The cell types in the endocrine pancreas of the human fetus

Summary Morphological and histochemical studies of the developing human islet cells are facilitated by the characteristic localization of the different islet cell types from about the third intrauterine month. By combining light microscopical analyses of silver impregnated and granule stained pancreatic sections with electron microscopy of osmium fixed material, the following four types of islet cells could be identified: (1) A₁ cells containing faint globular granules. These granules could be visualized only with the electron microscope. (2) A₂ cells containing electron-dense globular granules. It is uncertain whether the observation of a light and a dark variety of the A₂ cells reflects different stages of maturation or signifies cells with different secretion products. (3) B cells with irregular granules, which were often accumulated at the capillary pole of the cells. (4) Agranular islet cells. Mixed forms of A cells containing both faint and dense granules were also encountered. The difficulties in evaluating in the light microscope what may be called D cells in the human fetal islets were obvious from the observation of more cells stained with light-green than A₁ cells. Except for acid phosphatases, the histochemical tests for different phosphatases and esterases revealed rather weak or negative reactions in the islet cells. The development of phosphatase and esterase activities in the islets seemed far from complete, when morphologically differentiated islet cells could be recognized.Supported by the United States Public Health Service [grants RF-83(01) and TW-83(02)] and the Swedish Medical Research Council. The authors are indebted to Professor Carl Gemzell and Dr. Ulf Elvkull at the Department of Obstetrics and Gynecology, University Hospital, Uppsala, for the generous supply of the fetal pancreatic material.     

An immunocytochemical study of the endocrine pancreas in the Australian fat-tailed dunnart (Sminthopsis crassicaudata)

Summary The endocrine pancreas of the Australian fattailed dunnart, Sminthopsis crassicaudata, was investigated by means of electron-microscopic immunocytochemistry using the protein A-gold technique on London resin (LR) white-embedded tissue. The primary antibodies used were raised against insulin, glucagon, somatostatin and pancreatic polypeptide. The morphology of the secretory granules differed in the four cell types. The insulin cells are pleomorphic, and the secretory granules composed of an electron-dense core surrounded by an electron-lucen halo. The glucago cells possess granules with an electron-dense core usually surrounded by a halo of less dense granular material. Somatostatin cells have large, less dense secretory granules. The pancreatic polypeptide cells show small, dense secretory granules. In order for an ultrastructural study to be considered reliable for the definite identification of endocrine cell types, it is essential that it be corroborated by immunocytochemical data at the light-or preferably electron-microscopic level. Recent developments in immuno-electron-microscopic techniques have contributed to a better knowledge of cells responsible for the secretion of a wide variety of hormones, as in this study.     

An immunocytochemical and electron-microscopical study of endocrine cells in the gut and pancreas of a stomachless teleost fish,Barbus conchonius (Cyprinidae)

Summary Four immunoreactive endocrine cell types can be distinguished in the pancreatic islets of B. conchonius: insulin-producing B cells, somatostatin-producing A₁ (= D) cells, glucagon-producing A₂ cells and pancreatic poly-peptide-producing PP cells. The principal islet of this species contains only a few PP cells, while many PP cells are present in the smaller islets. Except for the B cell all pancreatic endocrine cell types are also present in the pancreatic duct.At least six enteroendocrine cell types are present in the gut of B. conchonius: 1. a cell type (I) with small secretory granules, present throughout the intestine, and possibly involved in the regulation of gut motility; 2. a C-terminal gastrin immunoreactive cell, probably producing a caerulein-like peptide; these cells are located at the upper parts of the folds, especially in the proximal part of the intestinal bulb; 3. a met-enkephalin-immunoreactive cell, present throughout the first segment; 4. a glucagon-immunoreactive cell, which is rare in the first segment; 5. a PP-immunoreactive cell, mainly present in the first half of the first segment; 6. an immunoreactive cell, which cannot at present be specified, located in the intestinal bulb. The latter four cell types are mostly located in the basal parts of the folds, although some PP-immunoreactive cells can also be found in the upper parts.Most if not all enteroendocrine cells are of the open type. The possible functions of all enteroendocrine cell types are discussed.Abbreviations BPP bovine pancreatic polypeptide - CCK cholecystokinin - GEP gastro-entero-pancreatic - GIP gastric inhibitory peptide or glucose-dependent insulin releasing peptide - PPP pig pancreatic polypeptide - VIP vasoactive intestinal polypeptide     

The ultrastructural basis for the identification of cell types in the pancreatic islets

Summary The pancreatic islets of rabbit, dog and opossum have been studied by light and electron microscopy. Silver-positive cells in the rabbit are predominantly sandwiched between the peripheral A and central B cells, and by electron microscopy are identified as D cells. Pancreatic islets in the tail of the dog pancreas have A, B, and D (silver-positive) cells, but the islets in the uncinate process of the dog pancreas lack phosphotungstic acid hematoxylin-positive A cells. By electron microscopy the characteristic D cells are found in both tail and uncinate process, but A cells are confined to the tail islets, confirming the identification of cell types. A unique cell type termed the F cell is found in the dog uncinate islets and it is characterized by secretory granules of angular profiles. In the opossum, the A cells contain considerable amounts of glycogen demonstrable by both light and electron microscopy. A unique cell type is also present in the opossum islets termed an E cell (Thomas, 1937), which has large secretory granules (400–500 m). The physiological implications of a multiplicity of cell types in pancreatic islets is discussed.This investigation was supported in part by United States Public Health Service research grants GM-10102 and GM-03784 from the Institute of General Medical Sciences, and AM-01226 from the Institute of Arthritis and Metabolic Diseases. The authors wish to acknowledge the valuable technical assistance of Mrs. Aileen Sevier and Mrs. Lidia Donahue.     

An excursion through the ultrastructural world of quick-frozen pancreatic islets

In this article we have presented a philosophical and historical perspective on quick freezing, freeze-drying, freeze-substitution, and immunocytochemical localization of pancreatic islet hormones. A compilation of our findings indicates that quick-freezing does not produce any gross distortion of islet tissue; the amount of usable islet tissue for ultrastructural analysis is approximately 13 micron deep from the frozen edge; three different cell types can be identified in quick-frozen tissue based on general morphological characteristics; freeze-substitution with tetrahydrofuran produces a unique ultrastructural appearance in which ribosomes are particularly striking; with the use of protein A-gold, insulin and glucagon can be localized immunocytochemically on silver-gray (50-nm-thick) sections treated with 1% ovalbumin at room temperature overnight; secretory granules of quick-frozen alpha and beta cells may exist in either a swollen or condensed state; swollen beta cell secretory granules contain a filamentous material that demonstrates immunogold labeling for insulin; insulin and glucagon can be localized within the cisternae of endoplasmic reticulum; our methods provide not only discrete immunocytochemical localization of hormone, but also well-preserved cellular compartments; energy electron loss spectroscopy (EELS) has shown that quantifiable nitrogen maps can be used as an index of hormone packaging in secretory granules; and the sectioning properties of secretory granules at the ultramicrotome change when islet tissue is unosmicated and sectioned on glycerol.     

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

The gastroenteropancreatic (GEP) endocrine system of bowfin (Amia calva) was described using light and electron microscopy and immunological methods. The islet organ (endocrine pancreas) consists of diffusely scattered, mostly small islets and isolated patches of cells among and within the exocrine acini. The islets are composed of abundant, centrally located B cells immunoreactive to bovine and lamprey insulin antisera and D cells showing a widespread distribution and specificity to somatostatin antibodies. A and F cells are present at the very periphery of the islets and are immunoreactive with antisera against glucagon (and glucagon-like peptide) and several peptides of the pancreatic polypeptide (PP)-family, respectively. The peptides of the two families usually collocates within the same peripheral islet cells and are the most common immunoreactive peptides present in the extra-islet tissue. Immunocytochemistry and fine structural observations characterised the granule morphology for B and D cells and identified two cell types with granules immunoreactive to glucagon antisera. These two putative A cells had similar granules, which were distinct from either B or D cells, but one of the cells had rod-shaped cytoplasmic inclusions within cisternae of what appeared to be rough endoplasmic reticulum. The inclusions were not immunoreactive to either insulin or glucagon antisera. Only small numbers of cells in the stomach and intestine immunoreacted to antisera against somatostatin, glucagon, and PP-family peptides. The paucity of these cells was reflected in the low concentrations of these peptides in intestinal extracts. The GEP system of bowfin is not unlike that of other actinopterygian fishes, but there are some marked differences that may reflect the antiquity of this system and/or may be a consequence of the ontogeny of this system in this species.     

Ultrastructure of endocrine pancreatic granules during pancreatic differentiation in the grass snake,Natrix natrix L. (Lepidosauria,Serpentes)

We used transmission electron microscopy to study the pancreatic main endocrine cell types in the embryos of the grass snake Natrix natrix L. with focus on the morphology of their secretory granules. The embryonic endocrine part of the pancreas in the grass snake contains four main types of cells (A, B, D, and PP), which is similar to other vertebrates. The B granules contained a moderately electron‐dense crystalline‐like core that was polygonal in shape and an electron‐dense outer zone. The A granules had a spherical electron‐dense eccentrically located core and a moderately electron‐dense outer zone. The D granules were filled with a moderately electron‐dense non‐homogeneous content. The PP granules had a spherical electron‐dense core with an electron translucent outer zone. Within the main types of granules (A, B, D, PP), different morphological subtypes were recognized that indicated their maturity, which may be related to the different content of these granules during the process of maturation. The sequence of pancreatic endocrine cell differentiation in grass snake embryos differs from that in many vertebrates. In the grass snake embryos, the B and D cells differentiated earlier than A and PP cells. The different sequence of endocrine cell differentiation in snakes and other vertebrates has been related to phylogenetic position and nutrition during early developmental stages.     

Exocyst Sec5 Regulates Exocytosis of Newcomer Insulin Granules Underlying Biphasic Insulin Secretion

The exocyst complex subunit Sec5 is a downstream effector of RalA-GTPase which promotes RalA-exocyst interactions and exocyst assembly, serving to tether secretory granules to docking sites on the plasma membrane. We recently reported that RalA regulates biphasic insulin secretion in pancreatic islet β cells in part by tethering insulin secretory granules to Ca²⁺ channels to assist excitosome assembly. Here, we assessed β cell exocytosis by patch clamp membrane capacitance measurement and total internal reflection fluorescence microscopy to investigate the role of Sec5 in regulating insulin secretion. Sec5 is present in human and rodent islet β cells, localized to insulin granules. Sec5 protein depletion in rat INS-1 cells inhibited depolarization-induced release of primed insulin granules from both readily-releasable pool and mobilization from the reserve pool. This reduction in insulin exocytosis was attributed mainly to reduction in recruitment and exocytosis of newcomer insulin granules that undergo minimal docking time at the plasma membrane, but which encompassed a larger portion of biphasic glucose stimulated insulin secretion. Sec5 protein knockdown had little effect on predocked granules, unless vigorously stimulated by KCl depolarization. Taken together, newcomer insulin granules in β cells are more sensitive than predocked granules to Sec5 regulation.     

Insulin secretion from perifused rat pancreatic pseudoislets

Summary Isolated adult rat pancreatic islets were dispersed into single cells and cultured free-floating for 3 to 4 d, during which time islet cells reaggregated spontaneously into spherical clusters or pseudoislets. The gross morphology of these tissues resembled nondissociated islets. Electron microscopy revealed well-preserved cell ultrastructure and intercellular membrane connections. Immunofluorescent localization of islet cell types showed that A cells tended to be peripherally distributed around a B cell core, with D cells scattered throughtout the aggregate, mass. The dynamics of insulin release from pseudoislets were evaluated in vitro by perifusion techniques. Pseudoislets exhibited clear biphasic dose-dependent insulin responses to 30 min glucose stimulation over the range 5.5 to 30 mM. Repeated 2-min pulses with 22 mM glucose elicited brief monophasic spikes of insulin release of, consistent magnitude.l-Arginine (5 to 20 mM) evoked biphasic insulin release but these responses were not dose-dependent. These data indicate that islet cells reaggregate into structures with close morphologic similarities to intact islets, and that pseudoislet B cells continue to secrete insulin in response to nutrient secretagogues, comparable to that seen with islets in vitro and in situ. This work was supported by grants from the Medical Research Council of New Zealand. D. W. H. was the recipient of a Novo Diabetes Research Scholarship.     

Polyamines, molecules necessary for cell division, colocalize with peptide growth factors

The naturally occurring polyamines spermidine and spermine are necessary for cell division and growth. By restaining experiments, using three independent polyamine cytochemical methods, together with peptide immunocytochemistry, we show that substantial amounts of polyamines occur in a number of peptide growth factor-producing cell types. These include submandibular granular convoluted duct cells producing epidermal growth factor (EGF), pancreatic islet cells producing insulin and anterior pituitary cells producing growth hormone (GH). Other cell types in these tissues display only weak or no polyamine reactivity. Also blood platelets, known to contain platelet-derived growth factor (PDGF), are strongly stained for polyamines. Moreover, in EGF cells, insulin cells and blood platelets, polyamines are clearly localized in secretory granules. The possibility that polyamines may be coreleased and act in concert with peptide growth factors is discussed.     

Cellular and subcellular expression of Golf/Gs and Gq/G11 alpha-subunits in rat pancreatic endocrine cells.

We studied the cellular and subcellular localization of Galpha-subunits in pancreas by immunocytochemistry. Golfalpha and G11alpha were specifically localized in islet insulin B-cells and glucagon A-cells, respectively. Gsalpha and Gqalpha labeling was more abundant in B-cells. The presence of Golfalpha in B-cells was confirmed by in situ hybridization. In B-cells, Golfalpha and Gsalpha were found in the Golgi apparatus, plasma membrane (PM) and, remarkably, in mature and immature insulin secretory granules, mainly at the periphery of the insulin grains. Gqalpha was detected on the rough endoplasmic reticulum (RER) near the Golgi apparatus. In A-cells, the Galpha-subunits were mostly within the glucagon granules: G11alpha gave the strongest signal, Gsalpha less strong, Gq was scarce, and Golf was practically absent. Gqalpha and Gsalpha immunoreactivity was detected in acinar cells, although it was much weaker than that in islet cells. The cell-dependent distribution of the Galpha-subunits indicates that the stimulatory pathways for pancreatic function differ in acinar and in islet B- and A-cells. Furthermore, the G-protein subunits in islet cell secretory granules might be functional and participate in granule trafficking and hormone secretion.     

Quantification of endocrine cells and ultrastructural study of insulin granules in the large intestine of opossum Didelphis aurita (Wied-Neuwied, 1826)

This study aimed to investigate the distribution of argyrophil, argentaffin, and insulin-immunoreactive endocrine cells in the large intestine of opossums (Didelphis aurita) and to describe the ultrastructure of the secretory granules of insulin-immunoreactive endocrine cells. Fragments of the large intestine of 10 male specimens of D. aurita were collected, processed, and subjected to staining, immunohistochemistry, and transmission electron microscopy. The argyrophil, the argentaffin, and the insulin-immunoreactive endocrine cells were sparsely distributed in the intestinal glands of the mucous layer, among other cell types of the epithelium in all regions studied. Proportionally, the argyrophil, the argentaffin, and the insulin-immunoreactive endocrine cells represented 62.75%, 36.26%, and 0.99% of the total determined endocrine cells of the large intestine, respectively. Quantitatively, there was no difference between the argyrophil and the argentaffin endocrine cells, whereas insulin-immunoreactive endocrine cells were less numerous. The insulin-immunoreactive endocrine cells were elongated or pyramidal, with rounded nuclei of irregularly contoured, and large amounts of secretory granules distributed throughout the cytoplasm. The granules have different sizes and electron densities and are classified as immature and mature, with the mature granules in predominant form in the overall granular population. In general, the granule is shown with an external electron-lucent halo and electron-dense core. The ultrastructure pattern in the granules of the insulin-immunoreactive endocrine cells was similar to that of the B cells of pancreatic islets in rats.     

Ultrastructural studies of the ontogeny of fetal human and porcine endocrine pancreas, with special reference to colocalization of the four major islet hormones.

Most, if not all, endocrine cells seem capable of synthesizing and storing more than one hormone. Such cellular colocalization of hormones can be due either to the presence of two or more specific granules within the cells or to colocalization of the hormones within a single granule. The present study was performed to clarify the subcellular localization of insulin, glucagon, somatostatin, and pancreatic polypeptide within the endocrine cells of the human and porcine pancreas during fetal development, with special reference to possible colocalization of the hormones. The tissue specimens were processed for ultrastructural cytochemistry using Lowicryl as embedding medium. An immunogold labeling technique was used with two parallel, but not interacting, antibody chains. Sections from each specimen were double labeled in different combinations giving a complete covering of the four major islet hormones. During fetal life (50-90 days prenatally in porcine pancreas, 14 weeks gestation in the human pancreas) several hormones were demonstrated, not only in the same endocrine cells, but also in the same secretory granules (polyhormonal granules). Costorage of insulin, glucagon, somatostatin, and pancreatic polypeptide was demonstrated in granules in pancreatic endocrine fetal cells. At an early fetal stage, the endocrine cells contained either dense, round granules or pale, heteromorphous granules. With increasing age and maturation of the endocrine cells, structural differentiation of the secretory granules was found to be associated with a gradual disappearance of the polyhormonal granules. The first genuine monohormonal cell to appear in the porcine fetus was the pancreatic polypeptide cell (at 70 days gestation); it was followed by the somatostatin-producing endocrine cell. Mature insulin- and glucagon-producing cells were only demonstrated after birth. Thus, in the adult pancreatic endocrine cells, each specific endocrine cell type produced only one of the four classical hormones. The present investigation demonstrated that the endocrine cells of the fetal, but not the adult, pancreas are able to synthesize all the major islet hormones, and that these peptides are costored in the same granule. The data obtained support the concept of a common precursor stem cell for pancreatic hormone-producing cells.     

New cell types of the pancreatic islets in the crucian carp,Carassius carassius

Summary The pancreatic islets ofCarassius carassius have been studied by electron microscopy. 1. Besides A-, B- and D-cells, two new cell types, the fourth and the fifth, have been identified. The fourth cell type is numerous; it occurs interposed among the other types of islet cells or in small clusters. The secretory granules (90–280 mg in diameter) are round or oval and usually with much lower electron density than α- and δ-granules. The secretory granules of the fifth type of cell (approximately 140–240 mμ in diameter) contain finely granular material and an electron dense core that is round or often tetra- or hexagonal. 2. The islet cells with clear cytoplasmic matrix generally contain large numbers of fine, agranular and cored vesicles 400–680 ? in diameter. They appear, in bead-like chains, or randomly scattered throughout the cytoplasm, or often clustered in aggregates close to the secretory granules and show evidence of incorporation into the secretory granules. The two types of vesicles may be formed by constriction or pinching-off of the tubular smooth endoplasmic reticulum.     

Insulin as a surface marker on isolated cells from rat pancreatic islets

Immunoreactive insulin was shown to exist as a surface molecule in the plasma membrane of dispersed rat pancreatic islet cells. The intact cells were stained by immunofluorescence with a guinea pig antisera specific for insulin. The hormone on the cell surface could not be accounted for by insulin bound to specific receptors or nonspecifically absorbed to cells. Thus, surface insulin was demonstrated to be a specific membrane antigen for islet cells. Furthermore, the proportion of islet cells with insulin on the cell surface was directly correlated with insulin secretion in several different settings. This correspondence was demonstrated by varying the glucose concentration in the medium, by withholding Ca2+, which inhibits secretion, and by adding theophylline, which potentiates secretion. Consequently, these results suggested that insulin as a membrane protein was a marker for cells that actively secreted the hormone and may have been derived in the fusion process of secretory granules with the plasma membrane.     

On the ultrastructural localization of noradrenaline in the central nervous system of the rat

Summary Isolated cells were prepared from microdissected pancreatic islets of guinea-pigs. Phase-contrast microscopy of the fresh islet cells suspended in a balanced salt solution displayed a number of cellular details, including cytoplasmic secretion granules. There was morphologic evidence of the survival of islet cells in monolayer cultures for up to 3 weeks. A moderate proliferation of cells occurred during the first 2 weeks after explantation. Different types of islet cells could not be distinguished in the phase-contrast microscope.Research carried out with the financial support of the Swedish Medical Research Council (B 67-12 X-109-03), the Medical Faculty of Uppsala University and the U. S. Public Health Service (Grant AM-05759-05). We thank Assistant Professor Björn Sandström, M. D., for much advice on the tissue-culture method.     

Immunohistochemical localization of monoamine oxidase type B in pancreatic islets of the rat.

Monoamine oxidase (MAO) is regarded as a mitochondrial enzyme. This enzyme localizes on the outer membrane of mitochondria. There are two kinds of MAO isozymes, MAO type A (MAOA) and type B (MAOB). Previous studies have shown that MAOB activity is found in the pancreatic islets. This activity in the islets is increased by the fasting-induced decrease of plasma glucose level. Islet B cells contain monoamines in their secretory granules. These monoamines inhibit the secretion of insulin from the B cells. MAOB is active in degrading monoamines. Therefore, MAOB may influence the insulin-secretory process by regulating the stores of monoamines in the B cells. However, it has not been determined whether MAOB is localized on B cells or other cell types of the islets. In the present study, we used both double-labeling immunofluorescence histochemical and electron microscopic immunohistochemical methods to examine the subcellular localization of MAOB in rat pancreatic islets. MAOB was found in the mitochondrial outer membranes of glucagon-secreting cells (A cells), insulin-secreting cells (B cells), and some pancreatic polypeptide (PP)-secreting cells (PP cells), but no MAOB was found in somatostatin-secreting cells (D cells), nor in certain other PP cells. There were two kinds of mitochondria in pancreatic islet B cells: one contains MAOB on their outer membranes, but a substantial proportion of them lack this enzyme. Our findings indicate that pancreatic islet B cells contain MAOB on their mitochondrial outer membranes, and this enzyme may be involved in the regulation of monoamine levels and insulin secretion in the B cells.     

New views on the identification of the various cell types in the pancreatic islets of the rat. An ultrastructural and morphometrical study

Islets of Langerhans taken from different parts of the pancreas have been studied ultrastructurally in adult rats. Five different islet cell types were identified in each islet with the aid of morphometrical analysis of their specific secretory granules. Previous immunohistochemical findings concerning the amount and location of insulin-, glucagon-, somatostatin- and pancreatic-polypeptide-containing cells and their ultrastructurally recognizable counterparts were compared, and it was possible to identify four main islet cell types with the electron microscope. Moreover, cells quite similar to the enterochromaffine cells described elsewhere in the exocrine pancreas and in the gastrointestinal tract were found to normally occur in the pancreatic islets of the rat.     

Metabolic and secretory effects of methylamine in pancreatic islets

The metabolic and secretory effects of methylamine in rat pancreatic islets were investigated. Methylamine accumulated in islet cells, was incorporated into endogenous islet proteins, and inhibited the incorporation of [2,5-³H] histamine into either N,N-dimethylcasein or endogenous islet proteins. Methylamine (2 mM ) did not affect the oxidation of glucose or endogenous nutrients or the intracellular pH in islet cells. Glucose did not affect the activity of transglutaminase in islet homogenates, the uptake of ¹⁴C-methylamine by intact islets or its incorporation into endogenous islet proteins. Methylamine inhibited insulin release evoked by glucose, other nutrient secretagogues, and non-nutrient insulinotropic agents such as L -arginine or gliclazide. The inhibitory effect of methylamine upon insulin release was diminished in the presence of cytochalasin B or at low extracellular pH. Methylamine retarded the conversion of proinsulin to insulin. Trimethylamine (0.7 mM ) was more efficiently taken up by islet cells than methylamine (2.0 mM ), and yet caused only a modest inhibition of insulin release. These findings suggest that methylamine interferes with a late step in the secretory sequence, possibly by inhibiting the access of secretory granules to their exocytotic site.     

3D FIB-SEM reconstruction of microtubule–organelle interaction in whole primary mouse β cells

Microtubules play a major role in intracellular trafficking of vesicles in endocrine cells. Detailed knowledge of microtubule organization and their relation to other cell constituents is crucial for understanding cell function. However, their role in insulin transport and secretion is under debate. Here, we use FIB-SEM to image islet β cells in their entirety with unprecedented resolution. We reconstruct mitochondria, Golgi apparati, centrioles, insulin secretory granules, and microtubules of seven β cells, and generate a comprehensive spatial map of microtubule–organelle interactions. We find that microtubules form nonradial networks that are predominantly not connected to either centrioles or endomembranes. Microtubule number and length, but not microtubule polymer density, vary with glucose stimulation. Furthermore, insulin secretory granules are enriched near the plasma membrane, where they associate with microtubules. In summary, we provide the first 3D reconstructions of complete microtubule networks in primary mammalian cells together with evidence regarding their importance for insulin secretory granule positioning and thus their supportive role in insulin secretion.