Morphogenesis of somatostatin- and insulin-secreting cells in the lamprey endocrine pancreas

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

Multiple forms of glucagon and somatostatin isolated from the intestine of the southern-hemisphere lamprey Geotria australis

Current views on Agnathan phylogeny favor the hypothesis that the genera of holarctic lampreys belong to a single family (Petromyzontidae) and form an interrelated progression in which Petromyzon is near to Ichthomyzon at the base of the phylogenetic tree and Lampetra is the most derived. A stock similar to that of contemporary Ichthomyzon is considered to have given rise to the southern hemisphere lamprey Geotria australis, the sole member of the Geotriidae. In the present study, two molecular forms of glucagon were isolated from an extract of G. australis intestine that differed in structure by six amino acid residues. One form shows two amino acid substitutions (Leu14 --> Met and Ala29 --> Ser) compared with the single molecular form of glucagon isolated from the sea lamprey Petromyzon marinus and the second form shows three substitutions (Asp15 --> Glu, Ser16 --> Ala, Ile24 --> Thr) compared with the single glucagon isolated from the river lamprey Lampetra fluviatilis. As Petromyzon and Lampetra glucagons differ by six amino acid residues, the data suggest that a duplication of the glucagon gene occurred prior to or early in lamprey evolution. Although both genes are strongly expressed in G. australis, the expression of one gene predominates in P. marinus while that of the other gene predominates in L. fluviatilis. Previous work has shown that, in the islet organ of G. australis, preprosomatostatin is processed almost exclusively to somatostatin-33. However, the present study demonstrates that somatostatin-14 is the major molecular form in G. australis intestine with somatostatin-33 present only as a minor component. This result demonstrates a tissue-dependent pathway of posttranslational processing of preprosomatostatin in the Geotria enteropancreatic system.

Effects of somatostatin on lipid metabolism of larvae and metamorphosing landlocked sea lamprey, Petromyzon marinus

This study was designed to examine the role of somatostatin in regulating changes in lipid metabolism of larvae and metamorphosing landlocked sea lamprey, Petromyzon marinus. Larvae and animals in late metamorphosis (stage 6 on a 7-stage scale) were injected intraperitoneally once per day for 2 days with either saline (0.6%) or somatostatin-14 (SS-14; 500 ng/g body wt). Injection of SS-14 into larval and stage 6 metamorphosing animals resulted in elevated plasma fatty acids levels. In larvae, SS-14-induced hyperlipidemia was supported by enhanced lipolysis, as indicated by increased triacylglycerol lipase (TGL) activity in the liver and kidney. Mobilization of larval renal lipid was accompanied by reduced TG synthesis, as indicated by decreased diacylglycerol acyltransferase (DGAT) activity. In stage 6 metamorphosing lamprey, SS-14 did not significantly affect TGL activity; however, SS-14 significantly reduced fatty acid synthesis, as measured by acetyl-CoA carboxylase activity, in kidney, liver, and muscle, as well as muscular TG synthesis. SS-14-stimulated lipid depletion is reminiscent of the pattern of lipid metabolism displayed by P. marinus during their spontaneous metamorphosis-an observation which suggests that somatostatin may play a role in metamorphosis-associated changes in lipid metabolism in this species.

Immunohistochemical studies of the endocrine cells within the gastro-entero-pancreatic system of Osteoglossomorpha, an ancient teleostean group

The identification and distribution of endocrine cells within the gastro-entero-pancreatic (GEP) system of five species of the Osteoglossomorpha (Osteoglossum bicirrhosum, Scleropages jardini, Pantodon buchholzi, Notopterus chitala and Gnathonemus petersii) were analyzed by immunohistochemistry. Four immunoreactive cell types were identified within the pancreatic islets (A, B, D, and F cells), using antisera directed against mammalian insulin (m-INS), somatostatins (SST-14, SST-25), and members of the pancreatic polypeptide (aPY, NPY, PYY) and glucagon (GLU, GLP) families. The B cells were located throughout the center of the islets in the five species and, in general, D cells had a similar distribution. However, immunoreactivity to anti-somatostatins varied between four of the species and G. petersii, which showed less intensely stained D cells in the islets, but greater SST immunoreactivity in both the intestinal and the stomach epithelia than in comparable epithelia of other species. For peptides of both the pancreatic polypeptide and the glucagon families, the immunoreactivity was detected at the periphery of the islets, and there was a suggestion of an interfamily colocalization of peptides in some cells. In addition, glucagon family peptides showed a scattered immunoreactivity throughout the central portion of the islets. A moderately abundant number of cells in the intestine were immunoreactive to the PP family antisera in all five species. However, immunoreactivities to GLU, GLP, SST, and m-INS antisera were variable in intestinal cells of the species. Immunoreactivity with sera raised against m-INS and PYY was also observed in the stomach of P. buchholzi. The significance of these findings is discussed in both ontogenetic and phylogenetic contexts with respect to the GEP system in actinopterygian fishes and with respect to the possibility of variable processing of prohormones in the different organs of these osteoglossomorphs.

An immunohistochemical study of the endocrine cells within the pancreas, intestine, and stomach of the gar (Lepisosteus osseus L.)

The distribution and identity of the various endocrine cell types were examined in the pancreas, stomach, and anterior intestine of the phylogenetically ancient actinopterygian, the gar (Lepisosteus osseus L.), using immunohistochemistry. Antisera used were directed against several insulins (INSs) and somatostatins (SSTs), and members of the pancreatic polypeptide (PP, aPY, NPY) and glucagon (GLUC, GLP) families. In the gar pancreas the most pronounced aggregation of islet tissue is among the exocrine acini near the union of extrahepatic common bile duct with the gastrointestinal junction. Four immunoreactive cell types were identified within well-defined islets (A, B, D, and F cells) but immunoreactive cell types were also seen isolated among the exocrine acini. Centrally located B cells were immunoreactive with mammalian and lamprey INS antisera whereas the widely dispersed D cells immunostained with anti-SST-14, -25, and -34. SST was also localized in the epithelium of the pancreatic ducts. There was a colocalization of immunoreactivity for each member of the PP and GLU families at the periphery of each islet to identify F and A cells, respectively. However, colocalization of peptides from both families is suspected for at least some cells. Although the gastric and intestinal mucosae showed a similar pattern of immunoreactivity to GLP and not GLU, they had contrasting immunoreactivity with the two INS antisera. SST immunoreactivity was restricted to the stomach, whereas three of the four PP-family peptides were only immunoreactive in the intestine. Immunoreactivity to the various antisera used in the study imply that there may be an organ-specific processing of preproinsulin, that the gar SST profile may be more similar to agnathan and bowfin rather than either elasmobranch or teleost SSTs, and that only the GLP portion of the preproglucagon gene is expressed in the gastrointestinal mucosa. Our results are consistent with other recent endocrine studies showing that the gar is a widely distinct actinopterygian.

Immunohistochemical localization of different forms of somatostatin in the gastrointestinal tract of the calf

The presence of two peptides that belong to the somatostatin family has been investigated in the calf gut. Somatostatin-14-like and Somatostatin-28-like peptides have been localized by a light microscopic immunohistochemical method. The method employed antibodies linked to colloidal gold particles that were revealed by a silver-enhancement step. Somatostatin-14-like peptide was only present in mucosal endocrine cells, which were detectable along the entire gut with the exceptions of the abomasal gastric proper glands and caecum. The cells were most abundant in cardiac and pyloric glands. Langerhans' islets also contained this type of endocrine cell. Somatostatin-28-like-immunoreactive endocrine cells were more abundant than the former cell type. They were present in the gastric proper glands and caecum where Somatostatin-14-like-immunoreactive cells were absent. They were as numerous as the former type of cell in the endocrine pancreas. The Somatostatin-28-like peptide was also detectable in the intramural nervous components of the abomasum and the intestine, in both perikarya and terminals. Our results show a possible heterogeneity of an endocrine cell type, which synthesizes and secretes somatostatin peptides. Our results also support the hypothesis that somatostatin-14 and somatostatin-28 peptides may have distinct functional roles, particularly in different species.

Development of the adult endocrine pancreas during metamorphosis in the sea lamprey, Petromyzon marinus L. I. Light microscopy and autoradiography

The results of this study were found to support the hypothesis put forth by Boenig (Z. Mikrosk-Anat. Forsch., 17:125-184, 1929) that the caudal pancreas of adult lamprey develops from the epithelium of the extrahepatic common bile duct in that the bile duct cells were found to undergo a great proliferation during the early stages of metamorphosis, with a large number of the cells incorporating 3H-thymidine. If the bile duct degenerated as suggested by Barrington (The Biology of Lampreys. Academic Press, London, pp. 135-169, 1972), this uptake would not be expected. The cranial pancreas was determined to develop in a similar manner to the larval islets, with formation of the islets taking place within the intestinal/diverticular epithelium. The newly formed islets would migrate into the surrounding connective tissue. During the later stages of metamorphosis a small number of cells was found to incorporate the tritiated thymidine within mature islets.

A histochemical and immunohistochemical study of digestive enzymes and hormones during the larval development of the sea bream, Sparus aurata L

The distribution of different hydrolytic enzymes and the localization of the hormones which regulate glucose metabolism during development of the digestive tract of the sea bream, Sparus aurata L., were studied. The yolk sac contains trypsin, glucose-6-phosphatase, ATPases and acid and alkaline phosphatase activities. Positive insulin, glucagon and somatostatin cells were observed in the pancreas and in the lumen of the intestinal tract during endogenous feeding. From hatching until 3 days later, the digestive tract of sea bream larvae shows no enzymatic activities. During exogenous feeding, the activities of the phosphatases and trypsin generally increase, as do the amounts of the hydrolytic enzymes and trypsin, as well as the pancreatic and intestinal hormones. The enzymatic activities gradually decrease from the anterior part towards the posterior part of the digestive tract.

Distribution of two forms of somatostatin and peptides belonging to the pancreatic polypeptide family in tissues of larval lampreys, Petromyzon marinus L.: an immunohistochemical study

Immunohistochemistry on tissues of larval lampreys, Petromyzon marinus L., was used to determine the distribution of invariant somatostatin-14 (SST-14) and lamprey somatostatin-34 (SST-34) in the brain while antisera against porcine peptide tyrosine tyrosine (PYY), human neuropeptide Y (NPY), anglerfish peptide YG (aPY), salmon glucagon-like peptide (GLP), SST-14, and SST-34 were used in studies of the pancreas and anterior intestine. In the brain, SST-14 is the major form of somatostatin. SST-14- and SST-34-immunoreactive nerve fibers are distributed throughout the telencephalon, diencephalon, and mesencephalon. In the latter region SST-14 immunoreactivity is concentrated in nerve tracts in the nucleus interpeduncularis. Nerve cells within the olfactory bulbs are immunoreactive only to anti-SST-34. Cells immunostained with anti-SST-14 were localized within the ependymal and subependymal layers of the pars ventralis hypothalami and the subependymal layers of the pars dorsalis thalami. SST-14-immunoreactive perikarya are also distributed within the tegmentum mesencephali. Nerve fibers and cells immunoreactive to anti-SST-34 are detected in the pars ventralis hypothalami but these cells do not colocalize SST-14. Pancreatic islets, distributed within the epithelium and in the submucosal connective tissue at the esophageal-intestinal junction, are only immunoreactive to anti-insulin. The antisera revealed three distinct cell types in the intestinal epithelium: type 1 colocalizes aPY, NPY, and PYY; type 2 colocalizes SST-14 and SST-34; and type 3 demonstrates immunoreactivity only to anti-SST-34. Immunoreactivity to anti-GLP is absent.

Immunoreactivity to peptides belonging to the pancreatic polypeptide family (NPY, aPY, PP, PYY) and to glucagon-like peptide in the endocrine pancreas and anterior intestine of adult lampreys, Petromyzon marinus: an immunohistochemical study

Immunoreactivity of antisera directed against human neuropeptide Y (NPY), anglerfish polypeptide YG (aPY), bovine pancreatic polypeptide (bPP), salmon pancreatic polypeptide (sPP), porcine peptide tyrosine tyrosine (PYY), and salmon glucagon-like peptide (GLP) was investigated in the endocrine pancreas and anterior intestine of adult lampreys, Petromyzon marinus, by immunohistochemical analysis. There was no immunoreactivity to anti-sPP and anti-bPP in any tissue and anti-GLP immunostaining was only present in the anterior intestine. The immunoreactivity to antisera raised against NPY, aPY, and PYY was colocalized within the same small number of cells in the caudal and cranial pancreas of juveniles and the caudal pancreas of upstream migrant adults. These antibodies did not immunostain B- or D-cells and thus, NPY, aPY, and PYY were likely localized in a third cell type (3a) in the lamprey pancreas. Immunostaining of a few cells with only anti-aPY suggested the possibility of a fourth cell type (3b). Immunoreactivity was similar in the cranial and caudal pancreas of male upstream migrants; however, in the female cranial pancreas, a few cells demonstrated intense immunoreaction to anti-aPY, while weaker immunostaining with this antiserum was observed in B-cells. In the intestine of juvenile and upstream migrant lampreys, positive immunostaining to GLP, NPY, aPY, and PYY antibodies was colocalized within the same cell. We believe that this cell may contain PYY/glucagon family peptides. Other intestinal cells immunostained with either GLP or somatostatin-34 antiserum.

Somatostatin concentrations in the pancreatic-intestinal tissues of the sea lamprey, Petromyzon marinus L., at various periods of its life cycle

1. Somatostatin concentrations were measured in homogenates of the pancreas-intestinal tissues from each period of the life cycle of Petromyzon marinus using radioimmunoassay. 2. Levels were very low in larva (4.0 pg/mg wet weight) and in the first three stages of metamorphosis, but increased from stage 4 onwards and reached a high in upstream-migrating adults (210.0 ng/mg). 3. These data correlate well with our previous morphological and immunohistochemical observations on the morphogenesis of somatostatin-containing D-cells during the life cycle and indicate that the increased concentration of hormone accompanies the development of the endocrine pancreas in lampreys.

Distribution of two forms of somatostatin in the brain, anterior intestine, and pancreas of adult lampreys (Petromyzon marinus)

The distribution of two major immunoreactive forms of somatostatin, somatostatin-14 and somatostatin-34, within the brain, pancreas and intestine of adult lampreys, Petromyzon marinus, was identified using antisera raised against these peptides. Immunostaining of the brain is similar in juveniles and upstream migrants, and somatostatin-14 is the major somatostatin form demonstrated. A few somatostatin-34-containing cells are localized within the olfactory bulbs, thalamus and hypothalamus, but cells immunoreactive to anti-somatostatin-34 in the hypothalamus and thalamus do not co-localize somatostatin-14. Immunostaining of pinealocytes within the pineal pellucida with anti-somatostatin-14 may infer a novel function for this structure. Somatostatin-14 and somatostatin-34 are co-localized within D-cells of the cranial pancreas and caudal pancreas of juveniles and upstream migrants. Numerous somatostatin-34-immunoreactive cells are distributed within the epithelial mucosa of the anterior intestine but not all of these cells cross-react with anti-somatostatin-14. It appears that somatostatin-34 is the major somatostatin in the pancreo-gastrointestinal system of adult lampreys.

Gut hormones in cyclostomes

The current state of knowledge about regulatory peptides in endocrine cells and nerves of the alimentary canal of lampreys and hagfishes is reviewed. Cyclostomes have a wide range of peptides similar immunochemically to those of higher vertebrates. They include, in the endocrine cells of the intestine, peptides resembling glucagon, gastrin/cholecystokinin, peptide YY (pancreatic polypeptide/neuropeptide Y), substance P, vasoactive intestinal peptide, somatostatin and, in the larval stages at least, insulin. The enteric nerves of some lamprey species contain peptides resembling bombesin (gastrin-releasing peptide) and calcitonin gene-related peptide, as well as serotonin. The occurrence of other peptides is less well documented.Little is known of the molecular structure or the biological roles of the enteric peptides in cyclostomes. Extraction, purification, sequencing and physiological experiments are greatly needed.