Heterogeneities of the islets in the rabbit pancreas and the problem of "paracrine" regulation of islet cells

Comparative aspects of rodent and nonrodent animal models for mechanistic and translational diabetes research

The prevalence of diabetes mellitus, which currently affects 387 million people worldwide, is permanently rising in both adults and adolescents. Despite numerous treatment options, diabetes mellitus is a progressive disease with severe comorbidities, such as nephropathy, neuropathy, and retinopathy, as well as cardiovascular disease. Therefore, animal models predictive of the efficacy and safety of novel compounds in humans are of great value to address the unmet need for improved therapeutics. Although rodent models provide important mechanistic insights, their predictive value for therapeutic outcomes in humans is limited. In recent years, the pig has gained importance for biomedical research because of its close similarity to human anatomy, physiology, size, and, in contrast to non-human primates, better ethical acceptance. In this review, anatomic, biochemical, physiological, and morphologic aspects relevant to diabetes research will be compared between different animal species, that is, mouse, rat, rabbit, pig, and non-human primates. The value of the pig as a model organism for diabetes research will be highlighted, and (dis)advantages of the currently available approaches for the generation of pig models exhibiting characteristics of metabolic syndrome or type 2 diabetes mellitus will be discussed.

The ductal origin of structural and functional heterogeneity between pancreatic islets

Islets form in the pancreas after the first endocrine cells have arisen as either single cells or small cell clusters in the epithelial cords. These cords constitute the developing pancreas in one of its earliest recognizable stages. Islet formation begins at the time the cords transform into a branching ductal system, continues while the ductal system expands, and finally stops before the exocrine tissue of ducts and acini reaches its final expansion. Thus, islets continuously arise from founder cells located in the branching and ramifying ducts. Islets arising from proximal duct cells locate between the exocrine lobules, develop strong autonomic and sensory innervations, and pass their blood to efferent veins (insulo-venous efferent system). Islets arising from cells of more distal ducts locate within the exocrine lobules, respond to nerve impulses ending at neighbouring blood vessels, and pass their blood to the surrounding acini (insulo-acinar portal system). Consequently, the section of the ductal system from which an islet arises determines to a large extent its future neighbouring tissue, architecture, properties, and functions. We note that islets interlobular in position are frequently found in rodents (rats and mice), whereas intralobularly-located, peripheral duct islets prevail in humans and cattle. Also, we expound on bovine foetal Laguesse islets as a prominent foetal type of type 1 interlobular neuro-insular complexes, similar to neuro-insular associations frequently found in rodents. Finally, we consider the probable physiological and pathophysiological implications of the different islet positions within and between species.

Dual origin, development, and fate of bovine pancreatic islets

Endocrine cells are evident at an early stage in bovine pancreatic development when the pancreas still consists of primitive epithelial cords. At this stage, the endocrine cells are interspersed between the precursor cells destined to form the ductulo-acinar trees of later exocrine lobules. We here demonstrate that, in bovine fetuses of crown rump length ≥ 11 cm, the endocrine cells become increasingly segregated from the developing exocrine pancreas by assembly into two units that differ in histogenesis, architecture, and fate. Small numbers of 'perilobular giant islets' are distinguishable from larger numbers of 'intralobular small islets'. The two types of islets arise in parallel from the ends of the ductal tree. Aside from differences in number, location, and size, the giant and small islets differ in cellular composition (predominantly insulin-synthesising cells vs. mixtures of endocrine cells), morphology (epithelial trabeculae with gyriform and rosette-like appearance vs. compact circular arrangements of endocrine cells), and in their relationships to intrapancreatic ganglia and nerves. A further difference becomes apparent during the antenatal period; while the 'interlobular small islets' persist in the pancreata of calves and adult cattle, the perilobular giant islets are subject to regression, characterised by involution of the parenchyma, extensive haemorrhage, leukocyte infiltration (myeloid and T-cells) and progressive fibrotic replacement. In conclusion, epithelial precursor cells of the ductolo-acinar tree may give rise to populations of pancreatic islets with different histomorphology, cellular composition and fates. This should be taken into account when using these cells for the generation of pancreatic islets for transplantation therapy.

Low insulin content of large islet population is present in situ and in isolated islets

The existence of morphologically distinct populations of islets in the pancreas was described over 60 years ago. Unfortunately, little attention has been paid to possible functional differences between islet subpopulations until recently. We demonstrated that one population, the small islets, were superior to large islets in a number of functional aspects. However, that work did not determine whether these differences were inherent, or whether they arose because of the challenge of isolation procedures. Nor, were there data to explain the differences in insulin secretion. We utilized immunohistochemistry, immunofluorescence, ELISA, and transmission electron microscopy to compare the unique characteristics found in isolated rat islet populations in situ and after isolation. Insulin secretion of small isolated islets was significantly higher compared to large islets, which correlated with higher insulin content/area in small islets (in situ), a higher density of insulin secretory granules, and greater insulin content/volume in isolated islets. Specifically, the core b-cells of the large islets contained less insulin/cell with a lower insulin granule density than peripheral b-cells. When insulin secretion was normalized for total insulin content, large and small islets released the same percentage of total insulin. Small islets had a higher density of cells/area than large islets in vitro and in situ. The data provide a possible explanation for the inferior insulin secretion from large islets, as they have a lower total cell density and the b-cells of the core contain less insulin/cell.

Pancreatic islet plasticity: interspecies comparison of islet architecture and composition

The pancreatic islet displays diverse patterns of endocrine cell arrangement. The prototypic islet, with insulin-secreting beta-cells forming the core surrounded by other endocrine cells in the periphery, is largely based on studies of normal rodent islets. Recent reports on large animals, including humans, show a difference in islet architecture, in which the endocrine cells are randomly distributed throughout the islet. This particular species difference has raised concerns regarding the interpretation of data based on rodent studies to humans. On the other hand, further variations have been reported in marsupials and some nonhuman primates, which possess an inverted ratio of beta-cells to other endocrine cells. This review discusses the striking plasticity of islet architecture and cellular composition among various species including changes in response to metabolic states within a single species. We propose that this plasticity reflects evolutionary acquired adaptation induced by altered physiological conditions, rather than inherent disparities between species.

Revascularization and remodelling of pancreatic islets grafted under the kidney capsule

The revascularization and the structural changes resulting from interactions between the graft and the host were investigated in transplanted pancreatic islets under the kidney capsule. Islets were isolated from mice pancreata and transplanted in syngeneic diabetic animals. Graft-bearing kidneys were collected on different days post-transplant and processed for light microscopy, immunohistochemistry and transmission electron microscopy. A numerical analysis was performed in order to compare the percentage number of the different types of cells in native islets and at different time points after the transplant. Recipient animals reversed diabetes within 4 days. An intraperitoneal glucose tolerance test was performed to determine islet functionality under stressful conditions. During the initial few days post-transplant, the islets showed peculiar shapes and the graft tended to aggregate along the vessels. Starting at days 4-7 post-transplant, islets were revascularized from vessels connected to both the cortical and the capsular vascular network of the kidney. From day 7-14 post-transplant, the vessels progressively appeared more similar in features and size to those of in situ pancreatic islets. Both the percentage number of the different cell types and the distribution of Alpha, Beta and Delta cells inside the graft were significantly different as compared with intact islets, demonstrating quantitative and structural changes after the engraftment. No concomitant proliferation of Beta cells was detected using a bromodeoxyuridin staining method. Despite the fact that quick revascularization preserved a large mass of tissue, the remodelling process of the graft and the newly formed vascularization led to a different organization of the endocrine tissue as compared with intact in situ islets. This constitutes the morphological basis for alterations of the normal intercellular interactions and may explain the altered secretory cell function often observed in transplant.

Morphological changes of isolated rat pancreatic islets: a structural, ultrastructural and morphometric study

Improved techniques for pancreatic islet extraction can yield a reasonable number of transplantable cells. However, the isolation and purification process may damage the islets and impair their physiological functions. The aim of this study was to determine the effect of the isolation procedure on the structure of isolated islets and to correlate this with their functionality. Islets were isolated from rat pancreata and purified by Eurocollins-Ficoll discontinuous density gradient processing, and then processed for light microscopy, and scanning and transmission electron microscopy. Morphometric analysis was also performed. Islet functionality was determined by reversal of streptozotocin-induced diabetes and the intraperitoneal glucose tolerance test in a syngeneic rat model of pancreatic islet transplantation. Fragments of variable size and shape comprised a relatively large proportion (26%) of the isolated endocrine tissue. Isolated islets showed slight alterations of cell ultrastructure. Major damage (including breakage of the plasma membrane) and loss of cells were observed in the peripheral cells of the isolated islets. An equal mass of islet equivalent (IEq, islets with an average diameter of 150 microm), but with a different islet equivalent/islet number ratio, was transplanted in diabetic animals. When larger and more complete islets were transplanted (higher ratio), better function of the graft was observed by reversal of hyperglycaemia and response to the glucose tolerance test as compared with the functionality and response of smaller (fragmented) islets transplanted (lower ratio). Digestion, trauma and hypoxia during isolation are responsible for qualitative and quantitative changes of isolated islets. Alterations in normal secretory function after the transplant were related to lower islet equivalent/islet number ratio. The incomplete integrity of the islets may explain the failure of the fine glycaemic metabolic regulation.

Endocrine tissue associated with the pancreatic ductal system: a light and electron microscopic study of the adult rat pancreas with special reference to a new endocrine arrangement

BACKGROUND

A substantial part of the endocrine pancreas has been previously described as being located either close to the excretory ducts as small clusters of endocrine cells and as Islets of Langerhans, or associated with the ducts as single endocrine cells scattered through the ductal epithelium.

METHODS

Four Wistar white adult rats were sacrificed and perfused via the thoracic aorta with 2.5% glutaraldehyde. After the usual treatment for the transmission electron microscopy, pieces of pancreas were sectioned consecutively for light microscopy. Consecutive ultrathin sections were performed in the most interesting cases.

RESULTS

The observations previously reported were confirmed. In addition, a new endocrine arrangement was detected and described as buds of endocrine cells (mainly B-cells) protruding from the ductal epithelium into the surrounding tissue.

CONCLUSIONS

The authors propose to explain the endocrine buds as components of the gastro-entero-pancreatic system or as a stage of an endocrine pancreatic "neo-histogenesis" occurring in the adult rat pancreas.

The endocrine pancreas of glucagon- and somatostatin-immunized rabbits. II. Electron microscopy

An active or passive immunization against hormones and the subsequent neutralization of hormones by circulating antibodies is a valuable tool for the identification of hormonal action. To recognize presumed local (autocrine, paracrine) effects exerted by pancreatic hormones, the endocrine pancreas of rabbits was investigated electron-microscopically after long-term immunization against glucagon or somatostatin. Glucagon immunization resulted in hyperplasia and hypertrophy of glucagon- (A-) cells and in their increased metabolic activities: They showed prominent nucleoli, increased amounts of endoplasmic reticulum, Golgi areas, and mitochondria. These changes were paralleled by alterations in secretion granules (increased size, decreased hormonal content), increased numbers of lysosomes (crinophagic bodies), and an increment of the filamentous system. Basically, these findings point to an autocrine regulation of A-cells. Following somatostatin immunization, somatostatin- (D-) cells were hyperplastic but unchanged in their metabolic state. Instead, insulin-(B-) cells and A-cells exhibited equivalents of increased cellular activities (parameters, see above). This stimulation most probably is caused by cancelled paracrine (inhibitory) effects of somatostatin. The changes observed after both immunizations were differently expressed in morphologically heterogeneous islet types (size, angioarchitecture, cellular composition, microtopology of the various cell types). It is concluded, therefore, that the regulation of islets is not uniform. Autocrine and paracrine effects exerted by islet hormones are of different significance in individual islets, or they interfere differently with other regulatory signals.

The endocrine pancreas of glucagon-and somatostatin-immunized rabbits. I. Light microscopy and immunohistochemistry

Peptide antibodies raised in rabbits are widely used in biology and medicine. During immunization of the animals, the respective antibodies may affect the endocrine cells physiologically responsible for the synthesis of peptides used as antigens. Since corresponding morphological data are still sparse, the rabbit endocrine pancreas was systematically investigated by light microscopy and immunocytochemistry after long-term immunization against glucagon and somatostatin. Both immunizations led to an increase in the number of islets (nesidioblastosis), to the development of giant islets (macronesia), and to changes in the relative proportions of the major types of endocrine cells or their hormonal content. The latter changes differed after either immunization: glucagon immunization resulted in hypertrophy and hyperplasia of glucagon cells and a decrease in their hormonal content; somatostatin immunization led to an increased proportion of somatostatin cells and a lowered hormonal content of insulin cells. The various alterations were expressed differently according to islet type; islets of the rabbit pancreas differ in size or angioarchitecture, and in the proportion and distribution of endocrine cells. The present findings point to autocrine or paracrine effects of the respective peptides. These effects, however, are obviously of differing significance in morphologically heterogeneous islets.

Pancreatic endocrine cells in sea bass (Dicentrarchus labrax L.) II. Immunocytochemical study of insulin and somatostatin peptides

Insulin (INS)- and somatostatin (SST)-immunoreactive cells were demonstrated by light immunocytochemistry in the endocrine pancreas of sea bass (Dicentrarchus labrax). INS-immunoreactive cells were identified using bovine/porcine, bonito, and salmon (s) INS antisera; the immunostaining was abolished when each antiserum was preabsorbed with its respective peptide but not with unrelated peptides. These cells also reacted with mammal (m) SST-28 (4-14) antiserum. The immunoreaction did not change when this antiserum was preabsorbed by bovine INS. INS-immunoreactive cells were located in the central part of the endocrine areas of the principal, intermediate, and small islets. Two SST-immunoreactive cell types (D1 and D2) were revealed. D1 cells, immunoreactive to SST 14 (562) and sSST-25 antisera, were located next to the glucagon-immunoreactive cells in the peripheral part of the endocrine areas. D2 cells, immunoreactive to SST-14 (562), SST-14 (566), and mSST-28 (4-14) antisera, were found in apposition to the INS-immunoreactive cells. The specificity controls showed that D1 cells expressed sSST-25-like peptides, while D2 cells might contain SST-14 and/or mSST-28-like peptides. The close topographic association between the different SST-immunoreactive cells and both glucagon- and insulin-immunoreactive cells might indicate the existence of a specific paracrine regulation of each endocrine cell type in the sea bass endocrine pancreas.

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.

Immunohistochemistry of opioid peptides in the guinea pig endocrine pancreas

Previous immunochemical investigations have demonstrated various opioid peptides in the pancreas. However, controversies exist related to the cellular localization of these peptides in the endocrine pancreas. Therefore, the guinea pig endocrine pancreas was immunohistochemically investigated for the presence of opioid peptides derived from pro-dynorphin, pro-enkephalin or pro-opiomelano-cortin. Immunoreactivities were demonstrated on serial semithin sections by the peroxidase anti-peroxidase technique. In routinely immunostained sections, immunoreactivities for dynorphin A and alpha-neo-endorphin were localized in pancreatic enterochromaffin cells, but not in islet cells. Immunoreactivity for Met-enkephalin was confined exclusively to B-cells and was localized only in some secretory granules. However, pre-treatment of semi-thin sections with trypsin and carboxypeptidase B led to a marked increase of Met-enkephalin immunoreactivity in B-cells. In addition, immunoreactivities for Met-enkephalin-Arg-Gly-Leu and bovine adrenal medulla dodecapeptide could be demonstrated in B- and A-cells, and beta-endorphin immunoreactivity was localized in A-cells. In no case, however, were immunoreactivities detected for bovine adrenal medulla docosapeptide, peptide F, corticotropin, melanotropin or dynorphin 1-32. The immunohistochemical findings indicate that opioids of different peptide families are present in the guinea pig endocrine pancreas. Since several opioid peptides of the corresponding pro-hormones could be demonstrated in the reference organs but not in the pancreas, it is concluded that the biosynthetic pathways of the respective precursors are different from those in the adrenal medulla or in the pituitary.

Vagal afferent fibers almost exclusively innervate islets in the rat pancreas as demonstrated by anterograde tracing

Anterograde tracing from the nodose ganglion with wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was utilized to investigate vagal afferent innervation of the rat pancreas. Labelled afferent fibers were consistently detected in islets in all animals. Only about 10% of islets were labelled even in best cases. In only 5 out of 16 rats, acini and excretory ducts received afferent innervation. Injections into the right nodose ganglion resulted in labelling preferentially within the splenic lobe, whereas injections into the left ganglion labelled fibers predominantly in the duodenal lobe. These results point to a non-random distribution of vagal afferent fibers within the pancreas, and suggest a role for these afferents in the regulation of endocrine pancreatic function.