Cytoarchitecture of surface layer cells of the teleost epidermis

Histological and ultrastructural characterization of the dorso-ventral skin of the juvenile and the adult starry puffer fish (Arothron stellatus, Anonymous 1798)

BACKGROUND

The starry puffer fish (Arothron stellatus, Anonymous, 1798) is a poisonous tetradontidae fish inhabiting the Red sea. The skin constitutes an important defense against any external effects. The study aims to characterize the dorso-ventral skin of the juvenile and the adult starry puffer fish using light and scanning electron microscopies. Twenty specimens of juvenile and adult fresh fishes were used.

RESULTS

The scanning electron microarchitecture of the skin of the juvenile and adult fish showed delicate irregular-shaped protrusions, and well-defined bricks-like elevations on the dorsal side and interrupted folds as well as irregular-shaped protrusions on the ventral side. In adult fish, the patterned microridges of the superficial and deep epithelial cells (keratinocytes) were larger and well-defined in the dorsal skin than in the ventral side, the contrary was seen in the juvenile fish. The microridges were arranged in a fingerprint or honeycomb patterns. The openings of the mucous cells were more numerous in the dorsal skin in both age stages but more noticeable in adult. Furthermore, the sensory cells were more dominant in the juveniles than the adults. The odontic spines were only seen in adult. Histologically, few taste buds were observed in the epidermis of the dorsal skin surface of the adult fish. Both mucous and club cells were embedded in the epidermis of the juvenile and adult fish with different shapes and sizes. Melanophores were observed at the dorsal skin of both juvenile and adult fishes while fewer numbers were noticed at the ventral surfaces. Several dermal bony plates with different shapes and sizes were demonstrated in the skin of both adult and juvenile fishes.

CONCLUSION

The structural variations of skin of the juvenile and adult fishes may reflect the various environmental difficulties that they confront.

Tools of the trade: studying actin in zebrafish

Actin is a conserved cytoskeletal protein with essential functions. Here, we review the state-of-the-art reagents, tools and methods used to probe actin biology and functions in zebrafish embryo and larvae. We also discuss specific cell types and tissues where the study of actin in zebrafish has provided new insights into its functions.

Keratins and the plakin family cytolinker proteins control the length of epithelial microridge protrusions

Actin filaments and microtubules create diverse cellular protrusions, but intermediate filaments, the strongest and most stable cytoskeletal elements, are not known to directly participate in the formation of protrusions. Here we show that keratin intermediate filaments directly regulate the morphogenesis of microridges, elongated protrusions arranged in elaborate maze-like patterns on the surface of mucosal epithelial cells. We found that microridges on zebrafish skin cells contained both actin and keratin filaments. Keratin filaments stabilized microridges, and overexpressing keratins lengthened them. Envoplakin and periplakin, plakin family cytolinkers that bind F-actin and keratins, localized to microridges, and were required for their morphogenesis. Strikingly, plakin protein levels directly dictate microridge length. An actin-binding domain of periplakin was required to initiate microridge morphogenesis, whereas periplakin-keratin binding was required to elongate microridges. These findings separate microridge morphogenesis into distinct steps, expand our understanding of intermediate filament functions, and identify microridges as protrusions that integrate actin and intermediate filaments.

Microridges are apical epithelial projections formed of F-actin networks that organize the glycan layer

Apical projections are integral functional units of epithelial cells. Microvilli and stereocilia are cylindrical apical projections that are formed of bundled actin. Microridges on the other hand, extend laterally, forming labyrinthine patterns on surfaces of various kinds of squamous epithelial cells. So far, the structural organization and functions of microridges have remained elusive. We have analyzed microridges on zebrafish epidermal cells using confocal and electron microscopy methods including electron tomography, to show that microridges are formed of F-actin networks and require the function of the Arp2/3 complex for their maintenance. During development, microridges begin as F-actin punctae showing signatures of branching and requiring an active Arp2/3 complex. Using inhibitors of actin polymerization and the Arp2/3 complex, we show that microridges organize the surface glycan layer. Our analyses have unraveled the F-actin organization supporting the most abundant and evolutionarily conserved apical projection, which functions in glycan organization.

Histochemical analysis of glycoproteins in the secretory cells in the epidermis of the head skin of Indian Major Carp, Labeo rohita

A series of histochemical procedures were employed to localise and characterise glycoprotein (GP) classes produced by the epithelial cells, the type A and the type B mucous goblet cells (MGCs) and the club cells in the epidermis of Labeo rohita. The epithelial cells secreted GPs with oxidizable vicinal diols and GPs with sialic acid residues without O-acyl substitution in low concentrations. The type A MGCs and the type B MGCs, in contrast, produced these GPs in high concentrations. Further, these MGCs produced GPs with O-sulphate esters as well. GPs with O-sulphate esters were produced in high concentration by the type A MGCs and in low concentration by the type B MGCs. The club cells produced GPs with oxidizable vicinal diols in trace amounts. Production of more than one type of GPs suggested a basis for functional discrimination in their role in the mucous secretions at the skin surface. This is considered an adaptation to environment inhabited by the fish and is discussed in relation to their role in lubrication, protection and inhibition of the invasion and proliferation of pathogenic micro-organisms.

The survival of monogenean (platyhelminth) parasites on fish skin

This review deals with the problems faced by those monogenean (platyhelminth) parasites that attach themselves to fish skin. The structure of the skin and the ways in which the posterior hook-bearing haptor achieves virtually permanent attachment to the skin are considered. Small marginal hooklets are specialized for attachment to superficial host epidermal cells, finding anchorage in the terminal web of keratinous tonofilaments, while large hooks (hamuli) may penetrate into and lodge in the collagenous dermis. The complementary roles of suction and sticky secretions in haptor attachment and the role of the pharynx in temporary attachment during feeding are also considered. During leech-like locomotion the haptor is briefly detached and, at this critical time, the anterior end is strongly fixed to the wet, current-swept and possibly slimy skin by a sticky secretion. This secretion is deployed on paired pads or discs, the latter sometimes backed up by suction. After attachment by the haptor is re-established, the special tegument covering the anterior adhesive areas may be instrumental in their instant release. The role of fish skin in the phenomenon of host specificity and in the generation of a defensive response against monogeneans is considered and site-specificity of parasites on the host's body is discussed. Possible selection pressures exerted by predatory ‘cleaner’ organisms are briefly evaluated.

Histochemical characterization of glycoproteins in the buccal epithelium of the catfish, Rita rita

Glycoproteins (GPs) elaborated by the buccal epithelium of the catfish, Rita rita, were analysed by a range of histochemical methods. These included methods for the characterization and simultaneous visualization of GPs with oxidizable vicinal diols, with O-acyl sugars, with O-sulphate esters and with sialic acid residues with and without O-acyl substitution at C7, C8 or C9. GPs elaborated at the surface of the buccal epithelium are primarily from two sources, the epithelial cells and the mucous goblet cells. They include GPs with O-sulphate esters, GPs with sialic acid residues without O-acyl substitution and GPs with oxidizable vicinal diols. Different classes of GPs have been associated with specific functions and are discussed in relation to their physiological significance, with special reference to their roles in lubrication, alteration in viscosity, trapping of food particles, buffering of fluids at the epithelial surface, prevention of proteoloytic damage to the epithelium, antimicrobial activity and defence against pathogens. The epithelium shows specialized modifications in the form of the buccal glands. These have been considered to increase the secretory surface that allows profuse secretion of mucus in a very short period of time. The secretions of these glands have been associated with multiple functions similar to those of saliva.

Wound management in teleost fish: biology of the healing process, evaluation, and treatment

Wound healing, although similar to terrestrial vertebrates, occurs faster in fish species in a proper aquatic environment. Wound management must involve identification and correction of potential noninfectious and/or infectious causes of disease. Wound evaluation includes assessment of the wound as well as systematic examination of the patient, water quality conditions, and husbandry practices. Diagnostic evaluation should, at a minimum, include physical examination, skin/cornea (if indicated) scraping, and lesions cultures. The clinician should also consider full thickness wound biopsies, gill and fin biopsies, blood collection, and radiographs for systemic evaluation as well. Local wound management should follow basic principles of wound care including cleaning and lavage, debridement, primary closure if indicated, and local and/or systemic antiinfectives. Monofilament absorbable or nonabsorbable sutures are commonly used for primary closure of wounds. Diagnostic results, patient and client compliance, and economic impact should guide systemic methods of treatment for wound care. Options for systemic therapy include oral or gavage, parenteral, bath, or prolonged immersion treatments. With bath or prolonged immersion therapies, aquatic systems must be properly prepared for treatment. Treatment of identified pathogens, proper water quality and husbandry, supportive care and nutrition must all be provided to promote an ideal wound healing environment for the teleost fish.

Cutaneous biology and diseases of fish

The normal structure and function of the piscine integument reflects the adaptation of the organism to the physical, chemical, and biological properties of the aquatic environment, and the natural history of the organism. Because of the intimate contact of fish with the environment, cutaneous disease is relatively more common in fish than in terrestrial vertebrates and is one of the primary disease conditions presented to the aquatic animal practitioner. However, cutaneous lesions are generally nonspecific and may be indicative of disease that is restricted to the integument or a manifestation of systemic disease. Regardless, a gross and microscopic examination of the integument is simple to perform, but is highly diagnostic and should always be included in the routine diagnostic effort of the aquatic animal practitioner, especially since various ancillary diagnostic procedures are either not practical or lack predictive value in fish. The purpose of this article is to provide an overview of normal cutaneous biology prior to consideration of specific cutaneous diseases in fish.

Nonylphenol provokes a vesiculation of the Golgi apparatus in three fish epidermis cultures

The aim of this work was to study the effects of nonylphenol and waste water on the cell ultrastructure of fish skin. Therefore, besides a recently established primary cell culture and a cell line, an epidermal tissue culture of fish was developed and tested. In all three systems a prominent vesiculation of the Golgi apparatus was observed after exposure to nonylphenol, which has not been described before and therefore strongly suggests an effect that might indicate exposure to nonylphenol and/or related substances. The Epithelial papulosum cyprini cell line was the most sensitive to nonylphenol, followed by the primary cell culture of epidermis cells and then the explant tissue culture. The vesiculation of the Golgi apparatus was accompanied by degenerative changes in the two cell cultures only. The lack of degenerative changes in the cells of the tissue culture was discussed with respect to the presence of differentiated cells that might better be able to protect themselves by mucous or by an activated xenobiotic metabolism. In a second type experiment, a waste water sample containing small concentrations of nonylphenol was applied to the cultures. It did not lead to a vesiculation of the Golgi apparatus, probably because the nonylphenol concentrations in the waste water were too low to induce the vesiculation. The cultures exposed to waste water revealed unspecific degenerative cellular changes. Additionally, explant cultures were prepared from fish that had survived a 6-month exposure to polluted river water. In these cultures a higher number of mitochondria containing myelin bodies were observed when compared to control cultures. Consequently, exposure to polluted water containing a mixture of substances in vitro and in vivo was found to lead to degenerative alterations in the ultrastructure of the cells.

The application of microridge analysis in the diagnosis of gastro-oesophageal reflux disease

BACKGROUND

The percentage of epithelial surface area covered by microridges (%MR) seen during scanning electron microscopy of oesophageal biopsy specimens has previously been shown to correlate with symptomatic reflux disease, a result < or = 35% being abnormal. The aim of this study was to compare %MR with endoscopy, light microscopy, and pH monitoring results.

METHODS

Sixty-seven patients with heartburn were divided into oesophagitis or none on the basis of endoscopy and light microscopy findings and into those with and without abnormal acid reflux on the basis of pH monitoring.

RESULTS

The endoscopic and light microscopic oesophagitis groups had significantly greater degrees of acid reflux than those without oesophagitis (p < 0.05), even though neither the specific %MR nor the number of patients below the 35% cutoff showed any difference between those with and without endoscopic oesophagitis, light microscopic oesophagitis or those with normal and abnormal acid reflux on pH monitoring.

CONCLUSION

Despite the significant relationship between endoscopic and light microscopic oesophagitis and abnormal pH monitoring microridge analysis did not correlate with any of these variables

Ultrastructure and histochemical aspects of the oropharyngeal epithelium of Siren intermedia Le Conte

The present electron microscopical and histochemical study of the oropharyngeal epithelium (OEPI) in Siren intermedia describes its various differentiated and developing cell types and cell contents. The OEPI is multilayered; it consists of a basal layer, a middle cell layer and a pavement layer. Three cell types are found in the pavement layer: pavement cells (PC), goblet cells (GC), and mitochondria-rich cells (MRC). These originate from basal cells and mature in the middle cell layer. The MRC is equipped with a tubule system that seems to be responsible for gaseous exchange. MRC and GC probably appear only in the caudal portions of the oropharynx. Electron microscopically, the goblet cell type appears uniform, except for the production of secretory granules; rostrad, these are largely PAS-positive and occipitad alcianophilic. The production of mucin granules is lower in the PC, where they accumulate as loose aggregations in the cell apex. Their contents react strongly to alcian blue. Only electron-lucent (loose) mucosubstances are exocytosed; they form the apically-situated acidic mucous coat of the PC. The possible osmoregulatory function of these mucins is discussed in relation to the model suggested by Kirschner (1982, in Gomme 1984).

Cytoskeleton in microridges of the oral mucosal epithelium in the carp, Cyprinus carpio

Microridges produce a characteristic fingerprint-like pattern on the surface of fish oral mucosa. The cytoskeleton in these microridges was examined by immunofluorescence microscopy and transmission electron microscopy after detergent extraction and decoration with myosin subfragment 1. The effect of cytochalasin B on microridges was probed with scanning electron microscopy. Immunofluorescence microscopy revealed that actin filaments were present throughout the periphery of the epithelial cells and were especially localized beneath the free surface of the epithelium. In thin sections treated with Triton X-100, the majority of filaments in the microridges and their bases were found to be actin filaments and a plexus of keratin filaments that underlay the network of actin filaments. A part of the plexus of keratin filaments entered the microridges. After extraction with Triton X-100 and decoration with myosin subfragment 1, decorated actin filaments were found in the microridge cores, connected to the keratin filaments. The keratin filaments aggregated in the pattern of microridges and a few of them protruded into the microridges. Treatment with cytochalasin B caused microridges to disappear or to become thinner and lower or to change short or microvillus-like microridges. When most microridges disappeared, the surface of the superficial cells was prominently swollen, but the cell boundaries were fastened, and the microridges in the periphery were preserved. On the basis of these observations, the possible roles of actin and keratin filaments in the maintenance and the formation of microridges are discussed.

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.

Persistent, directional motility of cells and cytoplasmic fragments in the absence of microtubules

Directional cell locomotion is displayed by many cell types both in vivo and in vitro. In many instances, persistency and directionality are imposed by external stimuli such as chemical attractants or substrate properties. Some cell types, such as fibroblasts or leukocytes, are capable of migrating in the absence of known stimuli in a pattern known as persistent random walk, where the direction of movement is maintained for at least one cell diameter before the cell performs a sudden directional change. In many examples of persistent motility, microtubules are believed to have a key role as elements that stabilize or even determine a cell's direction of movement. If disassembled, persistency is reduced or impaired. Despite some reports to the contrary, these and other observations have led to the widely accepted view that microtubules may be the overall organizers of cell geometry, polarity and motile activity. Here we report that rapid, directional locomotion of fish epidermal keratocytes is independent of the presence of microtubules. Moreover, small cytoplasmic fragments derived from the anterior lamella of these cells are capable of locomoting in a pattern indistinguishable from that of intact cells. Since these fragments contain no nucleus, microtubules or centrioles, the persistency-determining component must be sought in some other component(s) of the cytoplasm, possibly the motile machinery of the lamella itself.

Type I and type II keratins have evolved from lower eukaryotes to form the epidermal intermediate filaments in mammalian skin

We have traced the evolutionary origins of keratin-like sequences to the genomes of lower eukaryotes. The proteins encoded by these genes have evolved to form the intermediate filaments that comprise the backbone of vertebrate skin cells. Two related but distinct types of keratins encoded by two separate multigene subfamilies are expressed in the epidermal keratinocytes of vertebrate species from fish to human. Both at the level of protein and at the level of DNA, these two classes of keratins are coordinately conserved throughout vertebrate evolution, indicating the central role that both types of keratins must play in the assembly and structure of the 8-nm filament.

Dynamics of the cytoskeleton of epidermal cells in situ and in culture

The cytoskeleton of primary tissue-culture cells from the epidermis of Xenopus laevis tadpoles was investigated by phase-contrast, immunofluorescence, and electron microscopy. The connection between the arrangement of different types of filaments and the mechanical properties of the epidermis is discussed. The bilayered epidermis attains stability from thick bundles of tonofilaments interconnecting the basal desmosomes. Twisting of tonofilaments around each other can explain the occurrence of elastic filamentous curls forming a meshwork braced between rows of "small desmosomes" in the apical region of the epidermis. Actin is arranged as a diffuse meshwork and sometimes forms bundles intermingling with tonofilament bundles. Surface membranes and rows of "small desmosomes" are delineated by actin and contain alpha-actinin. Actin raises the tension for rounding and spreading of cells. Microtubules stabilize already well-developed lamellae.

The role of divalent cations in the regulation of microtubule assembly. In vivo studies on microtubules of the heliozoan axopodium using the ionophore A23187

Low concentrations of calcium and magnesium ions have been shown to influence microtubule assembly in vitro. To test whether these cations also have an effect on microtubules in vivo, specimens of Actinosphaerium eichhorni were exposed to different concentrations of Ca++ and Mg++ and the divalent cation ionophore A23187. Experimental degradation and reformation of axopodia were studied by light and electron microscopy. In the presence of Ca++ and the ionophore axopodia gradually shorten, the rate of shortening depending on the concentrations of Ca++ and the ionophore used. Retraction of axopodia was observed with a concentration of Ca++ as low as 0.01 mM. After transfer to a Ca++-free solution containing EGTA, axopodia re-extend; the initial length is reached after about 2 h. Likewise, reformation of axopodia of cold-treated organisms is observed only in solutions of EGTA or Mg++, whereas it is completely inhibited in a Ca++ solution. Electron microscope studies demonstrate degradation of the axonemal microtubular array in organisms treated with Ca++ and A23187. No alteration was observed in organisms treated with Mg++ or EGTA plus ionophore. The results suggest that, in the presence of the ionophore, formation of axonemal microtubules can be regulated by varying the Ca++ concentration in the medium. Since A23187 tends to equilibrate the concentrations of divalent cations between external medium and cell interior, it is likely that microtubule formation invivo is influenced by micromolar concentrations of Ca++. These concentrations are low enough to be of physiological significance for a role in the regulation of microtubule assembly in vivo.

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.