Epidermal Club Cells in Fishes: A Case for Ecoimmunological Analysis

Epidermal club cells in the cardinal tetra (Paracheirodon axelrodi): Presence, distribution, and relationship to antipredator behavior

Epidermal club cells (ECCs) are present in many species of teleost fish. In an attempt to justify their presence in the epidermis of fish, they have been associated with numerous functions. One proposed function is communication with conspecifics during a predation event, as these cells may passively release substances upon rupture, which may occur during predation. We identified the presence and distribution of ECCs in the body skin of adult cardinal tetra, Paracheirodon axelrodi (Schultz, 1956) and analyzed the animal's behavioral response to conspecific skin extract in a laboratory setting. The identification and distribution of ECCs in the epidermis of the animals were confirmed by conventional histology and immunohistochemistry. Our results demonstrated that: ECCs are present in the skin of the entire body; a high density is observed in the dorsal side from head to tail, in the insertion of the fins and in the epidermis covering them; and ventral distribution is less extensive and more dispersed than dorsal distribution. Treatment of P. axelrodi specimens with skin preparations of conspecifics resulted in behavioral changes in the animals: they showed erratic swimming movements, they showed avoidance of the area of stimulus application and they decreased the time spent moving. Overall, these results allow us to conclude that P. axelrodi possesses ECCs throughout the body, with a greater presence in areas of high exposure to predation events (dorsal area and fins). Animals exposed to conspecific skin extract showed a significant increase in behaviors described as anti-predatory in other species. This supports the hypothesis that ECCs may be the origin of chemical alarm cues that are passively released when skin damage occurs, alerting the rest of the group to the risk of predation.

Cellular patterning and cyto-architectural organization of the skin of electric catfish (Malapterurus electricus, Siluriformes) with a particular emphasis on its ampullary electroreceptor

The functional morphology of the skin of Malapteruridae is presumably evolved to cope with a diversified range of ambient physiological, environmental, and behavioral conditions. Herein, we firstly characterized the microstructures and intriguing patterning of the skin of twelve adult electric catfish (Malapterurus electricus, Malapteruridae) using histological, histochemical, immunofluorescent, and ELISA standard methodology. The skin comprises three sequentially-oriented layers: the epidermis, dermis, and hypodermis with a significantly increased thickness of the former. The epidermis contains four types of cells: the surface epithelial cells, mucous cells, granular cells, and club cells. We defined distinctive ampullary electroreceptors in the outer epidermis that possess flask-shaped sensory crypt containing electroreceptor cells together with vertical collagen rods. Dermis and hypodermis are composed of connective tissue; however, the former is much more coarse and dense with comparable reactivity for Masson-Goldner trichrome (MT). Placing our data in the context of the limited body of previous work, we showed subtle changes in the expression of mucin subunits together with cytoskeletal fractions of collagens, myosin, F-actin, keratins, and tubulins. Taken as a whole, our results convincingly showed that the skin of M. electricus shares some structural similarities to other Siluriformes, however, it has some functional modifications that are implicated in protection, defense, and foraging behavior.

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.

Acquired predator recognition via epidermal alarm cues but not dietary alarm cues by isolated pupfish

We tested whether Shoshone pupfish Cyprinodon nevadensis shoshone and Amargosa River pupfish C. n. amargosae respond behaviourally to conspecific chemical alarm cues released when epidermal tissue is damaged by a predator. We found that both subspecies reduced activity and vertical position in the water column in response to alarm cues. We then tested if pupfish can use alarm cue to acquire recognition of a novel predator. We trained pupfish with (1) water + odour of largemouth bass fed a diet of earthworms, (2) alarm cues from skin extract (epidermal alarm cues) + odour of bass fed a diet of earthworms, or (3) water + odour of bass fed a diet of pupfish (dietary alarm cues). Pupfish responded to epidermal alarm cues but not to dietary alarm cues. Pupfish were retested with the odour of bass that were fed an earthworm diet. Pupfish that had previously received epidermal alarm cues reduced vertical position and activity relative to the other two treatments. This is the first demonstration of acquired recognition of a novel predator by a pupfish, the first report of partial predator naiveté, and opens the possibility of predator-recognition training as a tool for management and conservation of endangered desert fishes.

Interspecific differences and ecological correlations between scale number and skin structure in freshwater fishes

Fish skin is mainly composed of the epidermis, dermis, and its derivative scales. There is a wide diversity in scale number in fishes, but the diversity of skin structure lacks systematic histological comparison. This research aimed to improve our understanding of the functional relationship between the scale number and the skin structure in freshwater fishes and to determine which ecological factors affect the scale number and skin structure. First, we presented a method to quantify skin structure in fish and histologically quantified the skin structure of 54 freshwater fishes. Second, we collected the scale number and habitat information of 509 Cyprinidae fishes in China and explored which ecological factors were related to their scale number. Third, common carp and scaleless carp were used as models to study the effects of scale loss on swimming. We found a strong negative correlation between scale thickness and scale number. The main factor affecting the skin structure of fishes was the species' water column position, and the skin of benthic fishes was the most well-developed (thicker skin layers (dermis, epidermis) or more/larger goblet cells and club cells). The scale number was related to two factors, namely, temperature and water column position, and cold, benthic and pelagic adaptation may have contributed to increased scale numbers. Only in benthic fishes, the more well-developed their skin, the more scales. In common carp, scale loss did not affect its swimming performance. In summary, we suggest that there is a rich diversity of skin structure in freshwater fishes, and the scales of fish with well-developed skin tend to degenerate (greater number/smaller size/thinner, or even disappear), but the skin of fish with degenerated scales is not necessarily well developed.

Epidermal threads reveal the origin of hagfish slime

When attacked, hagfishes produce a soft, fibrous defensive slime within a fraction of a second by ejecting mucus and threads into seawater. The rapid setup and remarkable expansion of the slime make it a highly effective and unique form of defense. How this biomaterial evolved is unknown, although circumstantial evidence points to the epidermis as the origin of the thread- and mucus-producing cells in the slime glands. Here, we describe large intracellular threads within a putatively homologous cell type from hagfish epidermis. These epidermal threads averaged ~2 mm in length and ~0.5 μm in diameter. The entire hagfish body is covered by a dense layer of epidermal thread cells, with each square millimeter of skin storing a total of ~96 cm threads. Experimentally induced damage to a hagfish's skin caused the release of threads, which together with mucus, formed an adhesive epidermal slime that is more fibrous and less dilute than the defensive slime. Transcriptome analysis further suggests that epidermal threads are ancestral to the slime threads, with duplication and diversification of thread genes occurring in parallel with the evolution of slime glands. Our results support an epidermal origin of hagfish slime, which may have been driven by selection for stronger and more voluminous slime.

Morpho-functional changes of lungfish Protopterus dolloi skin in the shift from freshwater to aestivating conditions

African dipnoi (Protopterus sp.) are obligate air-breathing fish that, during dry season, may experience a period of dormancy named aestivation. Aestivation is characterized by complete reliance on pulmonary breathing, general decrease of metabolism and down-regulation of respiratory and cardiovascular functions. To date, little is known about morpho-functional rearrangements induced by aestivation in the skin of African lungfishes. Our study aims to identify, in the skin of P. dolloi, structural modifications and stress-induced molecules in response to short-term (6 days) and long-term (40 days) aestivation. Light microscopy showed that short-term aestivation induces major reorganization, with narrowing of epidermal layers and decrease of mucous cells; prolonged aestivation is characterized by regenerative processes and re-thickening of epidermal layers. Immunofluorescence reveals that aestivation correlates with an increased oxidative stress and changes of Heat Shock Proteins expression, suggesting a protective role for these chaperons. Our findings revealed that lungfish skin undergoes remarkable morphological and biochemical readjustments in response to stressful conditions associated with aestivation.

The feasibility of skin mucus replacing exosome as a pool for bacteria-infected markers development via comparative proteomic screening in teleost

In fish, skin mucus forms a protective barrier between the body surface and the external water environment, thus providing the most direct and intuitive clues to monitor the subject's health condition. To explore the impact of the Vibrio harveyi pathogen on teleost, the proteome of epidermal mucus from control and sick Cynoglossus semilaevis were screened through iTRAQ followed with LC-MS/MS. 1531 credible proteins were obtained relating to structural, metabolic and immunological functions. 335 different expressed proteins (DEPs) were identified, with 166 up-regulated and 169 down-regulated in MS. 62 proteins were characterized, including 22 up-regulated proteins and 40 down-regulated proteins. Integrated analysis of DE-miRNAs and DEPs from miRomics and proteomics were conducted to show the indirect regulatory relationship. Comparative analysis of DEPs between mucus and exosomes demonstrated that exosomes contributed the most DEPs of all mucus DEPs. 125 proteins are DEPs only in exosomes, which presented minor difference in total mucus. Expression of Aminopeptidase (anpep), Calcium-transporting ATPase, Histone H2B and H2A confirmed implied fine discriminative power with infected C. semilaevis, among which Calcium-transporting ATPase and H2B also appeared in list of exosomal markers. This study might shed the light on effective biomarker digging at other extended screening scenarios.

Ignorance is not bliss: evolutionary naiveté in an endangered desert fish and implications for conservation

Predator naiveté has been invoked to explain the impacts of non-native predators on isolated populations that evolved with limited predation. Such impacts have been repeatedly observed for the endangered Pahrump poolfish, Empetrichthys latos, a desert fish species that evolved in isolation since the end of the Pleistocene. We tested Pahrump poolfish anti-predator responses to conspecific chemical alarm cues released from damaged epidermal tissue in terms of fish activity and water column position. Pahrump poolfish behavioural responses to conspecific alarm cues did not differ from responses to a dechlorinated tap water control. As a positive control, the well-studied fathead minnow, Pimephales promelas, showed significant alarm cue responses in terms of reduced activity and lowered water column position. The density of epidermal club cells, the presumptive source of alarm cues, was significantly lower in Pahrump poolfish relative to fathead minnows. Therefore, anti-predator competence mediated by conspecific alarm cues does not seem to be a component of the ecology of Pahrump poolfish. These findings provide a proximate mechanism for the vulnerability of Pahrump poolfish to non-native predators, with implications for the conservation and management of insular species.

Changes in marine turbot (Scophthalmus maximus) epidermis and skin mucus composition during development from bilateral larvae to juvenile flat fish

Alike other flat fish, marine turbot has the particularity that changes from larvae with bilateral symmetry to adult with asymmetry, in terms of the position of the eyes. As expected, the skin configuration of this species is also affected by the development and transformation suffered by fish during metamorphosis. In this context, changes in the epidermis of marine turbot were studied using conventional staining and histochemical techniques using six lectins (UEA-I, PNA, RCA-I, WGA, Con A and SBA). During development from larvae to juvenile (3-300 days post-hatching), the epidermis increased in both thickness and the number of cell layers. In fact, the simple cuboidal epithelium observed in larvae at day 3 already became stratified at days 10-12, which sequentially increase in thickness with fish development. Turbot epidermis is composed basically of four cell types: epithelial and mucous or secretory cells that are present through the development, and pigmented cells and a type that the authors described as club-like cells that appear during and post-metamorphosis. The Alcian blue-periodic acid Schiff (AB-PAS) histochemical method revealed the presence of neutral glycoconjugates in mucous and club-like cells at post-metamorphic stages of fish. Accordingly, lectin analysis showed mucous cells containing glycoproteins rich in fucose (UEA-I labelling) and glycoconjugates rich in the sequence galactose-N-acetyl galactosamine (PNA and RCA-I labelling) when this cell type appears. Interestingly, melanophores were observed in the dorsal epidermis of post-metamorphic juveniles. This type of cell contains a black-to-brown pigment that provides the skin the typical colour of this fish species. Changes in mucous coat composition were observed during fish development, which was attributed to different roles of the glycoconjugates.

Endocrine, Stemness, Proliferative, and Proteolytic Properties of Alarm Cells in Ruby-Red-Fin Shark (Rainbow Shark), Epalzeorhynchos frenatum (Teleostei: Cyprinidae)

The current study investigated the morphological, histochemical, and immunohistochemical characteristics of alarm cells and their precursors in ruby-red-fin shark (rainbow shark), Epalzeorhynchos frenatum (Teleostei: Cyprinidae). Precursor alarm cells were shown to be small, cuboidal, pyramidal, or round in shape, with eosinophilic cytoplasm, resting on the basement membrane of the epidermis. The cells later elongated to become columnar in shape. Subsequently, they enlarged and became large oval-shaped cells. They then underwent shrinkage and vacuolation. The superficial alarm cells were collapsed. Alarm cells were found to have an affinity for different histochemical stains, including bromophenol blue, iron hematoxylin, Sudan black, Mallory triple trichrome, Crossman's trichrome, Safranin O, and Weigert's stains, as well as lipase and alkaline phosphatase. Endocrine properties of the alarm cells were identified by silver staining and synaptophysin immunostaining. Alarm cells exhibited stemness activities and had a strong immunoaffinity for CD117. They had a proteolytic function, as identified by lysosome-specific staining with acridine orange and strong immunoaffinity for matrix metalloproteinase (MMP-9). They also exhibited proliferatively, reflected by immunological staining by proliferating cell nuclear antigen. In conclusion, alarm cells are unique epidermal cells with multiple functions. They play immunological, and endocrine, roles. They also retain stemness and proliferative properties.

Skin Wound Healing Rate in Fish Depends on Species and Microbiota

The skin is a barrier between the body and the environment that protects the integrity of the body and houses a vast microbiota. By interacting with the host immune system, the microbiota improves wound healing in mammals. However, in fish, the evidence of the role of microbiota and the type of species on wound healing is scarce. We aimed to examine the wound healing rate in various fish species and evaluate the effect of antibiotics on the wound healing process. The wound healing rate was much faster in two of the seven fish species selected based on habitat and skin types. We also demonstrated that the composition of the microbiome plays a role in the wound healing rate. After antibiotic treatment, the wound healing rate improved in one species. Through 16S rRNA sequencing, we identified microbiome correlates of varying responses on wound healing after antibiotic treatment. These findings indicate that not only the species difference but also the microbiota play a significant role in wound healing in fish.