Morphological characteristics of the digestive tract of Schizodon knerii (Steindachner, 1875), (Characiformes: Anostomidae): An anatomical, histological and histochemical study

Morphological and Molecular Functional Evidence of the Pharyngeal Sac in the Digestive Tract of Silver Pomfret, Pampus argenteus

The pharyngeal sac is a comparatively rare organ in the digestive tract among teleost fishes. However, our understanding of this remarkable organ in the silver pomfret (Pampus argenteus) is limited. In the present study, we examined the various morphological and histological characteristics of the pharyngeal sac using histochemical techniques and electron microscopy. The pharyngeal sac showed unique characteristics such as well-developed muscular walls, weakly keratinized epithelium, numerous goblet cells, and needle-like processes on the papillae. The porous cavity of the papillae contained numerous adipocytes and was tightly enveloped by type I collagen fibers. These structures might provide mechanical protection and excellent biomechanical properties for grinding and shredding prey. A comparison of gene expression levels between the pharyngeal sac and esophagus using RNA-seq showed that phenotype-associated genes (epithelial genes and muscle genes) were upregulated, whereas genes related to nutrient digestion and absorption were downregulated in the pharyngeal sac. These results support the role of the pharyngeal sac in shredding and predigesting food. Overall, these findings provide a clearer understanding of the pharyngeal sac morphology and explain the morphological adaptations of the digestive tract for feeding on gelatinous prey. To our knowledge, this is the first report on pharyngeal sac gene expression in P. argenteus.

Morphological study of the digestive tract of the cardinal tetra, Paracheirodon axelrodi (Characiformes: Characidae)

Abstract The cardinal tetra Paracheirodon axelrodi is a species of the family Characidae of great interest as an ornamental fish. Many aspects of the biology of this species are still unknown. The present work presents a complete description of the different components of the digestive tract of P. axelrodi, analyzing its global anatomical arrangement, the adjacent glands (liver and pancreas) and the previously ignored pyloric caeca, using histological and histochemical techniques with light microscopy and scanning electron microscopy. Three levels of analysis were performed: macroscopic anatomy, histology of the gastrointestinal tract and scanning electron microscopy of mucosal surfaces. The intestinal coefficient (Ci) of P. axelrodi calculated here (0.94) is within the expected range for a carnivorous species. The anatomy of the alimentary and the resulting Ci is consistent with carnivorous diet in line with dietary studies in the wild in this species. The digestive tract, including adjacent glands (liver and pancreas), presented a relatively conserved general pattern within that described for other species of the family Characidae, especially at the histological level.

Morphological characteristics of the blackspot seabream (Pagellus bogaraveo) tongue: A structural and immunohistochemical study

The blackspot seabream (Pagellus bogaraveo, Brünnich, 1768) is an omnivorous, predominantly carnivorous fish. In aquaculture, it is fed with pellets rich in proteins and fat. The morphological and functional aspects of the fish tongue, the feeding modality and the tasting capacity are strictly related. Therefore, the aim of this study was to describe by scanning electron, light and confocal laser microscopy, the morphological characteristics of the tongue in this species. It showed an apex, a body and a root. There were rows of teeth on the edges of the mouth and taste pores on all the tongue dorsal surface with folds and furrows. In addition, body and root showed several fungiform-like papillae in the mucosa of the folds, covered by a weakly keratinized stratified squamous epithelium, can be observed. The papillae were innervated by S100 positive fibres. In the apex, a mesenchymal tissue with vimentin positive star-shaped stem cells was evident. The results could give a support for a wider use of the blackspot seabream as a farmed species, considering the morphological data as correlated with the potentiality of food discrimination. This provides a basis for possible applications in feeding strategies. The presence, localization and characteristics of the mesenchymal stem cells, as seen also in previous studies, could represent a further basis for future applications in clinical trials.

Comparative analysis of the small and large intestines of Acará Geophagus brasiliensis (Quoy & Gaimard, 1824) (Pisces: Cichlidae)

Geophagus brasiliensis, popularly known as acará, is a common fish in lentic freshwater environments in South America. This species has a detritivorous-iliophagous or omnivorous feeding habit, with high food plasticity; however, there are no studies describing its intestinal tract histologically. Therefore, the present study analysed through histological and histochemical techniques the intestines of the acará. Adult specimens were collected with gillnets, anaesthetized and euthanized. Then, the fish were submitted to biometry and dissection to remove fragments of intestines. The samples were fixed in Bouin liquid for 12 hours and subjected to histological and histochemical techniques. Histologically, all samples of intestines were organized into four layers: mucosa, submucosa, muscular and serosa. The small intestine (foregut and midgut) was characterized by the presence of intestinal villi covered by simple prismatic epithelium with a striated border and goblet cells supported by the connective lamina propria. In the large intestine (hindgut), there was an absence of villi and an abundance of goblet cells. Positive reaction to Periodic Acid-Schiff (PAS) and Alcian Blue (AB) pH 2.5 reactions were detected in goblet cells, indicating the presence of mucosubstances. No lipids were detected in the intestine cells due to the negative reaction to the Sudan Black B. The results of the present study provide subsidies for a better understanding of the intestinal morphology of teleosts and provide valuable information for phylogenetic studies.

Digestive tube of an herbivorous fish (Rhytiodus microlepis) from the Amazonian floodplain lakes: A morphological and histochemical study

This study describes the morphology and histochemistry of the digestive tube of the herbivorous fish Rhytiodus microlepis, which is commonly found in the Amazonian floodplain lakes, Brazil. Thirty-eight fish were used in this study. The digestive tube was measured and dissected for anatomical description, and stained with (i) haematoxylin and eosin, for histological analyses; (ii) periodic acid-Schiff, for detected neutral mucins; and (iii) Alcian Blue (AB) pH 1.0 and 2.5 for acid and sulphated acid mucins. The oesophagus, with its secretory cells and taste buds, is adapted for lubrication of vegetable matter, resistance to friction and reception of stimuli. The stomach is U-shaped and rich in gastric glands, particularly in cardiac and fundic regions, which are adapted to receive large amounts of food, and promotes the digestion. The intestine comprises more than 70% of the digestive tube. All portions of the intestine, except the rectum, have the same pattern of mucosal folds, and the goblet cells reacted positively to all histochemical methods (PAS, AB 1.0 and 2.5), while rodlet cells reacted only to periodic acid-Schiff (PAS) staining. Neutral mucins and sulphated acid mucins predominated in the anterior portion of the intestine and a high concentration of carboxylated acid mucosubstances were present in the other portions. The rectum showed a strong reaction to all histochemical methods. The muscular layer of the intestine consists of three layers, which showed features presumably related to the rigid nature of the food. A simple squamous epithelium constitutes the serous layer over the entire length of the tube.

Morpho-histochemical adaptations of the digestive tract in Gangetic mud-eel Ophichthys cuchia (Hamilton 1822) support utilization of mud-dwelling prey

Freshwater mud-eel, Ophichthys cuchia is nocturnal, carnivorous and economically important fish, yet its digestive physiology is unknown. We therefore studied the gastrointestinal (GI) tract of O. cuchia using morphological, osteological, histological and histochemical approach to understand how the structural adequacy of GI tract helps in acquisition of bottom and mud-dwelling prey and supports utilization of uncommon food resources. Morphologically the GI tract showed typical features of carnivorous fishes in the form of sub-terminal mouth, short muscular esophagus, expendable stomach, short intestine and rectum. Osteological investigation clearly showed that the specialized arrangement of teeth in the oral cavity and pharyngeal region helps in digging and manipulation of bottom and mud-dwelling prey. Longitudinal mucosal folds, stratified squamous epithelium with numerous goblet cells of esophagus protect mucosa from mechanical harm and also allow easy transit of prey into the stomach. Large-sized rugae of stomach are complemented with numerous gastric glands which together increase the transit time of carnivorous food stuff and ensure its complete digestion. The pylorus specifically allows only digested and finely ground food stuff into the intestine and thus avoids the intestinal blockage due to entry of undigested food. The enterocytes and goblet cells of intestine are absorptive and lubricative in their functions respectively. A significant increase in the number of goblet cells and thickness of muscularis from the intestine to the rectum was evident which facilitates the easier transit of food, protection for the epithelium from abrasion, and helps in defecation. Histochemistry revealed that the mucins are secreted throughout the GI tract and thus catalyze faster digestion of carnivorous food stuff, providing protection to mucosal membrane from abrasion, acts as a co-factor to support digestion, absorption of proteins, ions, fluids and helps in defecation. The outcome of this study clearly supports the notion that structural adaptations in the digestive tract of fishes can be effectively used as a blueprint to understand why and how particular fish species feed and use unique food. Additionally, the structural and functional adequacy of the digestive tract helps the fishes to acquire and utilize novel habitats and food resources. The results presented in this study will serve as a reference point for future studies, which focus primarily on understanding the evolution of carnivory in Synbranchids.

Anatomical, histological and immunohistochemical study of the tongue in the rainbow trout (Oncorhynchus mykiss)

The rainbow trout (Oncorhynchus mykiss Walbaum, 1792) is a fish commercially farmed all over the world. These fish are usually fed, in aquaculture, with pellets rich in proteins and fat. It is well known that there are close relationships among the adaptation of vertebrates to their environment, the capacity and the modality of feeding and the oral cavity morphology, especially the tongue one. No data are so far available about the morphology of the rainbow trout tongue, and therefore, the aim of this study was to investigate by light, scanning electron and confocal laser microscopy, the morphological characteristics of the tongue. An apex, a body and a root can be distinguished in the tongue, and the presence of teeth, taste buds and fungiform-like papillae was demonstrated. Light microscopy shows the presence of an adipose tissue pad in the deeper layer of the apex and in the most superficial layer of the root. In the deeper layer of the body, a triangular-shaped pad consisting of fusiform cells immersed in abundant extracellular matrix of the mesenchymal tissue was observed. The confocal laser microscopy shows the presence of cells with a fibroblast-like morphology positive for vimentin. In the deepest layer of the tongue root, a large area of osteo-cartilaginous tissue was observed. The results, besides the description of the morphological characteristics of the tongue, related to studies regarding the feeding, could be considered for the eventual applications of the use of mesenchymal cells, observed in adult fish, in cell therapies in different pathologies.

Morphology of the Atlantic salmon (Salmo salar) tongue

The Atlantic salmon (Salmo salar) is a freshwater and marine fish of the family Salmonidae, widely farmed in aquaculture facilities in several countries. The salmon are carnivorous, but in aquaculture, alternative foods have been experienced. It is well known that feeding in captivity should cause adaptation and modifications of the morphological characteristics of the oral cavity, especially of tongue; therefore, the aim of this study was to investigate, by light, laser confocal and scanning electron microscopy, the morphological characteristics of the tongue dorsal surface, considering the importance of the correlations between feeding habits and the anatomy of the tongue. Scanning electron microscopy demonstrates the presence of caniniform teeth with oro-aboral orientation surrounded by numerous filiform papillae, single, fused or arranged in row. Oro-aborally, the papillae show an appearance like a rosette and they disappear at level of the root. Light and laser confocal microscopy demonstrates that the mucosa is covered by a non-keratinized stratified pavement epithelium with, in the deepest layer, the presence of a triangular structure whose apex is cranially directed and base facing aborally. In this structure, spindle-shaped cells are present, with a vimentin immunoreactivity, that for their characteristics could be adult mesenchymal stem cells. The obtained data could be useful not only for further studies on the nutrition, but it is interesting the detection of tissues typical of the embryo-fetal phase in the adult specimens tongue, thus giving a basis for studies of potential applications, if any, regarding cell therapies for different clinical indications.

Modifications in the gills of hill stream Moth catfish, Hara hara (Erethistidae, Siluriformes): A light and scanning electron microscope investigation

Present study reports significant modifications in surface ultrastructure, histological organization, and histochemical localization of glycoproteins (GPs) in the gills of a hill stream catfish, Hara hara. Punctate microridges on free surface of epithelial cells covering gill arches, gill rakers, gill filaments and secondary lamellae are considered to provide adaptive plasticity to gills in relation to the environment inhabited by fish. Short and stout gill rakers are considered to prevent food particles to pass in opercular chamber along with respiratory current that could damage delicate gill filaments. Mucous goblet cells show presence of different classes of glycoproteins. GPs with oxidizable vicinal diols are considered to control acidity of acidic GPs. GPs with carboxyl groups have been implicated with defensive mechanism against microorganisms. GPs with O-sulphate esters are associated to trap and to lubricate food particles for easy swallowing. Taste buds on gill arches and gill rakers function to select palatable food particles. Occurrence of taste buds on the gill filaments is regarded significant adaptation to analyse the chemical nature of water. This study could play a significant role to understand adjustment of gills in the hill stream fish.

Structural adequacy of the digestive tract supports dual feeding habit in catfish Pachypterus khavalchor (Siluriformes: Horabagridae)

Lepidophagy is comparatively rare amongst teleost fishes, yet our understanding of this specialization is lacking. Therefore we examined the digestive tract features of Pachypterus khavalchor using morphological, osteological, histological and histochemical techniques to comprehend and relate structural organization of digestive tract with scale eating habit. Morphologically, the alimentary canal is defined by a short and muscular esophagus, well-developed stomach and comparatively short intestine. Gut content analysis and intestinal coefficient value (0.53 ± 0.01) revealed that P. khavalchor exhibit both carnivory and lepidophagy. However, P. khavalchor primarily feeds on the scales (67.47%) and other chitin-rich material like aquatic insects (17.62%), aquatic larvae (8.66%) which affirms its solid association with chitinase producing endosymbionts in the gut. Lepidophagy is further supported by the osteological observations. The perfect segregation of the functions such as food capture, ingestion and processing amongst the different types of teeth located in the oral cavity and pharyngeal region thus could be taken as evolutionary adaptations in scale eaters to support lepidophagy. Specialized arrangement of the esophageal and stomach epithelial folds could be altogether taken as an adaptation with the end goal to frame the scale stacks and accordingly facilitate the handling and processing of chitin-rich bolus. The esophageal mucosa is simple squamous epithelium instead of stratified epithelium with numerous goblet cells to withstand the mechanical harm by hard-food stuff like scales. The cardiac and fundic regions exhibited large number tubular gastric glands with simple columnar epithelium. Surface cells of all three stomach regions stained positive for PAS staining. The intestine is without pyloric caeca and is divided into anterior and posterior region. Histologically it is characterized by simple columnar epithelium with brush border and numerous goblet cells throughout its length. Presence of large number microvilli on anterior and posterior intestine was noticeable. Intestinal goblet cells reacted positively to PAS, AB (pH 1) and AB (pH 2.5). Secretions of goblet cells are important for lubricating and protecting the epithelium. The results of present investigation improve the understanding of the digestive physiology of scale eaters in general and P. khavalchor in particular. Overall, our data indicates that though P. khavalchor predominantly feeds on scale, the digestive physiology is adapted to support dual feeding habit (lepidophagy and carnivory).

In vivo multivesicular bodies and their exosomes in the absorptive cells of the zebrafish (Danio Rerio) gut

Intercellular communication of gut epithelial cells is critical to gut mucosal homeostasis. Exosomes are important intercellular mediators in communication between cell to cell. Although many literature focus on the immunologic roles in the gut by the exosomes, the biological process of exosomes in the absorptive cells remains unknown. Uncovering the distribution, classification and formation process of multivesicular bodies (MVBs) and their exosomes in the absorptive cells of the zebrafish gut, is urgently needed to establish a platform for immunological research of fish gut exosomes. The expression levels of CD63 and TSG101 were different among the three segments of the gut, and they were enriched at the apex of the mid gut villi. The characteristics of MVBs and their exosomes in the absorptive cells were further revealed by transmission electron microscopy (TEM). Early endosomes (ee) were mainly present in the apical and basal cytoplasm of absorptive cells. Late endosomes (le) were mostly distributed with the supranuclear part of these cells. "Heterogeneous" MVBs were detected underlying the apical membranes of absorptive cells. Many exosomes with some MVB-like structures occurred in the lumen, indicating that the release process was mainly through apical secretion. Various MVBs with exosomes and the endosome-heterogeneous MVB-exosome complex existed widely in the mid gut absorptive cells, concluding that zebrafish as a potential model for in vivo MVBs and their exosomes research. All the results were summarized in a schematic diagram illustrating the morphological characteristics of gut MVBs and their exosomes in zebrafish.

The digestive tube of Piaractus brachypomus: gross morphology, histology/histochemistry of the mucosal layer and the effects of parasitism by Neoechinorhynchus sp

The objective of the present study was to describe the histology and histochemistry of the mucosal layer of the digestive tube of Piaractus brachypomus, and the histopathology associated with parasitism by Neoechinorhynchus sp. The digestive tube of P. brachypomus consists of three macroscopically distinct portions: short, rectilinear and elastic-walled ooesophagus, J-shaped siphon stomach and a long intestine with rectilinear and curved portions, defined by patterns of villi as foregut, midgut, and hindgut. Histological and histochemical differences were observed in the mucosal layers of the different digestive tube regions, such as intense production of neutral and acidic mucous substances in the pseudostratified mucosal epithelium of the oesophagus; positive periodic acid Schiff reagent (PAS)reactions at the apex of the columnar epithelial cells of the stomach and increased intensity of histochemical reactions in the hindgut region. Neoechinorhynchus sp. was present in 85.7% of specimens examined, with a mean intensity of 7.4 ± 6.2 (±) and abundance of 6.33. Good health of the fish indicated by high relative condition factor values ( _Kn ) and occurrence of only mild to moderate alteration in the mucosal layer indicated that Neoechinorhynchus sp. exhibits low pathogenicity towards P. brachypomus hosts in farming environments, with low levels of infection.

Structure of the alimentary tract in the Atlantic mudskipper Periophthalmus barbarus (Gobiidae: Oxudercinae): anatomical, histological and ultrastructural studies

The alimentary tract of oxudercine gobies is characterized by a lack of an anatomically distinct stomach, owing to which they are classified as stomachless. Since the environment, food requirements, and feeding habits have a significant impact on the anatomy of the alimentary tract of fish, it was assumed that predominantly carnivorous, semi-terrestrial mudskippers would have a stomach. In order to verify this hypothesis, anatomical, histological, histochemical and ultrastructural analysis of the alimentary tract of the Atlantic mudskipper Periophthalmus barbarus was performed. The results revealed that despite a lack of clear anatomical distinction within the alimentary tract, there were four well-distinguished sections visible at the histological level: oesophagus, stomach, intestine, and rectum. The division was enhanced by the presence of a pyloric sphincter and an ileorectal valve. The stomach contained tubular glands composed of oxynticopeptic cells. Gland cells had pepsinogen granules and a well-developed tubulovesicular network of smooth membranes, which indicates the secretion of gastric juice. The presence of neutral mucus in the apical region of surface epithelial cells as protection against hydrochloric acid as well as the presence of active pepsin also confirm gastric function. However, low pepsin activity seems to implies low protein digestion. The results of this study indicate that the Atlantic mudskipper P. barbarus has a functional stomach.

Morpho-histological characterisation of the alimentary canal of an important food fish, Asian seabass (Lates calcarifer)

Asian seabass (Lates calcarifer) is a food fish of increasing aquaculture importance. In order to improve our understanding on the digestive system and feeding of this species, morphological and histological features of the gut were studied. Morphologically, the Asian seabass gut is defined by a short and muscular esophagus, well-developed stomach and comparatively short intestine. Mucous secreting goblet cells reactive to PAS (Periodic Acid Schiff) and AB (Alcian Blue) stain were present throughout the esophagus. The stomach was sac-like and could be distinguished into the cardiac, fundic and pyloric regions. Gastric glands and mucus cells were predominately present in the cardiac and fundic regions. Five finger-like pyloric caeca were present between the stomach and intestine. The intestine was a short, tubular structure with no morphological differences between the various regions. Histologically, the intestinal regions were similar, the main difference being in the number of goblet cells that increased from anterior to posterior intestine, with 114 ± 9, 153 ± 7 and 317 ± 21 goblet cells in the anterior, mid and posterior regions, respectively. The intestinal epithelium stained positively for PAS, but the staining was stronger for acidic glycoproteins. The rectum was similar to intestine, except for increased goblet cell numbers (anterior rectum: 529 ± 26; posterior rectum: 745 ± 29). Gut morpho-histology did not respond to salinity changes, however, there was a significant reduction of mucosal height, goblet cell numbers and muscularis thickness upon food deprivation.