of the first and second gill arches, but were often replaced by horny spines on the third and fourth gill arches. Apical crypts of chloride cells occurred mostly on the surfaces of the gill filaments adjacent to the afferent artery of the filament. In seawater adapted killifish, crypts resembled narrow, deep holes along the borders of adjacent pavement cells, had openings of about 2 microns2, and occurred at a frequency of about 1 per 70 microns2 of surface area. In freshwater fish, the crypts usually had larger openings (about 10 microns2), occurred less frequently (1 per 123 microns2), and exhibited many cellular projections in their interiors. Changes in crypt morphology may be related to the ion transport function of chloride cells.

Morphology of taste buds on the gill arches of the mullet Mugil cephalus, and the killifish Fundulus heteroclitus

The morphology of taste buds on the gill arches of two euryhaline teleosts, the mullet Mugil cephalus, and the killifish Fundulus heteroclitus, were investigated using light microscopic and scanning and transmission electron microscopic techniques. On the mullet gill arches, taste buds were limited to the pharyngeal surfaces of the smooth-surfaced gill rakers. On the killifish gill arches, taste buds were located on the pharyngeal surfaces of all gill rakers and on the gill arch itself at the bases of the gill rakers. Despite dramatic differences in gill-raker structure between these two species, the taste buds themselves were similar ultrastructurally and closely resembled those described in other fishes. Cells within the taste buds included spindle-shaped dark and light cells and basal cells. Ultrastructural features of both the light and dark cells could support either receptor or transport functions. Tufts of microvilli, including one thick microvillus per light cell and numerous thin microvilli per dark cell, protruded at the apex of each taste bud between the ridged surface epithelial cells. Light cells contained numerous tubular membrane elements some of which appeared to open onto the apical surface of the taste bud. Dark cells contained numerous microtubules and apical, electron-lucent vesicles possibly involved in transport.

Morphology of the buccopharyngeal portion of the gill in the fathead minnow Pimephales promelas (Rafinesque)

Buccopharyngeal epithelium covering gill arches and gill rakers of the fathead minnow was studied by light microscopic, scanning, and transmission electron microscopic techniques. Mature mucous cells in goblet pattern and nonmucus containing cells were in the apical one-third of the tissue. The latter cells contributed to a surface microridge system which overlapped apices of goblet cells. The bottom of the epithelium was comprised of a continuous row of darkly stained basal epithelial cells. In this region, two to three epithelial cells of similar staining characteristics were piled up forming apical columns which partially encircled nests of lightly stained cells. A basal lamina and thick basement lamella of about 20 piles of orthogonally arranged collagen supported the epithelium. Numerous taste buds were seen in gill arches and rakers. Taste bud cellular components included marginal cells, light receptor cells, dark receptor cells, and basal cells. These were identical in all taste buds. Taste bud surface morphology differed between gill arch and raker. Pores of the former were depressed, while those of the latter were raised. Thick microvilli of taste pores were apical extensions of light cells, while smaller, more numerous microvilli were projections from dark cells.

Ultrastructure of marine teleost gill epithelia: SEM and TEM study of the chloride cell apical membrane

This paper deals with the structure of gill epithelia in the sole, Solea solea, as revealed by transmission and scanning electron microscopy. In this marine teleost the chloride cell and its accessory cell form a cellular complex. Apically the plasma membranes of these cells are loosely juxtaposed, thus forming a leaky epithelium covering a large part of the gill.

Microridge cells in the larynx of the male white rat. Investigations by reflection scanning electron microscopy

The laryngeal epithelium of male white rats is studied by reflection scanning electron microscopy (SEM). In addition to ciliated cells, microvilli cells, brush cells and goblet cells that are characteristic for normal respiratory epithelium the microridge or labyrinth cell can be seen in particular regions of the larynx. The apical surface of a typical labyrinth cell is characterized by a system of narrow standing microridges of about 0.05--0.15 micronm in diameter and interconnecting microridges with a diameter of about 0.01 micronm. The microridge system of a labyrinth cell originates from the fusion densely standing microvilli. Between microridge cells and microvilli cells all transition forms can be observed. The preferable localiziation of the microridge cell in the larynx and its possible function is discussed.

Identification of glycoproteins in goblet cells of epidermis and gill of plaice (Pleuronectes platessa L.), flounder (Platichthys flesus (L.)) and rainbow trout (Salmo gairdneri Richardson)

A quantitative analysis has been made of the glycoproteins present in the goblet cells of the epidermis, gill filaments and gill lamellae of three species of teleost fish. The glycoproteins have been identified by a combination of techniques, including the use of the enzyme sialidase followed by Alcian Blue staining, at PH 2.6 or 1.0, in combination with periodic acid-Schiff. The selected fish were representative of species living in marine, freshwater and estuarine environments. The range of glycoproteins identified in these fish was similar to that found in mammalian tissue in that both neutral and acid glycoproteins were present, the latter included both sialomucins sensitive and resistant to sialidase, and sulphomucin. A single goblet cell contained either neutral or acid glycoproteins alone or in combination. Only the epidermis of the plaice and rainbow trout contained uniform cell populations producing acid glycoproteins, the former sulphomucin and the latter mainly sialomucin. At each site in the flounder and in the gill epithelia of the plaice and rainbow trout, the goblet cell population was mixed, with cells producing each type of glycoprotein. The number of goblet cells producing each type of glycoprotein varied at each tissue site.

A proposed function for microridges on epithelial cells

Microridges (MR), also called microvillar ridges, microplicae and cytoplasmic folds, have been noted on many epithelial surfaces. Several functions have been proposed for these structures. In the present study we examine the mechanical role that microridges may play in holding mucus to the lumenal surface to the esophagus in the trout Salmo gairdneri. Our findings support the hypothesis that the microridges help hold a protective coat of mucus to the epithelium. In addition, the complex curved or whorled arrangement of microridges appears to facilitate the spread of mucus away from goblet cells.