An immunocytochemical and ultrastructural study of endocrine cells in the gut of a teleost fish, Sparus auratus L

Mucosal Hallmarks in the Alimentary Canal of Northern Pike Esox lucius (Linnaeus)

Simple Summary

In vertebrates, mucous cells are one of the main cellular components of the gut mucosal system, which secrete different mucin types involved in several functions. Endocrine cells are scattered in the epithelium of the gut mucosa, and they produce and release regulatory molecules affecting food intake and nutrition. The goal of this study was to obtain data on quantitative distribution of mucous and endocrine cell types in the alimentary canal of the northern pike (Esox lucius), using histochemistry and immunofluorescence. In the stomach of pike, there is a high abundance of mixed mucins, with the acid component contributing to the lubrication of mucosae, where they are associated with the rapid passage of digesta through the intestine. Neutral mucins increase in the intestine aborally. The distribution of endocrine cells of the diffuse endocrine system shows the presence of somatostatin and catecholamine-secreting endocrine cells and the lack of gastrin-secreting endocrine cells. We show a close regulatory relation between endocrine and mucous cells of the gut mucosal system involved in the physiology of fish nutrition. Results confirmed the relationship between the carnivorous diet and the gut mucins distribution of northern pike; indeed, our data provide very important information to ichthyologists who study dietary behavior of species.

Abstract

On the basis of trophic behavior, fish are classified as herbivores, carnivores, omnivores, or detritivores. Epithelial mucous cells secrete mucin types specific to diet and digestive function. Mucus secretion is regulated mainly by molecular modulators produced by epithelial endocrine cells in response to luminal or tissue stimuli. These modulators are involved in control of food intake and digestive functions. Immunohistochemical and immunofluorescence studies were conducted on 10 adult northern pike (Esox lucius Linnaeus, 1758) from Lake Piediluco (Central Italy) to quantify distribution of sub-types of mucous and endocrine cells in alimentary mucosal epithelium. Neutral mucins predominated in the esophagus, and mixed and acidic mucins predominated in stomach and intestine. The gastric epithelium contained endocrine cells secreting somatostatin, tyrosine hydroxylase, and substance P. Mucous cells secreting neutral mucins increased in number from proximal to distal intestine, with endocrine cells containing substance P in the proximal intestine and those containing Leu-enkephalin throughout the intestine. Lectin histochemistry of gut sections revealed an abundance of N-acetyl-glucosamine and N-acetyl-galactosamine as carbohydrate residues on the mucin chain. The quantity and content of endocrine and mucous cells in the alimentary canal of E. lucius showed a direct relationship with its diet.

Cholecystokinin: how many functions? Observations in seabreams

A short overview on the regional distribution of the gastro-intestinal peptide hormone cholecystokin (CCK) in fish is presented. In particular, the results of molecular and immunological studies on seabreams, Diplodus puntazzo and Diplodus sargus, are reported, which, by demonstrating CCK in the hindgut, open new questions regarding the functional role of this hormone in that part of the intestine. The putative involvement of hindgut CCK in the feedback control of digestive processes was tested by measuring CCK gene and protein expression in fed and fasted fish. The results of this study led to hypothesize different roles for the two CCK isoforms in D. sargus, one of which related to regulation of digestive processes from pyloric caeca through hindgut. On the other hand, a functional role alternative to regulation of digestive processes may be inferred for the other isoform.

Cholecystokinin in white sea bream: molecular cloning, regional expression, and immunohistochemical localization in the gut after feeding and fasting

BACKGROUND

The peptide hormone cholecystokinin (CCK), secreted by the midgut, plays a key role in digestive physiology of vertebrates including teleosts, by stimulating pancreatic secretion, gut motility, and gallbladder contraction, as well as by delaying gastric emptying. Moreover, CCK is involved in the regulation of food intake and satiation. Secretion of CCK by the hindgut is controversial, and its biological activity remains to be elucidated. The present paper addresses the regional distribution of intestinal CCK in the white sea bream, Diplodus sargus, as well as the possible involvement of hindgut CCK in digestive processes.

METHODOLOGY/PRINCIPAL FINDINGS

Full-lengths mRNAs encoding two CCK isoforms (CCK-1 and CCK-2) were sequenced and phylogenetically analyzed. CCK gene and protein expression levels in the different gut segments were measured 3 h and 72 h after feeding, by quantitative real-time RT-PCR and Western blot, respectively. Moreover, endocrine CCK cells were immunoistochemically detected. Fasting induced a significant decrease in CCK-2 in all intestinal segments, including the hindgut. On the other hand, no significant difference was induced by fasting on hindgut CCK-1.

CONCLUSIONS/SIGNIFICANCE

The results demonstrated two CCK isoforms in the hindgut of D.sargus, one of which (CCK-2) may be involved in the feedback control of uncompleted digestive processes. On the other hand, a functional role alternative to regulation of digestive processes may be inferred for D.sargus CCK-1, since its expression was unaffected by feeding or fasting.

Hormonal regulation of the fish gastrointestinal tract

The gastrointestinal tracts (GIT) of fish and other vertebrates are challenged with a diversity of functional demands caused by changes and differences in dietary inputs and environmental conditions. This contribution reviews how hormonal regulation plays an essential role in modulating the GIT functions of fish to match changes in functional demands. Exemplary is how hormones produced by the GIT, the associated organs (e.g., pancreas), and other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads) modulate the digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs. Hormones regulate the other GIT functions of osmoregulation (secretion and absorption of electrolytes and water), immunity, endocrine secretions, metabolism, and the elimination of toxic metabolites and environmental contaminants to match changes in environmental conditions and physiological states. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.

Occurrence of ACTH- and enkephalin-like peptides in the developing gut of Dicentrarchus labrax L

We carried out immunohistochemical tests in the developing gut of the sea bass Dicentrarchus labrax to follow the appearance and distribution of the immunoreactivity (IR) to antibodies against POMC-derived, adrenocorticotropic hormone (ACTH), alpha-melanocyte stimulating hormone (alpha-MSH) and beta-endorphin (beta-End), and against two enkephalins, with the aim to study a possible involvement of these molecules in the early neuro-immune-endocrine integration. Our data show that IR to antibodies against some molecules involved in the stress response, such as ACTH and enkephalins, are present in the sea bass gut from an early larval stage (4 days after hatching), before transition to the exotrophic feeding. Moreover, the present study demonstrates for the first time the presence of ACTH-like immunoreactive material in developing gut of a fish. The possible roles of tested molecules are discussed.

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.

Influence of enteric helminths on the distribution of intestinal endocrine cells belonging to the diffuse endocrine system in brown trout, Salmo trutta L

The presence of intestinal helminths in the alimentary canal of brown trout, Salmo trutta L., can alter the number of cells that synthesize modulatory peptides. A total of 167 brown trout were collected from tributaries of the River Brenta (northern Italy), of which 119 (71.3%) specimens were infected with enteric helminths, 28 with the acanthocephalan Pomphorhynchus laevis Müller, 1776 with intensity of infection ranging from 1 to 162 (18.57 +/- 30.79) worms per host and 67 fish with the cestode Cyathocephalus truncatus Pallas, 1781. Intensity of infection with C. truncatus ranged from 1 to 85 (6.87 +/- 12.59) per fish. In 24 fish there were concurrent infections of both species of helminths. The caecal and middle regions of the intestine were the most heavily parasitized. Immunohistochemical tests showed a decrease in endocrine cells (ECs) of the diffuse endocrine system (DES) positive to gastrin, cholecystokinin-8, bombesin and secretin antisera in the intestine of the infected trout. The number of ECs immunoreactive to anti-glucagon serum did not show differences in the digestive tract of uninfected brown trout and in conspecifics parasitized with P. laevis. The density of cells containing glucagon-like material was low in the fish parasitized with C. truncatus. The results suggest that endoparasitic helminths induce alterations in the DES of infected S. trutta.

Glucagon-like peptide isolated from the eel intestine: effects on atrial beating

A new glucagon-like peptide was isolated from the intestine of the eel Anguilla japonica. The primary structure was determined by sequence analysis after cleavage with lysyl endopeptidase, quantitative amino acid analysis and fast atom bombardment mass spectrometry as HSQGTFTNDY10SKYLETRRAQ20DFVQWLMNSK30RSGGPT. Since its structure is similar to that of oxyntomodulins (OXMs) reported in various vertebrates, we named this peptide eel oxyntomodulin (eOXM). We found that eOXM enhanced the contractile force and the beating rate of the eel atrium in a dose-dependent manner. These effects of eOXM were not inhibited by betaxolol, a β1-adrenoceptor antagonist, indicating that the actions of eOXM were independent of those of adrenaline. eOXM enhanced the intracellular Ca2+ concentration of the myocardium. The contractility of the eel atrium was greatly reduced after omitting Ca2+ from the bathing medium or after treatment with verapamil, a Ca2+ channel blocker. After inhibiting Ca2+ entry under these conditions, the inotropic effect of eOXM was markedly reduced, but the chronotropic effect was not altered significantly. These results indicate that the inotropic effect of eOXM is via a stimulation of Ca2+ influx but that the chronotropic effect may be independent of extracellular Ca2+.

Development of cholecystokinin and pancreatic polypeptide endocrine systems during the larval stage of Japanese flounder, Paralichthys olivaceus

To understand the developmental process of the endocrine system, which controls the pancreatic exocrine function in Japanese flounder, Paralichthys olivaceus, the expression patterns of cholecystokinin (CCK) and pancreatic polypeptide (PP) during the larval stage were analyzed by immunohistochemistry and in situ hybridization. Expression of CCK in the intestinal epithelia started at 2 days posthatching (dph), 1 day prior to the first feeding. Endocrine pancreatic cells containing insulin were present in the pancreas primodium at hatching, and these endocrine cells formed an islet at 3 dph and developed into the principle islet neighboring gallbladder at 20 dph. However, PP cells were not contained in the principle islet even after metamorphosis. PP cells were contained in the accessory islets differentiated around the proximal part of the pyloric appendages at 30 dph (early metamorphosis stage). Therefore, we infer that the stimulative regulation of pancreatic enzyme secretion by CCK starts to function at the first feeding, whereas the restrictive regulation by PP develops about 1 month later in flounder larvae. In addition, we observed that CCK immunoreactive cells appeared in the accessory islets at 30 dph, similar to PP cells, even though CCK mRNA expression could not be detected in cells from the islets. This indicates the possibility that a peptide that is cross-reacted with the CCK antibody, i.e., gastrin, is synthesized in the flounder islets.

Co-localization of Trk neurotrophin receptors and regulatory peptides in the endocrine cells of the teleostean stomach

Recently it has been observed that a subpopulation of gut endocrine cells in vertebrates express Trk-like proteins, suggesting that neurotrophins could regulate the synthesis and storage of amines and peptides of these cells. Nevertheless, the peptides and amines present in the endocrine cells that express Trks have not been characterized. In this study we used immunohistochemistry to investigate the occurrence of Trk-like proteins (TrkA-like, TrkB-like and TrkC-like) and the possible co-localization of these with peptides and/or biogenic amines in the endocrine cells of the stomach of three teleost (bass, gilt-head and scorpionfish). No TrkA-like immunoreactivity (IR) was detected in the stomach of these species, whereas TrkB-like IR and TrkC-like IR were observed in numerous cells of the gastric epithelium. TrkB-like immunoreactive cells were present in all three species examined, and were particularly abundant in the blind sac. Conversely, TrkC-like immunoreactive cells were found only in the bass stomach, apparently co-localized with TrkB-like IR. TrkB-like IR was found co-localized with somatostatin IR in scorpionfish, and with somatostatin and CGRP IR in gilt-head and bass. Gastric endocrine cells expressing 5-HT, glucagon, insulin, met-, leu-enkephalin, substance P, PYY, VIP, CCK, NPY, bombesin and motilin were unreactive for Trk-like proteins. The present results provide direct evidence for the occurrence of Trk-like neurotrophin receptor proteins in a subpopulation of the teleostean gastric endocrine cells and suggest that neurotrophins could regulate, as in neurons, the expression of some neuropeptides such as somatostatin and CGRP.

Glucagon- and NPY-related peptide-immunoreactive cells in the gut of sea bass (Dicentrarchus labrax L.): a light and electron microscopic study

Glucagon and peptide of the neuropeptide Y (NPY) family immunoreactivities were studied in the gut of sea bass (Dicentrarchus labrax) using antisera against bovine/porcine glucagon, porcine glucagon, glicentin (10-30), bovine pancreatic polypeptide (PP), peptide tyrosine tyrosine (PYY), salmon PYY (sPYY), and NPY. Glucagon-, glicentin-, PYY-, and NPY-immunoreactive (ir) cells were detected in the stomach, and glucagon-, PP-, PYY-, sPYY-, and NPY-ir cells in the intestine. PP, PYY, and NPY immunoreactivities coexisted in intestinal endocrine cells (NPY-like peptide containing cells), in some of which there was also glucagon immunoreactivity. Preabsorption tests indicated that different products of the glucagon gene(s) are probably expressed in the stomach and intestine of sea bass and that the peptides belonging to the NPY family in the endocrine cells of the intestine are more similar to NPY than to other peptides of this family. Glucagon-ir cells in the stomach, and glucagon/NPY-like containing cells in the intestine, were characterized by conventional and immunogold electron-microscopic techniques. The glucagon cells had secretory granules with a clotted content, the gold particles being observed in both the core and the halo. Glucagon/NPY-like cells showed two types of secretory granules differing in size, both of which were immunogold labeled with anti-NPY and anti-sPYY; the smaller granules were weakly immunogold labeled with anti-glucagon.

Ontogeny of the endocrine cells of the intestine and rectum of sea bass (Dicentrarchus labrax L.): an ultrastructural study

Several endocrine cell types were ultrastructurally characterized during the differentiation of the intestine and rectum of sea bass (Dicentrarchus labrax L.) larvae. Only one cell type (type I) was found in the posterior region of the undifferentiated gut of 5-day-old larvae (phase I). Types V and VI were found in both the intestine and rectum, types II, III and IV in the intestine, and types VII and VIII in the rectum of 9- and 12-day-old larvae (phase II), the rectum alone showing signs of functional differentiation. In phase III larvae, in which both the intestine and rectum were differentiated, types IX, X, XI, XII, XIII, XIV and XV were found in the intestine, only types X, XI and XII being seen in the rectum. Besides these, a new cell type, XVI, was observed in the intestine of 55- and 60-day-old larvae (phase IV), in which the digestive tract was completely differentiated. The endocrine cells appearing in phases I and II showed very scarce secretory granules and the ultrastructural features of undifferentiated cells. Some endocrine cell types in the earliest developmental stages were related to some of those found later. A maturational process of the endocrine cell types paralleled the differentiation of the intestine and rectum, with an apparent increase in the number of secretory granules accompanying organelle development.

Structure and function of a pentapeptide isolated from the gut of the eel

A novel peptide, H-Gly-Phe-Trp-Asn-Lys-OH, was isolated from eel guts. This peptide, termed eel intestinal pentapeptide (EIPP), enhanced the frequency of the spontaneous contractions and increased the basal tone of the circular muscle of the esophagogastric junction. Furthermore, EIPP enhanced the spontaneous contractions of the longitudinal muscle strips of the gut and stomach, and of the circular muscle of the gastro-intestinal junction. The peptide may be a physiological regulatory peptide in the gastro-intestinal tract of the eel.

Light and electron microscopic immunocytochemical demonstration of the coexistence of somatostatin 14- and somatostatin 25-like peptides in endocrine cells of the stomach of Sparus aurata (Teleost)

An immunofluorescence double-staining method colocalized somatostatin 14 (SST 14)- and somatostatin 25 (SST 25)-like immunoreactivities in endocrine cells located in the depth of gastric folds and upper part of the stomach glands of Sparus aurata (gilthead sea bream). An immunogold method identified somatostatin-like peptides in the secretory granules of the previously described Type IV endocrine cells. Appropriate preabsorption controls demonstrated two different granule populations with somatostatin-like immunoreactivity. SST 14-like peptides seemed to be located in the most commonly found granules, which showed a fibrillar content, whereas SST 25-like peptides were identified in more scarce and denser granules.

Maturation of the endocrine pancreas in the sea bass, Dicentrarchus labrax L. (Teleostei): an immunocytochemical and ultrastructural study. I. Glucagon-producing cells

The structure of the endocrine pancreas in the sea bass (Dicentrarchus labrax L.) was studied with special reference to glucagon-immunoreactive cells. As described in most of the teleosts, the sea bass was found to have a diffuse pancreas. In the adult, endocrine cells were clustered in a principal islet and numerous accessory islets where the glucagon A cells were localized peripherally. Under electron microscopy, the A cells displayed a clear hyaloplasm with granules having typical spherical or polyhedral cores, as in other vertebrates. The maturation of the endocrine pancreas was monitored under rearing conditions. The endocrine pancreas appeared during the prelarval stage, 3 days after hatching, and consisted of a single cluster of morphologically similar cells, containing very small cytoplasmic granules. During the larval stage, cytodifferentiation resulted in modifications of cell shape and increased granule size. Typical granules appeared in 8-mm-long larvae. Cells immunoreactive with mammalian glucagon antibodies appeared only at the beginning of the juvenile stage (3 months/20 mm). Electron microscope observations revealed that the storage of hormone in numerous cytoplasmic granules began at this stage.

Distribution of somatostatin and glucagon immunoreactive cells in the gastric mucosa of some cartilaginous fishes

The comparative distribution of somatostatin- and glucagon-like-containing cells in the histomorphologically different gastric mucosae of the cartilaginous fishes Heptranchias perlo, Raja asterias, Scyliorhinus canicula, Squatina aculeata, and Torpedo marmorata was immunocytochemically studied to demonstrate a possible interrelationship between these endocrine cells and/or other endocrine or nonendocrine cells. In the gastric mucosa, these open-type glucagon and somatostatin immunoreactive cells show a double localization with different morphology and interrelationships. At the bottom of gastric pits, which corresponds to a proliferative zone, spindle or pear-shaped immunopositive cells appear rather numerously and are often in close proximity to each other. In gastric glands, triangular or oval immunopositive cells never in contact with each other were detected; their numeric ratio seems to be rather constant even if their numeric frequency and distribution vary according to the histomorphological aspect of selachian gastric glands. Glucagon immunoreactive cells seem to be more related to pepsinogenic cells, while somatostatin immunoreactive cells seem to be more ubiquitous. Both cell types can present basal cytoplasmic processes. From our results we can suggest a possible regulative role exerted by these two peptides on gastric secretion and cell proliferation.

The endocrine cells in the gut of Mugil saliens Risso, 1810 (Teleostei): an immunocytochemical and ultrastructural study

The endocrine cells in the gut of Mugil saliens Risso, 1810 (leaping grey mullet) were investigated by immunocytochemical and electron microscopic techniques. Gastrin-, glucagon-, and somatostatin-immunoreactive cells were identified in the cardiac and cecal stomach regions, located mainly in the lower part of the gastric folds and in the upper part of the glands. Substance P-, somatostatin-, and pancreatic polypeptide (PP)-immunoreactive cells were found between epithelial cells in the pyloric stomach region. Gastrin-, cholecystokinin (CCK)-, gastric inhibitory polypeptide (GIP)-, substance P-, Met-enkephalin- and PP-immunoreactive cells were observed throughout the intestine while only the last three of these appeared in the posterior intestine. Nine types of gastroenteroendocrine cells were ultrastructurally characterized; some of them were related to the cell types immunocytochemically identified.

A comparative immunocytochemical study of the gastro-entero-pancreatic (GEP) endocrine system in a stomachless and a stomach-containing teleost

The gastro-entero-pancreatic (GEP) endocrine system of a stomach-containing and of a stomachless teleost, Sparus auratus and Barbus conchonius, respectively, are studied immunocytochemically using different antisera against mammalian hormones. Insulin-, glucagon-, somatostatin-, and pancreatic polypeptide (PP)-immunoreactive cells are identified in the endocrine pancreas of both species. Only the distribution of PP-immunoreactive cells differed strongly; in the principal islet of both fishes, few PP-immunoreactive cells are present, whereas in the smaller ones many of them are observed in S. auratus and none in B. conchonius. In the digestive tract of S. auratus 10 endocrine cell types can be distinguished: neurotensin-, secretin-, serotonin-, somatostatin-, and two types of substance P-immunoreactive cells exclusively in the stomach, and C-t-gastrin/CCK-, glucagon-, Met-enkephalin-, PP-, and only one type of substance P-immunoreactive cells in the intestinal epithelium. With the exception of substance P-immunoreactive cells, the other four intestinal endocrine cells, as well as an unspecific immunoreactive cell, can also be found in B. conchonius. Coexistence of glucagon- and PP-like immunoreactivity is observed in the pancreas of S. auratus and in the gut of B. conchonius. Pancreatic and gut endocrine cells showing only PP- or glucagon-like immunoreactivity are found, too.