Characterization of integrin expression in islets isolated from hamster, canine, porcine, and human pancreas.

The reasons for the failure of clinical islet transplantation remain obscure. Islet isolation, however, exposes the islet to variety of cellular stresses, including disruption of the cell-matrix relationship, an event associated with apoptosis. The cell-matrix relationship is characterized by an interaction between cell surface integrin receptors and matrix molecules of the surrounding basement membrane (BM). The purpose of this study was to characterize integrin expression and the distribution of the peri-insular BM in human, porcine, canine, and hamster pancreas, and after routine islet isolation. Whereas islets in the porcine pancreas do not have a demonstrable BM, islets in the human, canine, and hamster pancreas have an almost continuous BM with very little direct exocrine to endocrine cell-cell contact. After islet isolation, the BM was destroyed, only to be reestablished during the period of culture. In the pancreas of all four species, integrin alpha3 was expressed only on islet cells, and integrin alpha5 was present on islet cells as well as on acinar, centroacinar, and duct cells. Integrin alphaV was detected only in human and canine pancreas. Integrin beta1 was demonstrated only in the human pancreas. In isolated islets, integrin alpha3, alpha5, and alphaV expression decreased during the culture period and the intensity of the staining was observed to be coincident with the distribution of the BM. In summary, this is the first report of integrin expression in hamster, canine, porcine, and human islets. After islet isolation, the altered islet cell-matrix relationship is reflected both in the decrease in integrin expression and in the destruction of the peri-insular BM. These profound changes will need to be considered as the process of islet isolation for transplantation is refined. (J Histochem Cytochem 47:499-506, 1999)     

Pancreatic polypeptide cells of rat pancreas after chronic ethanol feeding

Male Wistar rats, (2 months old) were randomly divided into two groups according to the diet offered (C-control and E-ethanol treated rats). Final body weight was significantly increased but pancreatic weight as a percentage of body weight was decreased in ethanol treated rats. Volume density, number of pancreatic poly peptide (PP)-cells per islet and per micron 2 of islet were significantly increased. PP-cells were abundant and occupied the whole periphery of islets in the splenic part of the pancreas. Those cells showed strong immunopositivity. At the ultrastructural level PP granules had predominantly less electron density. The mean diameter of PP granules was significantly increased and the number of granules of larger diameter was greater in the E group of rats, than in the controls.     

Involvement of cellular calcium in exocrine pancreatic insufficiency during streptozotocin-induced diabetes mellitus

This study investigates the effects of the islet hormones insulin (Ins), glucagon (Glu), and somatostatin (Som) with nerve stimulation (EFS) acetylcholine (ACh) and cholecytokinin-octapeptide (CCK-8) on amylase secretion and intracellular free calcium concentration [Ca(2+)](i) in the pancreas of age-matched control and diabetic rats. Either Ins, Glu or Som elicited small increases in amylase secretion from the pancreas of age-matched control animals compared to a much larger increase in amylase secretion with either EFS, ACh or CCK-8. Combining the islet hormones with either EFS, ACh or CCK-8 resulted in marked potentiation of amylase output. In the diabetic pancreas, the islet hormones had no effect on amylase secretion compared to diabetic control. Moreover, either EFS, ACh or CCK-8 evoked a much smaller increase in amylase output compared to age-matched control. In addition, the islet hormones failed to potentiate the secretory effects of either EFS, ACh or CCK-8. In fura-2 loaded acinar cells from age-matched control pancreas either Ins or Glu elicited a small increase in [Ca(2+)](i) whereas Som had no effect. Both ACh and CCK-8 evoked large increases in [Ca(2+)](i) compared to control. Combining either Ins, Glu or Som with either ACh or CCK-8 resulted in a marked elevation in [Ca(2+)](i) compared to the responses obtained with either the islet hormones, ACh or CCK-8 alone. In diabetic fura-2 loaded pancreatic acinar cells, the islet hormones had no effect on [Ca(2+)](i) compared to control and moreover, the responses were much smaller than those obtained in acinar cells from age-matched control. Both ACh and CCK-8 induced large increases in [Ca(2+)]( i) in diabetic acinar cells. However, combining the islet hormones with either ACh or CCK-8 failed to enhance [Ca(2+)](i) compared to the reponses obtained in acinar cells from age-matched control. The results suggests that [Ca(2+)](i) homeostasis is deranged during diabetes mellitus and this in turn is probably associated with reduced pancreatic amylase secretion.     

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.     

Stem cell-based approaches to solving the problem of tissue supply for islet transplantation in type 1 diabetes

Type 1 diabetes is a debilitating condition, affecting millions worldwide, that is characterized by the autoimmune destruction of insulin-producing pancreatic islets of Langerhans. Although exogenous insulin administration has traditionally been the mode of treatment for this disease, recent advancements in the transplantation of donor-derived insulin-producing cells have provided new hope for a cure. However, in order for islet transplantation to become a widely used technique, an alternative source of cells must be identified to supplement the limited supply currently available from cadaveric donor organs. Stem cells represent a promising solution to this problem, and current research is being aimed at the creation of islet-endocrine tissue from these undifferentiated cells. This review presents a summary of the research to date involving stem cells and cell replacement therapy for type 1 diabetes. The potential for the differentiation of embryonic stem (ES) cells to islet phenotype is discussed, as well as the possibility of identifying and exploiting a pancreatic progenitor/stem cell from the adult pancreas. The possibility of creating new islets from adult stem cells derived from other tissues, or directly form other terminally differentiated cell types is also addressed. Finally, a model for the isolation and maturation of islets from the neonatal porcine pancreas is discussed as evidence for the existence of an islet precursor cell in the pancreas.     

Assessment of human pancreatic islet architecture and composition by laser scanning confocal microscopy.

The recent success of pancreatic islet transplantation has generated considerable enthusiasm. To better understand the quality and characteristics of human islets used for transplantation, we performed detailed analysis of islet architecture and composition using confocal laser scanning microscopy. Human islets from six separate isolations provided by three different islet isolation centers were compared with isolated mouse and non-human primate islets. As expected from histological sections of murine pancreas, in isolated murine islets alpha and delta cells resided at the periphery of the beta-cell core. However, human islets were markedly different in that alpha, beta, and delta cells were dispersed throughout the islet. This pattern of cell distribution was present in all human islet preparations and islets of various sizes and was also seen in histological sections of human pancreas. The architecture of isolated non-human primate islets was very similar to that of human islets. Using an image analysis program, we calculated the volume of alpha, beta, and delta cells. In contrast to murine islets, we found that populations of islet cell types varied considerably in human islets. The results indicate that human islets not only are quite heterogeneous in terms of cell composition but also have a substantially different architecture from widely studied murine islets.     

Effect of alloxan on rat pancreatic polypeptide (PP) cells

Summary Injection of alloxan caused an almost total disappearance of insulin cells in the rat pancreas. Planimetric analysis revealed a 50 per cent reduction of the mean islet volume. The number of immunoreactive pancreatic polypeptide (PP) cells per sectioned islet was significantly increased, and the PP cell volume per islet doubled. Assuming an unchanged number of islets, the results indicate an increase in total PP cell mass following alloxan administration.     

Islet Formation during the Neonatal Development in Mice

The islet of Langerhans is a unique micro-organ within the exocrine pancreas, which is composed of insulin-secreting beta-cells, glucagon-secreting alpha-cells, somatostatin-secreting delta-cells, pancreatic polypeptide-secreting PP cells and ghrelin-secreting epsilon-cells. Islets also contain non-endocrine cell types such as endothelial cells. However, the mechanism(s) of islet formation is poorly understood due to technical difficulties in capturing this dynamic event in situ. We have developed a method to monitor beta-cell proliferation and islet formation in the intact pancreas using transgenic mice in which the beta-cells are specifically tagged with a fluorescent protein. Endocrine cells proliferate contiguously, forming branched cord-like structures in both embryos and neonates. Our study has revealed long stretches of interconnected islets located along large blood vessels in the neonatal pancreas. Alpha-cells span the elongated islet-like structures, which we hypothesize represent sites of fission and facilitate the eventual formation of discrete islets. We propose that islet formation occurs by a process of fission following contiguous endocrine cell proliferation, rather than by local aggregation or fusion of isolated beta-cells and islets. Mathematical modeling of the fission process in the neonatal islet formation is also presented.     

Collection protocol for human pancreas

This dissection and sampling procedure was developed for the Network for Pancreatic Organ Donors with Diabetes (nPOD) program to standardize preparation of pancreas recovered from cadaveric organ donors. The pancreas is divided into 3 main regions (head, body, tail) followed by serial transverse sections throughout the medial to lateral axis. Alternating sections are used for fixed paraffin and fresh frozen blocks and remnant samples are minced for snap frozen sample preparations, either with or without RNAse inhibitors, for DNA, RNA, or protein isolation. The overall goal of the pancreas dissection procedure is to sample the entire pancreas while maintaining anatomical orientation. Endocrine cell heterogeneity in terms of islet composition, size, and numbers is reported for human islets compared to rodent islets. The majority of human islets from the pancreas head, body and tail regions are composed of insulin-containing β-cells followed by lower proportions of glucagon-containing α-cells and somatostatin-containing δ-cells. Pancreatic polypeptide-containing PP cells and ghrelin-containing epsilon cells are also present but in small numbers. In contrast, the uncinate region contains islets that are primarily composed of pancreatic polypeptide-containing PP cells. These regional islet variations arise from developmental differences. The pancreas develops from the ventral and dorsal pancreatic buds in the foregut and after rotation of the stomach and duodenum, the ventral lobe moves and fuses with the dorsal. The ventral lobe forms the posterior portion of the head including the uncinate process while the dorsal lobe gives rise to the rest of the organ. Regional pancreatic variation is also reported with the tail region having higher islet density compared to other regions and the dorsal lobe-derived components undergoing selective atrophy in type 1 diabetes. Additional organs and tissues are often recovered from the organ donors and include pancreatic lymph nodes, spleen and non-pancreatic lymph nodes. These samples are recovered with similar formats as for the pancreas with the addition of isolation of cryopreserved cells. When the proximal duodenum is included with the pancreas, duodenal mucosa may be collected for paraffin and frozen blocks and minced snap frozen preparations.     

牛蛙胃肠胰系统内分泌细胞的免疫组织化学鉴定与定位

应用过氧化物酶标记的链霉卵白素(S-P)免疫组织化学方法对牛蛙(Rana catesbeiana)胃肠胰系统5种内分泌细胞进行了鉴定与定位.在消化道中检测到了5-羟色胺(5-HT)、生长抑素(SS)、胃泌素(Gas)和胰高血糖素(Glu)细胞.5-HT细胞主要分布于胃幽门部和空肠,食道中偶见.SS细胞主要分布于胃,幽门部较密集,小肠各段少量,直肠和食道偶见.Gas细胞主要分布于小肠各段,胃和直肠中偶见,食道中未检测到.Glu细胞主要分布于胃和直肠,小肠各段偶见,食道中未检测到.在胰腺中检测出了5-HT、SS、Gas、Glu和胰多肽(PP)细胞.SS、Glu和PP细胞数量较多,成簇分布于胰岛中,5-HT和Gas细胞少量,散在分布于胰腺腺泡之间.胃腺部和胰腺内分泌细胞多呈圆形、椭圆形或形态不规则,有的可见明显胞突伸向邻近细胞,胃肠道上皮中的内分泌细胞多呈梭形、楔形或锥形,有的可见明显胞突伸向消化腔.与其它两栖动物相比,牛蛙胃肠胰系统内分泌细胞的存在与分布有一些共性,也存在着种间差异.     

乌龟胃肠胰系统内分泌细胞的免疫组织化学研究

应用过氧化物酶标记的链霉卵白素(Streptavidin peroxidase,简称S-P法)免疫组织化学技术,使用六种特异性胃肠激素抗血清对乌龟胃肠胰系统内分泌细胞的种类、定位、分布密度及形态进行了研究。在乌龟胰腺中检测出5-羟色胺、生长抑素、胰高糖素和胰多肽等4种内分泌细胞,生长抑素、胰高糖素细胞多成簇大量分布于胰岛中;5-羟色胺、胰多肽细胞多散在少量分布于胰腺腺泡之间。在乌龟消化道中共检测出5-羟色胺、生长抑素、胃泌素、胰高糖素和P物质等5种内分泌细胞:5-羟色胺细胞在消化道各段均有分布,以十二指肠处分布密度最高(30.7±4.2),空肠其次,回肠、直肠处最低(12.0±1.0/11.2±3.0);生长抑素细胞仅分布于食道和胃中各段;胃泌素细胞分布于胃幽门部和十二指肠处;胰高糖素细胞分布于胃体至空肠段,以胃幽门部分布密度较高(11.3±1.1);P物质细胞仅布于胃幽门部;消化道各段均未检出胰多肽细胞。与其他爬行动物比较,乌龟胃肠胰系统内分泌细胞的分布既存在着一定共同点,又显示了较大的种间差异。       

A comparison of cross sectional surface area densities between adult and juvenile porcine islets of Langerhans.

The object of this study was to evaluate differences in islet diameters, their distribution and both cross sectional surface areas and densities of insulin containing islets between adult and juvenile porcine pancreata using a computerised image analysis system (Improvision). Five adult (A) (2-3 yrs) and 5 juvenile (J) (< 12 mths) Large White pancreata were assessed. Biopsies were taken from 5 different regions (posterior lobe, duodenal lobe, along with the head, body and tail regions of the splenic lobe) of the pancreas and tissue sections stained for insulin. In both A and J pancreata islet numbers increased with decreasing islet diameter, showing a skewed distribution. There was no statistical significance between the cross sectional surface area within A (mean 5.04 x 10(3) microm2) or J (mean 5.99 x 10(3) microm2) pancreata. Assuming islets are spherical, extrapolation from pir2 showed that the mean diameter for A was 80 microm and 87 microm in J. These compared with A 77 microm and J 86 microm diameters using conventional microscopic techniques. The percentage islet volume density relative to exocrine tissue, derived from the principle of Delesse (Area density = volume density), did not significantly differ between each of the 5 areas studied, either in A or J. The percentage islet volume densities did show a significant difference between A (mean 1.83%) (P = 0.001) but not between J pancreata (mean 2.13 %). In conclusion poor islet yields can be attributed to differences in islet volume density of islets within porcine pancreata. These results also suggest that the posterior and duodenal lobes should be used along with the splenic lobe in order to improve porcine islet yields. Furthermore, the current practise of reporting porcine islet yields and the isolation index relative to 150 microm (IEQs) needs to be redefined, based on the assumption that the average size of an adult porcine islet is 80 microm.     

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.     

Development of endocrine pancreatic islets in embryos of the grass snake Natrix natrix (Lepidosauria,Serpentes)

Differentiation of the pancreatic islets in grass snake Natrix natrix embryos, was analyzed using light, transmission electron microscopy, and immuno-gold labeling. The study focuses on the origin of islets, mode of islet formation, and cell arrangement within islets. Two waves of pancreatic islet formation in grass snake embryos were described. The first wave begins just after egg laying when precursors of endocrine cells located within large cell agglomerates in the dorsal pancreatic bud differentiate. The large cell agglomerates were divided by mesenchymal cells thus forming the first islets. This mode of islet formation is described as fission. During the second wave of pancreatic islet formation which is related to the formation of the duct mantle, we observed four phases of islet formation: (a) differentiation of individual endocrine cells from the progenitor layer of duct walls (budding) and their incomplete delamination; (b) formation of two types of small groups of endocrine cells (A/D and B) in the wall of pancreatic ducts; (c) joining groups of cells emerging from neighboring ducts (fusion) and rearrangement of cells within islets; (d) differentiated pancreatic islets with characteristic arrangement of endocrine cells. Mature pancreatic islets of the grass snake contained mainly A endocrine cells. Single B and D or PP–cells were present at the periphery of the islets. This arrangement of endocrine cells within pancreatic islets of the grass snake differs from that reported from most others vertebrate species. Endocrine cells in the pancreas of grass snake embryos were also present in the walls of intralobular and intercalated ducts. At hatching, some endocrine cells were in contact with the lumen of the pancreatic ducts.     

Porcine islet isolation, cellular composition and secretory response

Porcine islets were isolated by infusion of a warm collagenase solution into whole pancreata followed by static incubation at 37 degrees C for 15 minutes. The pancreata were then chopped into small pieces and the free islets purified by filtration and centrifugation over a ficoll gradient. The insulin:amylase ratio of the islets compared to that in the intact pancreas was determined in 19 pancreata and indicates that the isolated islets were of a high degree of purity. The distribution of insulin, glucagon, somatostatin and pancreatic polypeptide containing cells in pig pancreas sections was compared with that in rat. Porcine islets were much smaller and less well defined than rat islets with infiltration of acinar material even into the islet core. The levels of insulin, glucagon and somatostatin in porcine pancreas and isolated porcine islets were measured using conventional radioimmunoassay techniques. The ratio of these hormones in the pancreas was 105.1:5.8:1 respectively, and in the islets 105.1:0.68:0.087 respectively. Fragmentation of the islets during the isolation may have led to the loss of glucagon and somatostatin-containing cells. Islets cultured overnight and tested with a range of glucose concentrations for one hour did not show a significant stimulation of insulin secretion in the presence of 8.3 mM or 16.7 mM glucose compared to that in 2.8 mM glucose. However freshly isolated islets challenged with 8.3 mM, 13.9 mM and 22.2 mM glucose showed a 1.8 fold, 2.0 fold and 2.3 fold response respectively, over that in 2.8 mM glucose.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Coexistence of pancreatic polypeptide (PP)-and glucagon-immunoreactivity in pancreatic endocrine cells of mouse

Immunocytochemical double staining techniques were used to study PP- and glucagon-like-immunoreactivity in pancreatic endocrine cells of mouse. An antiserum against FMRFamide appeared to react with all PP-immunoreactive endocrine cells. With fluorescence microscopy most PP/FMRFamide-immunoreactive cells also showed glucagon-immunoreactivity, but cells containing only PP- or glucagon-like substances were found as well. The proportion of cells containing PP-, glucagon, and both immunoreactivities varied strongly from islet to islet in all parts of the pancreas. Using an electron microscopical immunogold double staining procedure on Lowicryl-embedded pancreas, PP/FMRFamide- and glucagon-immunoreactivity appeared to be present in the majority of endocrine A cells; both immunoreactivities were randomly distributed within the granules of these cells. Cells containing only PP/FMRFamide- or glucagon-immunoreactivity were also found. Glucagon- and a faint FMRFamide-immunoreactivity was also observed in osmicated epon-embedded tissue. Independent of their immunoreactivity all positive cells showed the same round electron dense secretory granules.     

Coexistence of pancreatic polypeptide (PP)-and glucagon-immunoreactivity in pancreatic endocrine cells of mouse

Summary Immunocytochemical double staining techniques were used to study PP- and glucagon-like-immunoreactivity in pancreatic endocrine cells of mouse. An antiserum against FMRFamide appeared to react with all PP-immunoreactive endocrine cells. With fluorescence microscopy most PP/FMRFamide-immunoreactive cells also showed glucagon-immunoreactivity, but cells containing only PP-or glucagon-like substances were found as well. The proportion of cells containing PP-, glucagon, and both immunoreactivities varied strongly from islet to islet in all parts of the pancreas.Using an electron microscopical immunogold double staining procedure on Lowicryl-embedded pancreas, PP/FMRFamide-and glucagon-immunoreactivity appeared to be present in the majority of endocrine A cells; both immunoreactivities were randomly distributed within the granules of these cells. Cells containing only PP/FMRFamide-or glucagon-immunoreactivity were also found. Glucagon-and a faint FMRFamide-immunoreactivity was also observed in osmicated epon-embedded tissue. Independent of their immunoreactivity all positive cells showed the same round electron dense secretory granules.     

Ultrastructure of islet ghrelin cells in the human fetus

Ghrelin is a peptide hormone predominantly produced in the stomach. Ghrelin expression has also been reported in other tissues including the pancreas. We have reported that ghrelin cells constitute a novel endocrine cell type in the human and the developing rat islets. The cells are most numerous pre- and neonatally and, in humans, constitute 10% of all islet cells from mid-gestation to birth. Since gastric ghrelin expression is low before birth, the islets may be the main source of circulating ghrelin during this time. In the present investigation, we have performed an ultrastructural analysis of pancreatic ghrelin cells in human fetuses by using transmission electron microscopy and immunogold labelling. In addition, morphometrical analysis of secretory granules size was performed. Our data provide evidence for the unique ultrastructural features of ghrelin cells versus other islet cells. Notably, the secretory granules of ghrelin cells were of small size with a mean dense-core diameter of 110 nm. We conclude that ghrelin cells constitute a novel islet cell type, distinct from the previously hormonally characterised islet cell types.This work was supported by grants from the Swedish Medical Research Council (Project No. 4499), the Royal Physiographic Society and the Novo Nordic, Påhlsson and Gyllenstiernska Krapperup Foundations     

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.