Keratinization of fish skin with special reference to the catfish Bagarius bagarius

Epidermal modifications in a hill stream catfish, Hara hara in relation to its natural habitat: A scanning electron microscope and histochemical investigation

In the present study, the epidermis of the hill stream fish Hara hara has been investigated employing scanning electron microscope, histology, histochemistry and immunofluorescence techniques. The epidermis is characteristically differentiated into plaques separated from each other by deep furrows. In plaques, the epidermis is keratinized. In contrast, in furrows, it is mucogenic. Surface epithelial cells in plaques get modified into characteristic spine-like unculi. At the distal ends of these unculi, we find tree-like branched dendritic structures. The keratinized epithelial cells in plaques together with unculi frequently exfoliate at the surface. The epidermis in furrows is equipped with secretory glandular cells, that is, mucous goblet cells, sacciform cells and club cells; and sensory structures, that is, the taste buds. These glandular cells are involved in the elaboration of different types of carbohydrate and protein moieties. Further, in the epidermis of both, plaques and furrows, melanophores are frequently interspersed between the epithelial cells. In the plaque epidermis, in addition to melanophores, melanin granules are observed in epithelial cells undergoing keratinization as well as in those sloughing at the surface. Sloughing of keratinized epithelial cells together with spine-like unculi at the surface of the plaques; the secretions of the glandular cells, the distribution of melanophore and the taste buds interspersed between the epithelial cells and the presence of melanin granules in the keratinized epithelial cells have been associated with different functional roles. These include hydrodynamic advantage, protection from mechanical stress, pathogens, UV radiation, localization of food accurately and so on in relation to the natural habitat of the fish.

An Overview of the Biological Features, Distribution, and Conservation of a Critically Endangered Riverine Catfish, Bagarius bagarius (Hamilton, 1822), in the Natural Waters of Bangladesh

Bagarius bagarius (Hamilton, 1822) is widely distributed in South and Southeast Asian countries, including Bangladesh. This species is economically important as a game and food fish. The abundance of this fish is declining around the world, especially in Bangladesh, due to a variety of meteorological and mostly anthropogenic factors, which is potentially generating concern among the conservationists. Therefore, this species has already been declared a critically endangered species by IUCN Bangladesh. Although there is no specific conservation initiative for B. bagarius in Bangladesh, various measures are there to conserve fisheries resources, which may have an impact on conserving B. bagarius in this country. This study reviews the biology and ecology with its distribution throughout the country as well as the world, threats, conservation measures, and finds out the gaps in research on this fish. Moreover, this review suggests a suitable conservation framework to improve the conservation strategy for this critically endangered fish that can be replicated in other countries for the same purpose.

Insights regarding skin regeneration in non-amniote vertebrates: Skin regeneration without scar formation and potential step-up to a higher level of regeneration

Skin wounds are among the most common injuries in animals and humans. Vertebrate skin is composed of an epidermis and dermis. After a deep skin injury in mammals, the wound heals, but the dermis cannot regenerate. Instead, collagenous scar tissue forms to fill the gap in the dermis, but the scar does not function like the dermis and often causes disfiguration. In contrast, in non-amniote vertebrates, including fish and amphibians, the dermis and skin derivatives are regenerated after a deep skin injury, without a recognizable scar remaining. Furthermore, skin regeneration can be compared with a higher level of organ regeneration represented by limb regeneration in these non-amniotes, as fish, anuran amphibians (frogs and toads), and urodele amphibians (newts and salamanders) have a high capacity for organ regeneration. Comparative studies of skin regeneration together with limb or other organ regeneration could reveal how skin regeneration is stepped up to a higher level of regeneration. The long history of regenerative biology research has revealed that fish, anurans, and urodeles have their own strengths as models for regeneration studies, and excellent model organisms of these non-amniote vertebrates that are suitable for molecular genetic studies are now available. Here, we summarize the advantages of fish, anurans, and urodeles for skin regeneration studies with special reference to three model organisms: zebrafish (Danio rerio), African clawed frog (Xenopus laevis), and Iberian ribbed newt (Pleurodele waltl). All three of these animals quickly cover skin wounds with the epidermis (wound epidermis formation) and regenerate the dermis and skin derivatives as adults. The availability of whole genome sequences, transgenesis, and genome editing with these models enables cell lineage tracing and the use of human disease models in skin regeneration phenomena, for example. Zebrafish present particular advantages in genetics research (e.g., human disease model and Cre-loxP system). Amphibians (X. laevis and P. waltl) have a skin structure (keratinized epidermis) common with humans, and skin regeneration in these animals can be stepped up to limb regeneration, a higher level of regeneration.

Keratinization and mucogenesis in the epidermis of an angler catfish Chaca chaca (Siluriformes, Chacidae): A Histochemical and fluorescence microscope investigation

The present study describes keratinization and mucogenesis in the epidermis of an angler catfish Chaca chaca, using a series of immunochemical, fluorescence and histochemical methods. The epidermis is primarily mucogenic and shows characteristic specialised structures at irregular intervals. These structures are identified keratinized in nature. The superficial layer epithelial cells in the keratinized structures often detach from the underlying epithelial cells and exfoliate either singly or in the form of sheet. This is associated to provide protection by removing silty depositions, pathogens, and debris along with exfoliated keratinized cells/sheets periodically to keep the skin surface clean. Mucogenic epidermis is equipped with the mucous goblet cells and the club cells. Nevertheless, these cells are not discernible in the keratinized structures. This suggests an inverse relationship between mucogenesis and keratinization in the epidermis of the fish. The mucogenic epidermis is involved in the secretion of different classes of glycoproteins. These include glycoproteins with oxidizable vicinal diols, glycoproteins with O-sulphate esters and glycoproteins with sialic acid residues without O-acyl substitution. Secretion of these glycoproteins on the surface are associated to control the acidity of the acidic glycoproteins, to protect the skin surface against bacterial, viral infection and other pathogens, and help in lubrication to protect against abrasion during burrowing. Epidermal keratinization and glycoprotein characterization are associated with the physiological adaptations in relation to the characteristic habit and habitat of the fish.

A functional evaluation of feeding in the surgeonfish Ctenochaetus striatus: the role of soft tissues

Ctenochaetus striatus is one of the most abundant surgeonfishes on Indo-Pacific coral reefs, yet the functional role and feeding ecology of this species remain unclear. This species is reported to possess a rigid structure in its palate that is used for scraping, but some authors have reported that this element is comprised of soft tissue. To resolve the nature and role of this structure in the feeding ecology of C. striatus we examined evidence from anatomical observations, scanning electron microscopy, histology, X-ray micro-computed tomography scanning, high-speed video and field observations. We found that C. striatus from the Great Barrier Reef possess a retention plate (RP) on their palates immediately posterior to the premaxillary teeth which is soft, covered in a thin veneer of keratin with a papillate surface. This RP appears to be used during feeding, but does not appear to be responsible for the removal of material, which is achieved primarily by a fast closure of the lower jaw. We infer that the RP acts primarily as a 'dustpan', in a 'dustpan and brush' feeding mechanism, to facilitate the collection of particulate material from algal turfs.

Immune relevant molecules identified in the skin mucus of fish using -omics technologies

This review will give an overview of immune relevant molecules in fish skin mucus. The skin of fish is continuously exposed to a water environment, and unlike that of terrestrial vertebrates, it is a mucosal surface with a thin epidermis of live cells covered by a mucus layer. The mucosa plays an important role in maintaining the homeostasis of the fish and preventing the entry of invading pathogens. This review provides an overview of proteins, RNA, DNA, lipids and carbohydrates found in the skin mucus of studied species. Proteins such as actin, histones, lectins, lysozyme, mucin, and transferrin have extracellular immune relevant functions. Complement complement molecules, heat shock molecules and superoxide dismutase present in mucus show differential expression during pathogen challenge in some species, but their functions in mucus, if any, need to be shown. RNA, DNA, lipids, carbohydrates and metabolites in mucus have been studied to a limited extent in fish, the current knowledge is summarized and knowledge gaps are pointed out.

Tgm1-like transglutaminases in tilapia (Oreochromis mossambicus)

Among the adaptations of aquatic species during evolution of terrestrial tetrapods was the development of an epidermis preventing desiccation. In present day mammals, keratinocytes of the epidermis, using a membrane-bound transglutaminase (Tgm1), accomplish this function by synthesizing a scaffold of cross-linked protein to which a lipid envelope is attached. This study characterizes the abilities of two homologous transglutaminase isozymes in the teleost fish tilapia to form cross-linked protein structures and their expression in certain tissues. Results indicate they are capable of membrane localization and of generating cellular structures resistant to detergent solubilization. They are both expressed in epithelial cells of the lip, buccal cavity and tips of gill filaments. Adaptation of transglutaminase use in evolution of terrestrial keratinocytes evidently involved refinements in tissue expression, access to suitable substrate proteins and activation of cross-linking during terminal differentiation.

Scale morphology of Prochilodus lineatus with emphasis on the scale epithelium

The fish body is entirely covered by a thin, smooth and glandular epidermis, closely attached to the scales inserted on the dermis. The descriptive work on this tissue dates to twenty or thirty years ago, bears very little photographic record and does not focus on the scale epithelium, despite the fact that it is in direct contact with the environment. Thereupon, the present study characterizes the scale epithelium of Prochilodus lineatus, a robust species of fish. The observations show that the scale is completely covered by epithelium thicker on the proximal end of the scale, multilayered on the dorsal surface and undifferentiated on the ventral surface, and covered by mucous producing cells, mostly acid mucous. The scale is formed by plywood-like collagen matrix of collagen type III and supported by a network of elastic fibers on the ventral face. Differentiated cellular types are present, such as club cells, considered to be responsible for the release of alarm substances, which suggests possible use in environmental assessment as a non-invasive technique.

An Overview of the Immunological Defenses in Fish Skin

The vertebrate immune system is comprised of numerous distinct and interdependent components. Every component has its own inherent protective value, and the final combination of them is likely to be related to an animal’s immunological history and evolutionary development. Vertebrate immune system consists of both systemic and mucosal immune compartments, but it is the mucosal immune system which protects the body from the first encounter of pathogens. According to anatomical location, the mucosa-associated lymphoid tissue, in teleost fish is subdivided into gut-, skin-, and gill-associated lymphoid tissue and most available studies focus on gut. The purpose of this paper is to summarise the current knowledge of the immunological defences present in skin mucosa as a very important part of the fish immune system, serving as an anatomical and physiological barrier against external hazards. Interest in defence mechanism of fish arises from a need to develop health management tools to support a growing finfish aquaculture industry, while at the same time addressing questions concerning origins and evolution of immunity in vertebrates. Increased knowledge of fish mucosal immune system will facilitate the development of novel vaccination strategies in fish.

Epidermal barrier dysfunction and cutaneous sensitization in atopic diseases

Classic atopic dermatitis is complicated by asthma, allergic rhinitis, and food allergies, cumulatively referred to as atopic diseases. Recent discoveries of mutations in the filaggrin gene as predisposing factors for atopic diseases have refocused investigators' attention on epidermal barrier dysfunction as a causative mechanism. The skin's barrier function has three elements: the stratum corneum (air-liquid barrier), tight junctions (liquid-liquid barrier), and the Langerhans cell network (immunological barrier). Clarification of the molecular events underpinning epidermal barrier function and dysfunction should lead to a better understanding of the pathophysiological mechanisms of atopic diseases.

Essence of keratin in lips and associated structures of a freshwater fish Puntius sophore in relation to its feeding ecology: histochemistry and scanning electron microscope investigation

Morphological specializations in the lips and associated structures of Puntius sophore were examined by scanning electron microscopy and histochemically. The upper lip (UL), in P. sophore, is associated with the horny upper jaw sheath (HUJS) on its ventral side and with the rostral cap (RC) on its dorsal side through a thin and extensive fold of skin (FSUR). The lower lip (LL) is greatly enlarged, conspicuous and associated with horny lower jaw sheath (HLJS) on the dorsal side and ventrally continues with ventral head skin (VHS). On the lateral sides there is a thin and extensive fold of skin (FSLS) between the lower lip and VHS. In contrast to the mucogenic epithelia of the UL, LL, the RC and fold of skins, the horny jaw sheaths are keratinized in nature and surface epithelial cells are characteristically modified into unculi. The UL and the LL are equipped with epithelial cells (EC), mucous cells (MC) and taste buds (TB) while in addition to these cells club cells (CC) are also present in the RC. Keratin found in unculi is an extremely strong protein which is tough and insoluble, they form the hard but un-mineralized structures. Keratin in unculi could be regarded as an adaptation for browsing or scraping food materials from the substrate as the fish grubs about the bottom. The elaboration of mucus is considered to lubricate the surface and protect the epithelia from abrasions. Taste buds are associated to locate and select palatable food and to trigger a 'pick-up' reflex.

Histochemistry and functional organization of the dorsal skin of Ancistrus dolichopterus (Siluriformes: Loricariidae)

The structural organization and histo-cytochemical features of dorsal skin of Ancistrus dolichopterus (acari bodo) are the main focus of this work. The epidermis, dermis and subcutis are the principal layers of the skin. The epidermis mainly consists of epithelial and mucous cells. Interspersed between them are lymphocytes, pigment cells, eosinophilic granular cells (EGC), and the taste buds as sensory structures. The high number of EGCs is implicated in general and specific immunological defense from pathogenic bacteria and multicellular parasites. The epithelial cells and mucous cells contain glycoproteins with oxidizable vicinal diols, carboxyl groups and O-sulphate esters and their high secretory activity is correlated with the bottom dwelling habit of this species. A thick stratum laxum contains overlapping osteoderms bearing denticles, and the stratum compactum make the integument thicker to help the fish in negative buoyancy for maneuvering near the bottom and protection. The entire body surface is covered by conical, backwardly directed denticles. These are composed of a dentine cone, surrounding a pulp cavity with the top covered by mineralized cap, and are the true homologues of teeth. These structures provide effective protection from abrasion and enemies. These structural peculiarities and histochemical features indicate additional physiological role of the skin of A. dolichopterus.

External antigen uptake by Langerhans cells with reorganization of epidermal tight junction barriers

Outermost barriers are critical for terrestrial animals to avoid desiccation and to protect their bodies from foreign insults. Mammalian skin consists of two sets of barriers: stratum corneum (SC) and tight junctions (TJs). How acquisition of external antigens (Ags) by epidermal Langerhans cells (LCs) occur despite these barriers has remained unknown. We show that activation-induced LCs elongate their dendrites to penetrate keratinocyte (KC) TJs and survey the extra-TJ environment located outside of the TJ barrier, just beneath the SC. Penetrated dendrites uptake Ags from the tip where Ags colocalize with langerin/Birbeck granules. TJs at KC–KC contacts allow penetration of LC dendrites by dynamically forming new claudin-dependent bicellular- and tricellulin-dependent tricellular TJs at LC–KC contacts, thereby maintaining TJ integrity during Ag uptake. Thus, covertly under keratinized SC barriers, LCs and KCs demonstrate remarkable cooperation that enables LCs to gain access to external Ags that have violated the SC barrier while concomitantly retaining TJ barriers to protect intra-TJ environment.

Histochemical analysis of glycoconjugates in the skin of a catfish (arius tenuispinis, day)

A histochemical study using conventional carbohydrate histochemistry (periodic-acid staining including diastase controls, alcian blue staining at pH 1 and 2.5) as well as using a battery of 14 fluorescein isothiocyanate (FITC)-labelled lectins to identify glycoconjugates present in 10 different areas of the skin of a catfish (Arius tenuispinis) was carried out. The lectins used were: mannose-binding lectins (Con A, LCA and PSA), galactose-binding lectins (PNA, RCA), N-acetylgalactosamine-binding lectins (DBA, SBA, SJA and GSL I), N-acetylglucosamine-binding lectins (WGA and WGAs), fucose-binding lectins (UEA) and lectins which bind to complex carbohydrate configurations (PHA E, PHA L). Conventional glycoconjugate staining (PAS staining, alcian blue at pH 1 and 2.5) showed that the mucous goblet cells contain a considerable amount of glycoconjugates in all locations of the skin, whereas the other unicellular gland type, the club cells, lacked these glycoconjugates. The glycoproteins found in goblet cells are neutral and therefore stain magenta when subjected to PAS staining. Alcian blue staining indicating acid glycoproteins was distinctly positive at pH 1, but gave only a comparable staining at pH 2.5. The mucus of the goblet cells therefore also contains acid glycoproteins rich in sulphate groups. Using FITC-labelled lectins, the carbohydrate composition of the glycoproteins of goblet cells could be more fully characterized. A distinct staining of the mucus of goblet cells was found with the mannose-binding lectins LCA and PSA; the galactosamine-binding lectins DBA, SBA and GLS I; the glucosamine-binding lectin WGA; and PHA E which stains glycoproteins with complex carbohydrate configurations. No reaction occurred with the fucose-binding lectin UEA and the sialic acid-specific lectin SNA. In addition, the galactose-binding lectins PNA and RCA showed only a weak or completely negative staining of the mucus in the goblet cells. The specificity of the lectin staining could be proved by inhibiting binding of the lectins by competitive inhibition with the corresponding sugars. From these data, we can conclude that the mucus produced by the epidermal goblet cells of A. tenuispinis is rich in mannose, N-acetylgalactosamine and N-acetylglucosamine residues.

Fine structure of the organ of attachment of the teleost, Garra gotyla gotyla (Ham)

We describe the morphology of the attachment organ (AO) of the teleost, Garra gotyla gotyla (Cyprinidae). It is located ventrally around the mouth opening and used by the species for attachment to submerged rocks in sub-Himalayan streams and rivers where it lives. The AO consists of three crescentic parts and a central callus part. Scanning electron microscopy (SEM) shows the former to possess numerous tubercles, each of which bears about 23-27 curved spines. Light microscopy shows the epidermis of the tuberculated parts to possess one type of cell arranged into 7-8 rows. Transmission electron microscopy (TEM) reveals these cells to contain abundant tonofilaments (hence called the filament cells). The epidermis of the callus part possesses the filament cells and additionally mucous cells, which are absent in the tuberculated parts. The superficial epidermis is apparently keratinized (thickness: 5-8 microm), and a part of the cells of the outer row is modified into spines. These cells show a thick plasma membrane envelope and possess mucous granules (diameter: 0.1-0.3 microm) and bundles of tonofilaments. The cells of the inner two to four rows possess similar organelles and additionally, prominent Golgi bodies and rough endoplasmic reticulum. Immunohistochemically, the cells of the outer row and the spines stain positively for cytokeratin. The cells of the innermost rows (five to eight) possess few tonofilaments and no mucous granules. It is evident that the filament cells of the mid- to upper epidermis are specialized for the production of mucous granules and tonofilaments, which is unique for the teleost epidermis concerned. It appears that the tuberculated parts with spines assist in anchorage and interlocking with the substratum, while the central callus part probably utilizes both suction and frictional mechanisms, and mucous secretion protects the spines from damage during anchorage and abrasion.

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.

Sealing the live part of the skin: the integrated meshwork of desmosomes, tight junctions and curvilinear ridge structures in the cells of the uppermost granular layer of the human epidermis

In the literature the question of whether a system structurally and functionally related to the barrier function of the tight junctions (TJs) of polarized epithelia exists in the epidermis has been and still is controversially discussed. We have systematically addressed this question in a study of the granular layer of fetal and adult human epidermis, combining different light and electron microscopic methods. We show that the lateral membranes of the cells of the stratum granulosum are connected by an extended subapical complex system integrating desmosomes and TJ structures identified as sites of close membrane-membrane contact and as regions of membrane-to-membrane apposition that in immunoelectron microscopy are positive for TJ marker proteins, notably occludin, indicative of an extended, probably continuous TJ barrier. In addition, we have noted in freeze-fractures of the apical membrane attaching this layer to the basalmost membrane of the stratum corneum an extended system integrating desmosomes with intramembraneous ridge configurations that appear as strands, circles, lariats or complex meshworks showing numerous continuities with the desmosomes. In some regions this system interconnecting desmosomes with curvilinear ridge structures occupies the major part of the plasma membrane. The molecular organizations and possible functional contributions of both structural systems positioned at the border between the living portion of the epidermis and the corneal layer are discussed, in particular in relation to the formation of a stable association between the two layers and of a barrier to the paracellular flow of molecules and particles. It is also discussed whether similar structures occur in other keratinizing stratified squamous epithelia, in squamous metaplasias and in tumors derived from such tissues.

Evolution of tissue-specific keratins as deduced from novel cDNA sequences of the lungfish Protopterus aethiopicus

Lungfishes are possibly the closest extant relatives of the land vertebrates (tetrapods). We report here the cDNA and predicted amino acid sequences of 13 different keratins (ten type I and three type II) of the lungfish Protopterus aethiopicus. These keratins include the orthologs of human K8 and K18. The lungfish keratins were also identified in tissue extracts using two-dimensional polyacrylamide gel electrophoresis, keratin blot binding assays and immunoblotting. The identified keratin spots were analyzed by peptide mass fingerprinting which assigned seven sequences (inclusively Protopterus K8 and K18) to their respective protein spot. The peptide mass fingerprints also revealed the fact that the major epidermal type I and type II keratins of this lungfish have not yet been sequenced. Nevertheless, phylogenetic trees constructed from multiple sequence alignments of keratins from lungfish and distantly related vertebrates such as lamprey, shark, trout, frog, and human reveal new insights into the evolution of K8 and K18, and unravel a variety of independent keratin radiation events.

Skin-specific expression of ictacalcin, a homolog of the S100 genes, during zebrafish embryogenesis

Full-length cDNA coding for the ictacalcin gene, a homolog of the S100 genes, was isolated in zebrafish and mapped on linkage group 16 using the LN54 radiation hybrid panel. The homology and phylogenetic analyses, based on the deduced amino acid sequences, showed the orthologous relationship of ictacalcin genes between zebrafish and other fish species. However, ictacalcin genes constitute an out-group with respect to other members of the S100 gene family. This result supports the findings that fish ictacalcin genes are new members of the S100 gene family and may have evolved after the divergence of teleosts and tetrapods. The zebrafish ictacalcin gene was zygotically transcribed from 12 hours postfertilization onward and was stably expressed throughout adulthood. During zebrafish embryogenesis, the ictacalcin gene was specifically expressed in striated epidermal cells covering the entire embryo. The ictacalcin staining in keratinocytes of striated epithelia was absent in the cytoplasm surrounding the nuclei, but it was highly concentrated in the peripheral margin. Tissues enriched with epithelia folds, such as olfactory epithelium, hatching gland, pectoral fin buds, urogenital opening, and pharynx, showed a robust ictacalcin expression. The strikingly heavy staining of ictacalcin in the pharyngeal region provides us with an early marker to follow the pharynx formation in zebrafish embryos.

Immunocytochemical localization of keratins, associated proteins and uptake of histidine in the epidermis of fish and amphibians

Keratinization and the role of histidine in some species of fish and amphibians have been analyzed by immunocytochemistry and autoradiography. In cartilaginous and bony fishes, staining of acidic (AE1-positive) and basic (AE3-positive) keratins was strong and their distribution patterns were uniform in all epidermal layers. The AE2 antibody (for keratins K1 and K10 that are typical for keratinization) did not produce any positivity. This was also observed in lungfish epidermis but the AE2 antibody often produced some positivity in the more keratinized layers. In the axolotl (urodele), that is adapted to aquatic conditions, as well as in other species of urodele (newts) that are more adapted to terrestrial conditions, the same pattern was present as in fish. In the latter, the AE2 antibody non-specifically stained all epidermal layers. In more terrestrially-adapted anurans (frog and toad) AE1 immunopositivity was mainly found in basal layers, the AE3 antibody stained the entire epidermis, and AE2 immunopositivity was often localized in the external layers of the epidermis. This pattern resembled that in the epidermis of amniotes. Administration of tritiated histidine to goldfish epidermis showed that at 1, 4 and 24 h after injection, labelling was low and uniformely distributed in all epidermal layers. In newt and toad epidermis, histidine labelling increased from 1 to 4 h after injection but tended to remain evenly distributed throughout the epidermis. However, from 4 up to 24 h after injection, labelling became concentrated in the upper intermediate and replacement layers, suggesting that turnover proteins were produced. Histidine was probably converted into other metabolites at 4-24 h after injection. Whether the newly synthetized proteins were a form of keratin or a specific histidine-rich protein remains to be determined biochemically. Uptake of tritiated thymidine in newt epidermis indicated that keratinocytes move into the uppermost stratum intermedium within 4 days, and reach the replacement layer in approximately 6 days. Taken together, the data obtained with tritiated histidine and thymidine suggest that most histidine is taken up in the upper intermedium and replacement layer at 4-24 h after injection. Neither a granular layer nor crossreaction with filaggrin and loricrin were observed in fish and amphibian epidermis. Although the cell membrane of superficial corneous cells of amphibian epidermis became thicker, the absence of loricrine immunolabelling suggests that a cell corneous envelope containing this protein is not present or undetectable.

Keratinization of the epidermis of the Australian lungfish Neoceratodus forsteri (dipnoi)

The differentiation of the epidermis in sarcopterigian fish may reveal some trend of keratinization followed by amphibian ancestors to adapt their epidermis to land. Therefore, the process of keratinization of the epidermis of the Australian lungfish Neoceratodus forsteri was studied by histochemistry, electron microscopy, and keratin immunocytochemistry. The epidermis is tri-stratified in a 2-3-month-old tadpole but becomes 6-8 stratified in young adults. Keratin filaments increase from basal to external cells where loose tonofilament bundles are present. This is shown also by the comparison of positivity to sulfhydryl groups and increasing immunoreactivity to alpha-keratins in more external layers of the epidermis. Two broad-spectrum anti alpha-keratin monoclonal antibodies (AE1 and AE3) stain all epidermal layers as they do in actinopterigian fish. In the adult epidermis, but not in that of the larva, the AE2 antibody (a marker of keratinization in mammalian epidermis) often immunolabels more heavily the external keratinized layers where sulfhydryl groups are more abundant. Mucous granules are numerous and concentrate on the external surface of the epidermis to be discharged and contribute to cuticle formation. Keratin is therefore embedded in a mucus matrix, but neither compact keratin masses nor cell corneous envelope were seen in external cells. It is not known whether specific matrix proteins are associated with mucus. There was no immunolocalization of the keratin-associated proteins, filaggrin and loricrin, which suggests that the epidermis of this species lacks the matrix and cell corneus envelope proteins characteristic of that of amniotes. In conclusion, while specific keratins (AE2 positive) are probably produced in the uppermost layers as in amphibian epidermis, no interkeratin, matrix proteins seem to be present in external keratinocytes of the lungfish other than mucus.

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.

Brown bullhead (Ameiurus nebulosus) skin carcinogenesis

Alternative models using fish species have been tested in liver toxicity and carcinogenesis bioassays. Similar models have not been developed for skin. The brown bullhead (Ameiurus nebulosus) has shown potential as a model for skin carcinogenesis studies due to its sensitivity to environmental chemical pollutants. The present study is an initial morphologic and biochemical characterization of the normal and neoplastic brown bullhead skin to assess its suitability as a model of skin carcinogenesis. Brown bullhead were removed from Back River in the Chesapeake Bay region, an area historically polluted with heavy metals and polycyclic aromatic hydrocarbons. Histology, histochemistry, and electron microscopy were used to stage the morphologic development and progression of neoplasia in skin. The distribution of keratin, a family of structural proteins with altered expression in mammalian tumorigenesis, was analyzed with one and two dimensional gel electrophoresis and nitrocellulose blots of extracts from normal skin. Keratin expression in skin and other organs was also assessed with immunohistochemistry using AE1, AE3, and PCK 26 antibodies, and the proliferation index in skin and neoplasms with PCNA antibody. Skin lesions appeared to progress from hyperplasia through carcinoma, and the proliferation index was increased in papilloma. Also in papilloma, intercellular interdigitations appeared increased and desmosomes decreased which may in future studies correlate with changes in expression of other molecular markers of neoplastic progression. Both Type I and Type II keratin subfamilies were detected in skin using gel electrophoresis with the complimentary keratin blot-binding assay. For further development of the brown bullhead model, future studies can compare and relate these baseline data to alterations in expression of keratin and other markers in fish neoplasms and to molecular events which occur in man.

Molecular cloning of ictacalcin: a novel calcium-binding protein from the channel catfish, Ictalurus punctatus

Calcium is essential for a variety of functions in animals, including signal transduction, transmission of nerve impulses, and bone and scale growth. In freshwater adapted teleosts, blood calcium levels are maintained constant (2-4 mM) even at low external calcium concentration (< 0.01 mM). Epithelial cells in skin and gill have been implicated in calcium homeostasis. We have cloned a cDNA from Ictalurus punctatus, the channel catfish, that codes for ictacalcin, a novel member of the S100 family of calcium-binding protein. In-situ hybridization demonstrates ictacalcin mRNA is abundant in epithelial cells of olfactory rosette, barbel, skin and gill but not brain or muscle. The presence of ictacalcin protein in these tissues was confirmed by immuno-blot analysis. Tissue extracts and recombinant ictacalcin bind calcium with attendant changes in electrophoretic mobility indicative of changes in protein conformation. The calcium-binding activity and abundant localization of ictacalcin in epithelial cells of several tissues indicates that this protein plays an important role in catfish calcium homeostasis.

Fine structure of the skin of the amphibious fishes, Boleophthalmus pectinirostris and Periophthalmus cantonensis, with special reference to the location of blood vessels

The skin of the amphibious fishes, Periophthalmus cantonensis and Boleophthalmus pectinirostris, was investigated by transmission electron microscopy (TEM). In both species the epidermis consists of superficial nonvacuolated epithelial cells, swollen epithelial cells with membrane limited cytoplasmic vacuoles, and basal epithelial cells in a single layer. Unicellular mucous glands, but no chloride cells are found in the epidermis of B. pectinirostris; in contrast there are chloride cells and no unicellular mucous cells in P. cantonensis. Intraepidermal blood vessels are a notable feature in the epidermis of P. cantonensis. Capillaries are distributed near the epidermal surface, offering an air-blood barrier of sufficient thinness (2-4 μm) for cutaneous respiration. The large blood vessels (arterioles and venules) are seen in the middle stratum of the epidermis and seem to be able to regulate blood flow in the skin. In B. pectinirostris, the dermal capillaries lie immediately below the superficial epithelial cells at the apex of a dermal bulge where the air-blood barrier is almost as thin as that of P. cantonensis despite locally different histology. In the spacious dermal bulges, the dermal tissue such as blood vessels, pigment cells, fibroblasts, and collagen fibers are scattered. Melanophores and two other types of chromatophores occur in the part adjacent to the blood vessel wall. The organization of the epidermis and blood vessels of the skin is discussed with regard to terrestrial life in these amphibious fishes. © 1992 Wiley-Liss, Inc.

Studies on the structure and histochemistry of the epidermis in the marine catfish Plotosus lineatus (Thunberg, 1791) (Plotosidae, Pisces)

The structure and the histochemical characteristics of the epidermis of the marine catfish Plotosus lineatus were determined with a series of recent techniques employed either alone or in combination with other procedures to detect selectively sialic acid containing glycoproteins or other acidic carbohydrates. The epidermis consists of a multilayered epithelium where are recognizable various cell types namely, the epithelial cells, the club cells and the mucous or goblet cells the latter concentrated mainly in the outermost layer. The goblet cells and the epithelial cells in the outer epidermis produced both neutral and acidic glycoproteins. The epithelial cells store also acidic glycoproteins containing high amounts of sulphate esters (sulphomucins). The absence of the sialic acid is also discussed.