The endocrine cells in the gastroenteropancreatic system of the bowfin, Amia calva L.: an immunohistochemical, ultrastructural, and immunocytochemical analysis

Anatomy of the endocrine pancreas in actinopterygian fishes and its phylogenetic implications

The anatomy and organisation of the endocrine pancreas in ray-finned fishes vary widely. The two main morphoanatomical character states are diffuse versus compact pancreatic tissue. The latter are called Brockmann Bodies (BBs), or principal islets. The present study is the first comprehensive survey on the anatomy of the endocrine pancreas (diffuse versus compact) across 322 actinopterygian species in 39 orders and 135 families based on literature, specimen dissections, and Magnetic Resonance Imaging (MRI). The data obtained show that large endocrine pancreatic islets (BB) have appeared several times in teleost evolution: in some ostariophysian clades and within the Salmoniformes and Neoteleostei. Acanthomorpha (spiny-rayed fishes) is the largest clade of the Neoteleostei. Within this clade, an absence of BBs is only observed in flying fishes (Exocoetidae), parrotfishes (Scarinae), and some of the scarine relatives, the Labridae. The presence of BBs in examined jellynose fish species from the Ateleopodiformes indicates support for its sister-group relationship to the Ctenosquamata (Myctophiformes + Acanthomorpha). More investigations are still needed to corroborate the presence or absence of BBs as a putative synapomorphy for a clade comprising Ateleopodiformes and Ctenosquamata.

Structural and immunohistochemical characterization of pancreas of Molly fish (Poecilia sphenops), with a special reference to its immune role

Recently, teleost species have been considered important model systems for investigating different research areas including immunologic one. The available literature provides poor data about the localization and the structure of pancreas in Molly fish. Moreover, little attention has been paid to the immunologic role of pancreatic tissue of teleost, particularly Molly fish; therefore, this study aimed to highlights the description of pancreatic tissue in Molly fish using light- and electron- microscopy, focusing on the role of pancreatic immune cells and pancreatic acinar cells in immune responses. Microscopic analysis revealed that the pancreas of Molly fish was composed of intrahepatic, disseminated and compact parts. Exocrine pancreatic tissue was diffusely extended within the hepatic tissue forming hepatopancreas. The disseminated pancreas appeared as several irregular nodules of pancreatic tissue localized within the mesenteric adipose tissue. The compact pancreas appeared as an oval shaped body embedded within the mesenteric adipose tissue between the spleen and the intestinal loops. Several telocytes and melanomacrophages were detected within the disseminated pancreatic nodules. Moreover, dendritic cells were found in a close association to the exocrine pancreatic acini. The pancreatic acinar cells showed strong immunoreactivity to APG5, TGF-β, IL-1β, NF-κB, Nrf2, and SOX9 in both hepatopancreas and disseminated pancreas of Molly fish. S100 protein revealed a strong expression in the exocrine pancreatic acinar cells of disseminated pancreas and also in the endocrine cells of the compact pancreas. In conclusion, findings of this study suggest the potential role of the pancreas of the Molly fish in cell proliferation and differentiation, proinflammatory cytokines stimulation, and regulation of both innate and adaptive immunity. RESEARCH HIGHLIGHTS: Telocytes and melanomacrophages were detected in the disseminated pancreatic nodules of the Molly fish. In Molly fish, dendritic cells were found in a close association to the exocrine pancreatic acini. Strong immunoreactivity of the pancreatic acinar cells of the Molly fish to APG5, TGF-β, IL-1β, NF-κB, Nrf2, SOX9, and S100.

The insulin gene as an energy homeostasis biomarker in Yangtze sturgeon (Acipenser dabryanus)

Insulin plays an important role in maintaining energy homeostasis and has the potential to be an indicator of energy homeostasis in the Yangtze sturgeon, Acipenser dabryanus. In this study, the Yangtze sturgeon insulin (Adinsulin) was cloned and characterized. To evaluate the possibility of insulin as an energy state assessment indicator, quantification real-time PCR (qRT-PCR) was used to evaluate expression changes in different tissues (the whole brain, esophagus, cardiac stomach, pyloric stomach, pyloric caeca, duodenum, valvula intestine, rectum, liver, pancreas, spleen, kidney, heart, muscle, gill and eye) from 6 fish (average weight 325.7 ± 22.3 g) and in three experiments including postprandial, fasting and re-feeding, and glucose tolerance treatment in which fish were divided into two groups including a group that administered a glucose solution (1 ul/g body weight) and another group that administered sterile water as control. In these three experiments, 6 fish were sampled, respectively, then been used to evaluate expression changes of insulin. All fish in feeding groups were fed in tanks (60.0 cm × 50.0 cm × 40.0 cm) with a commercial diet (crude protein ≥ 40%, crude fat ≥ 12%, coarse fiber ≤ 6%, crude ash ≤ 18%; TONGWEI CO., LTD, China) once a day at 16:00. The result showed that Adinsulin was highly expressed in the pancreas, which was the basis for the next experiment to use the pancreas as the test target. Adinsulin expression significantly increased 1 h after feeding and decreased rapidly after 3 h of feeding, but it was still significantly higher than that of the group without feeding (P < 0.01). Compared to the feeding group, the expression of Adinsulin was significantly reduced in the fasting group of 3 days (P < 0.01), 6 days (P < 0.01), 10 days (P < 0.05), 11 days (P < 0.05) and 13 days (P < 0.01) and was no significant difference in re-feeding for 1st day, 2nd day and 4th day, but there was difference between re-feeding group and fasting group. After glucose tolerance treatment, serum glucose levels increased significantly (P < 0.05), accompanied by a significant increase (P < 0.001) in insulin expression. This study result shows that insulin has the capacity to measure the energy homeostasis of Yangtze sturgeon. Further development of detection methods for sturgeon plasma or serum insulin will avoid slaughtering animals and is more practical in energy homeostasis assessment.

Unique localization of disseminated pancreas in the oesophagus of catfish (clarias gariepinus) with reference to sexual dimorphism

BACKGROUND

The fish pancreas has been reported to be composed of two portions: compact and disseminated. However, little has been elucidated in catfish. The present study describes a unique localization of the disseminated pancreas in African catfish.

METHODS

The sections were obtained and used for either routine histological examination following staining with haematoxylin and eosin (H & E), periodic acid-Schiff's, or were subjected to immunohistochemical staining for detection of both insulin-producing β cells and glucagon-producing alpha cells.

RESULTS

Our investigation showed that the pancreas of catfish consisted of both compact and disseminated portions. The compact pancreas was embedded in the mesenteric adipose tissue between the spleen, stomach and liver. However, the disseminated one showed unique localization in the tunica adventitia of the middle portion of the oesophagus. The pancreas consisted of two portions, exocrine and endocrine. Furthermore, in both types of pancreas, the female showed a significantly higher ratio for the endocrine islet area/pancreatic tissue area than that of the male and also a significantly higher ratio for both insulin- and glucagon-positive area/islet area in the female pancreas (compact and disseminated) than that of the male.

IN CONCLUSION

The present study provides evidence on a unique localization of the disseminated pancreas in the oesophagus of catfish. Furthermore, we revealed sex-related difference in the endocrine portion in both pancreatic tissues with more development in the female. The study suggests that sex hormones could be contributed to such sexual dimorphism. However, further investigation is required to compare the degree of development during the spawning and resting seasons.

Immunohistochemical distribution of insulin-, glucagon- and somatostatin-containing cells in the pancreas of Lake Van fish (Alburnus tarichi Güldenstädt, 1814) (Cyprinidae)

The Lake Van fish (Alburnus tarichi) is a species that is endemic to Turkey’s Lake Van basin. In this study, the regional distribution, volume density, and relative frequency of some pancreatic endocrine cells in Lake Van fish were investigated via immunohistochemistry using specific mammalian antibodies. The pancreatic tissue was observed to be surrounded by adipose tissue, which was adjacent to the gall bladder or extrahepatic bile duct, or dispersed in the adipose tissue ranked among coils of post-esophageal swelling and intestine. The pancreatic endocrine cells were examined, including the islets, exocrine pancreas, and pancreatic ducts. According to the modified aldehyde fuchsin staining and immunohistochemistry, insulin-secreting beta cells were observed to localize throughout the islets. Glucagon immune-reactive (IR) cells were observed to be situated moderately on the islet periphery, and were rarely determined in the islet central region. A small number of somatostatin- IR cells were observed in the islet centers and peripheries. Similar distributions of those three endocrine cells were also determined in the secondary islets. Additionally, the endocrine cell percentages did not differ between the primary and secondary islets; insulin-, glucagon- and somatostatin-IR cells comprised approximately 54%, 29%, and 11% of the endocrine cells in the principal islets, whereas they comprised 52%, 27%, and 14% in the secondary islets, respectively. Insulin-, glucagon- and somatostatin-IR cells were also determined among the epithelium and subepithelial connective tissue in the pancreatic ducts or exocrine areas of the pancreas. With this study, the existence, regional distribution, and relative frequency of the insulin-, glucagon- and somatostatin-IR cells were first investigated in the pancreatic tissue of Lake Van fish and the results were discussed.

Effects of Enteromyxum scophthalmi experimental infection on the neuroendocrine system of turbot, Scophthalmus maximus (L.)

Enteromyxum scophthalmi is an intestinal myxosporean parasite responsible for serious outbreaks in turbot Scophthalmus maximus (L.) culture, in North-western Spain. The disease affects the digestive tract, provokes severe catarrhal enteritis, emaciation and high rates of mortality. The digestive parasitization triggers a response with the coordinate participation of immune and neuroendocrine systems through the action of peptides released by enteroendocrine cells and present in nervous elements, acting as neuro-immune modulators. The present study was designed to assess the response of the turbot neuroendocrine system against E. scophthalmi infection. Immunohistochemical tests were applied to sections of the gastrointestinal tract of uninfected and E. scophthalmi-infected turbot to characterize the presence of bombesin (BOM), glucagon (GLUC), somatostatin (SOM), leu-enkephalin (LEU) and met-enkephalin (MET). The occurrence of E. scophthalmi in the turbot gastrointestinal tract increased the number of enteroendocrine cells immunoreactive to SOM, LEU and MET. On the other hand, BOM and GLUC immunoreactive cells were less numerous in the gastrointestinal tract of the parasitized turbot. Scarce immunoreactivity to BOM, GLUC and SOM was observed in nerve fibres and neurons of the myenteric plexus of control and infected fish. The results indicate that E. scophthalmi infection in turbot induced changes in the neuroendocrine system, with the diminution of the anorexigenic peptides BOM and GLUC; the increase of enkephalins, related to pro-inflammatory processes; and the increase of SOM, which may cause inhibitory effects on the immune response, constituting a compensatory mechanism to the exacerbated response observed in E. scophthalmi-infected turbot.

New insights into the signaling system and function of insulin in fish

Fish have provided essential information about the structure, biosynthesis, evolution, and function of insulin (INS) as well as about the structure, evolution, and mechanism of action of insulin receptors (IR). INS, insulin-like growth factor (IGF)-1, and IGF-2 share a common ancestor; INS and a single IGF occur in Agnathans, whereas INS and distinct IGF-1 and IGF-2s appear in Chondrichthyes. Some but not all teleost fish possess multiple INS genes, but it is not clear if they arose from a common gene duplication event or from multiple separate gene duplications. INS is produced by the endocrine pancreas of fish as well as by several other tissues, including brain, pituitary, gastrointestinal tract, and adipose tissue. INS regulates various aspects of feeding, growth, development, and intermediary metabolism in fish. The actions of INS are mediated through the insulin receptor (IR), a member of the receptor tyrosine kinase family. IRs are widely distributed in peripheral tissues of fish, and multiple IR subtypes that derive from distinct mRNAs have been described. The IRs of fish link to several cellular effector systems, including the ERK and IRS-PI3k-Akt pathways. The diverse effects of INS can be modulated by altering the production and release of INS as well as by adjusting the production/surface expression of IR. The diverse actions of INS in fish as well as the diverse nature of the neural, hormonal, and environmental factors known to affect the INS signaling system reflects the various life history patterns that have evolved to enable fish to occupy a wide range of aquatic habitats.

Immunohistochemical study of the principal pancreatic islet of the toadfish, Halobatrachus didactylus (Pisces: Batrachoididae)

The endocrine pancreas of the toadfish, Halobatrachus didactylus, consists of one large circular principal islet (Brockman body) located in the dorsal side or neck region of the gallbladder, along with various accessory islets of variable sizes and shapes, embedded in the exocrine tissue located within the digestive organs connecting mesenteries. Islet cells showed variable shapes, angular or fusiform, with long cytoplasmic processes, granular cytoplasm, and a large eccentric nucleus. Cells were found scattered or as aggregates or cords. Four primary endocrine cell types immunoreactive for glucagon (α cells), insulin (β cells), somatostatin (δ cells), and pancreatic polypeptide (F cells) were identified within the toadfish principal islet. The α, δ, and F cells were located both at the periphery and in the central regions, while β cells, which were the predominant type, were present only in the central core. α and δ cells were found in moderate frequencies, while F cells were the least abundant. Macroscopically, the Brockman body of H. didactylus is visible as a milky white nodule separated from the exocrine tissue. Its size, location, and ease of extraction suggest that H. didactylus is suitable as experimental subject for biochemical, immunological, and physiological studies of the endocrine pancreas including in vitro investigations of hormone production, storage, and release.

Insulin expression in the brain and pituitary cells of tilapia (Oreochromis niloticus)

While the presence of immunoreactive insulin in the central nervous system of many vertebrate species is well known, the origin of brain insulin is still debated. In this study, we applied RT-PCR, quantitative RT-PCR (qRT-PCR), and Northern hybridization to examine expression of the insulin gene in different tissues of an adult teleost fish, the Nile Tilapia (Oreochromis niloticus). We found that the insulin gene is transcribed at a high level in Brockmann bodies (pancreatic islet organs) and at a low level in the brain and pituitary gland. In the brain, insulin transcripts were detected in all areas by qRT-PCR and in situ hybridization. The highest level of insulin mRNA was found in the hypothalamus. The level of insulin transcription in the pituitary gland was 6-fold higher than that in the brain and 4.6-fold higher than that in the hypothalamus. Furthermore, insulin mRNA and immunoreactive insulin-like protein was detected in the pituitary gland using in situ hybridization, immunohistochemistry, and Western blot analysis. Our results indicate that in adult tilapia insulin expression is not restricted to the endocrine pancreatic cells, but also occurs in endocrine cells of the pituitary gland and in the neuronal cells of the brain, suggesting that the brain/pituitary gland might represent extrapancreatic origin of insulin production.

The fish endocrine pancreas: review, new data, and future research directions in ontogeny and phylogeny

The literature on the ontogeny and phylogeny of the endocrine pancreas of ray-finned fishes is summarized since the latest review in fish [Youson, J.H., Al-Mahrouki, A.A., 1999. Review. Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fishes. Gen. Comp. Endocrinol. 116, 303-335]. A basic description and a demonstration of the diversity of the fish islet organ is provided through new immunohistochemical data on islet tissue from a basal teleost, an osteoglossomorph, and a more derived teleost, a perciforme. Unlike the previous review, the present report provides a review and discussion of the utility of sequence data of insulin, somatostatin, and NPY- and glucagon-family peptides in phylogenetic analyses of jawed and jawless fishes. The present study also provides the first comparative analysis of sequences of preprohormones of endocrine peptides from closely related basal teleost species. Some nucleotide and deduced amino acid sequence data for preprosomatostatins (PPSS-I and/or -II) are compared for four species of bonytongues, Osteoglossomorpha, and with PPSSs of the white sucker, Catostomus commersoni, representing Cypriniformes, a more generalized teleost order. Phylogenetic analysis of deduced amino acid sequences of the PPSSs of these species and others from databases indicates good support for the monophyly of Osteoglossomorpha and some support for the present taxonomic grouping of the osteoglossomorphs examined, and also the white sucker. However, PPSS may have limited phylogenetic utility due to the relative short sequence, particularly in resolving relationships among lineages that diverged over a short period of time. Since in the few fish species examined we have just touched the surface in describing the diversity of structure of the islet organ, and likely the nature of the products of its cells, this report promotes the continued study of this organ.

Regulation of somatostatins and their receptors in fish

The multifunctional nature of the somatostatin (SS) family of peptides results from a multifaceted signaling system consisting of many forms of SS peptides that bind to a variety of receptor (SSTR) subtypes. Research in fish has contributed important information about the components, function, evolution, and regulation of this system. Somatostatins or mRNAs encoding SSs have been isolated from over 20 species of fish. Peptides and deduced peptides differ in their amino acid chain length and/or composition, and most species of fish possess more than one form of SS. The structural heterogeneity of SSs results from differential processing of the hormone precursor, preprosomatostatin (PPSS), and from the existence of multiple genes that give rise to multiple PPSSs. The PPSS genes appear to have arisen through a series of gene duplication events over the course of vertebrate evolution. The numerous PPSSs of fish are differentially expressed, both in terms of the distribution among tissues and in terms of the relative abundance within a tissue. Accumulated evidence suggests that nutritional state, season/stage of sexual maturation, and many hormones [insulin (INS), glucagon, growth hormone (GH), insulin-like growth factor-I (IGF-I), and 17beta-estradiol (E2)] regulate the synthesis and release of particular SSs. Fish and mammals possess multiple SSTRs; four different SSTRs have been described in fish and several of these occur as isoforms. SSTRs are also wide spread and are differentially expressed, both in terms of distribution of tissues as well as in terms of relative abundance within tissues. The pattern of distribution of SSTRs may underlie tissue-specific responses of SSs. The synthesis of SSTR mRNA and SS-binding capacity are regulated by nutritional state and numerous hormones (INS, GH, IGF-I, and E2). Accumulated evidence suggests the possibility of both tissue- and subtype-specific mechanisms of regulation. In many instances, there appears to be coordinate regulation of PPSS and of SSTR; such regulation may prove important for many processes, including nutrient homeostasis and growth control.

Morphology of the air-breathing stomach of the catfish Hypostomus plecostomus

Histological and ultrastructural investigations of the stomach of the catfish Hypostomus plecostomus show that its structure is different from that typical of the stomachs of other teleostean fishes: the wall is thin and transparent, while the mucosal layer is smooth and devoid of folds. The epithelium lining the whole internal surface of the stomach consists of several types of cells, the most prominent being flattened respiratory epithelial cells. There are also two types of gastric gland cells, three types of endocrine cells (EC), and basal cells. The epithelial layer is underlain by capillaries of a diameter ranging from 6.1-13.1 microm. Capillaries are more numerous in the anterior part of the stomach, where the mean number of capillary sections per 100 microm of epithelium length is 4, compared with 3 in the posterior part. The cytoplasm of the epithelial cells, apart from its typical organelles, contains electron-dense and lamellar bodies at different stages of maturation, which form the sites of accumulation of surfactant. Small, electron-dense vesicles containing acidic mucopolysaccharides are found in the apical parts of some respiratory epithelial cells. Numerous gastric glands (2 glands per 100 microm of epithelium length), composed of two types of pyramidal cells, extend from the surface epithelium into the subjacent lamina propria. The gland outlets, as well as the apical cytoplasm of the cells are Alcian blue-positive, indicating the presence of acidic mucopolysaccharides. Zymogen granules have not been found, but the apical parts of cells contain vesicles of variable electron density. The cytoplasm of the gastric gland cells also contains numerous electron-dense and lamellar bodies. Gastric gland cells with electron-dense cytoplasm and tubulovesicular system are probably involved in the production of hydrochloric acid. Fixation with tannic acid as well as with ruthenium red revealed a thin layer of phospholipids and glycosaminoglycans covering the entire inner surface of the stomach. In regions of the epithelium where the capillaries are covered by the thin cytoplasmic sheets of the respiratory epithelial cells, a thin air-blood barrier (0.25-2.02 microm) is formed, thus enabling gaseous exchange. Relatively numerous pores closed by diaphragms are seen in the endothelium lining the apical and lateral parts of the capillaries. Between gastric gland cells, solitary, noninnervated endocrine cells (EC) of three types were found. EC are characterized by lighter cytoplasm than the surrounding cells and they contain dense core vesicles (DCV) with a halo between the electron-dense core and the limiting membrane. EC of type I are the most abundant. They are of an open type, reaching the stomach lumen. The round DCV of this type, with a diameter from 92-194 nm, have a centrally located core surrounded by a narrow halo. EC of type II are rarely observed and are of a closed type. They possess two kinds of DCV with a very narrow halo. The majority of them are round, with a diameter ranging from 88-177 nm, while elongated ones, 159-389 nm long, are rare. EC of type III are numerous and also closed. The whole cytoplasm is filled with large DCV: round, with a diameter from 123-283 nm, and oval, 230-371 nm long, both with a core of irregular shape and a wide, irregular halo. EC are involved in the regulation of digestion and probably local gas exchange. In conclusion, the thin-walled stomach of Hypostomus plecostomus, with its rich network of capillaries, has a morphology suggesting it is an efficient organ for air breathing.