Bridging the Gap: Pancreas Tissue Slices From Organ and Tissue Donors for the Study of Diabetes Pathogenesis

For decades, information regarding the human endocrine pancreas and its functional unit, the islet of Langerhans, has been somewhat limited; a facet largely related to structural anatomy and islets being obtained from autopsy or organ donor cases under somewhat uncontrolled conditions (1–3). As a result, a majority of data on the function and physiology of the human endocrine pancreas have been derived nearly exclusively from systemic metabolic studies from living subjects. Specifically, analysis of blood plasma samples following the application of various standardized metabolic tests (e.g., glucose tolerance tests or mixed meal tests) have, and remain, a standard method to provide a valuable picture of pancreatic function within the systemic environment (4). This representation of endocrine pancreas function includes the integrative response of all endocrine cells within the organ, but in addition, their modulation and regulation via the nervous and vascular system (5). Within this setting, the complexity of the obtained readout is further increased by the potential alteration through other organs and tissues (e.g., through changes in insulin resistance, insulin clearance, or glucose uptake) (6). Thus, investigations of systemic metabolism are multifaceted, and the challenge is to pinpoint the contribution and cellular mechanisms of the endocrine pancreas to the systemic status. Nevertheless, systemic studies are indispensable for successful implementation of interventions and therapeutic approaches into clinical use as they offer a unique and important option to illustrate effects on the complete organism.

However, understanding human islet and pancreas pathogenesis in detail ultimately requires studies of viable human tissue to enable analysis and direct assessment of islet cell function, regulation, and their cellular interactions. Importantly, animal models, although useful for experimental manipulations and basic mechanistic studies, display fundamental differences and have provided fairly limited translational value (7–16). This situation has sparked the formation of several initiatives over the last two decades, efforts that aimed to expand access for researchers to human pancreatic specimens (17–21). As a result, the number of studies investigating pancreas histopathology and function of human islet biology has grown immensely.

Specifically, frozen and fixed sections of intact human pancreatic tissues have enabled the detailed study of pancreas morphology, cell identity, and cell status (3). This said, investigations of endocrine pancreas function and physiology have predominantly been performed on isolated islets from brain-dead organ donors (22,23). For this purpose, islets are isolated from the organ donor pancreas by chemical and mechanical methods. The resulting preparation allows for investigation of endocrine pancreas cell physiology while maintaining the three-dimensional (3D) cellular structure of the islets as a functional unit (24). Depending on the technologies used, isolated islets can be investigated in large numbers simultaneously to provide knowledge on the integrated function and status of the endocrine pancreas, or individually to address potential heterogeneities between islets (25). In addition, certain methods, including electrophysiology and live-cell imaging, also allow for assessment of single-cell physiology within the context of an islet (26). Thus, isolated human islets facilitate the study of biological processes in pancreatic endocrine cells within a preserved juxtacrine and paracrine signaling environment. In this way, isolated islets have and will continue to occupy a central role in acquiring knowledge on the human endocrine pancreas in health and disease.

However, there are a number of limitations one should consider in terms of study design and data interpretation, with the notion that isolated islets are highly susceptible to the preparation procedure itself. Naturally, extraction of islets from their surrounding tissue prevents one from assessing any islet-extrinsic aspects of endocrine pancreas physiology and pathophysiology. More importantly, the isolation process has been shown to alter islet morphology, gene expression, and cell biology (27,28). The necessary recovery period following their isolation and extended culture times might lead to an additional adaptation of the islet to its in vitro environment. Finally, an isolation procedure based on chemical and mechanical properties of the tissue is susceptible to a selection bias for islets within a specific range of physical and biochemical properties. This, along with the enzymatic and mechanical stress imparted during the isolation process, could lead to loss or destruction of islets that have been structurally compromised in pathophysiological processes (e.g., a type 1 diabetes autoimmune attack).

In addition to investigating intact isolated islets, the study of pancreatic endocrine cell biology can be resolved to a cellular level by dispersing islets into single cells in vitro. This process further eases cell handling and the application of additional technologies, including high-resolution imaging or electrophysiology. In recent years, their use in the constantly growing field of single-cell “omics” technologies has become an increasingly important focus of attention (29,30). These approaches allow for the assessment of cell status at an extraordinarily detailed level and, therefore, have vastly increased our information on human islet cell biology and heterogeneity (25).

To provide a platform for the study of islet cell physiology and pathophysiology that circumvents many, but not all, of the known methodological limitations of existing technologies and bridges the methodological gap between isolated islets and systemic metabolism, we developed the human pancreas tissue slice platform (Fig. 1) (31,32). Herein, we discuss the potential use of human pancreas tissue slices for the study of islet physiology and function from single-cell to organ tissue level, the added value and limitations of studies on tissue slices, and the considerations that must be made depending on the source of human pancreatic tissue.

The preparation of tissue slices was initially established and successfully applied to several other organs, including intestine, colon, liver, brain, and heart (33–35). For the study of the pancreas, the tissue slice platform was originally developed to investigate the endocrine pancreas of the mouse, which as noted earlier, has proven its value by contributing to an increased understanding of islet physiology and pathophysiology in diabetes (36). Recently, we were able to transfer this technology to the study of the human endocrine pancreas by using pancreas tissue obtained from patients undergoing pancreatectomy or from organ donors (31,32,37). By immersing pieces of viable human pancreas tissue in low-temperature gelling agarose for stabilization, small tissue blocks (∼2–5-mm edge length) can be readily cut into slices of variable thickness (80–200 µm) using a vibratome (38). Although the slicing procedure will inevitably result in some partially damaged islets, especially at the cutting surface, the injured cells are washed off shortly after preparation. Subsequently, islets compromised by the slicing procedure can be easily identified and excluded in imaging or electrophysiological studies. In addition, digestive enzymes leaked from damaged exocrine cells due to slicing or secreted during culture will need to be inactivated by application of enzyme inhibitors to allow islet studies after slice culture (37). The main benefit of tissue slices for the study of pancreas physiology is related to the apparent preserved morphological arrangement of the different cellular compartments within viable organ tissue (38). In addition, the fast and minimally destructive preparation procedure imposes no enzymatic and only limited mechanical stress. This also promotes the almost immediate use of tissue slices for functional assessment, circumventing the need for a resting culture period and reducing a potential adaption of cells to the culture conditions (39). Thus, tissue slices facilitate the investigation of a close reflection of in vivo pancreas cell physiology and pathogenesis from individual cells to organ tissue.

Similar to isolated islets, pancreas tissue slices enable the study of islet physiology at different levels, from individual cells within islets to groups of islet cells or several islets simultaneously (Fig. 2). A number of studies on mouse pancreas tissue slices have used electrophysiological techniques to provide single-cell resolution of islet physiology. Among others, the tissue slice approach was the first preparation to demonstrate K_ATP channel regulation by physiological intracellular ATP concentrations (40). In addition, tissue slices have allowed for the study of various aspects of β-cell biology using whole-cell patch-clamp measurements, including functional maturation (41) and modulation of coordinated islet cell function by gap junctions (42). Electrophysiological investigations of α-cell function have also been performed in living tissue slices of streptozotocin-induced diabetes, where the tissue slice technique is particularly advantageous as the damaged islets of this mouse model cannot be easily isolated or result in an extremely low yield of isolated islets (43,44). An important display of the in situ character of pancreas tissue slices was achieved by combining different imaging methods for studies of β-cell polarity. This was possible due to the preserved 3D endocrine-vascular niche in tissue slices (45,46). Studies using 3D confocal, two-photon, and block-face serial electron microscopy in mouse and human samples were able to identify three distinct β-cell domains, and the basal domain facing the vasculature was suggested to be responsible for insulin secretion into the circulation (46,47).

Zooming out from individual cells to the simultaneous assessment of multiple endocrine cells, live-cell imaging methods have provided important data from studies on islets in tissue slices (Fig. 2). Glucose-stimulated metabolic flux was investigated in a newly designed microfluidic device, where a clear increase in NAD(P)H autofluorescence was observed in endocrine cells with only minimal changes in the surrounding exocrine tissue in living pancreas tissue slices from rats (48). As in other preparations, the most commonly used parameter to investigate islet cell physiology by live-cell imaging in slices has been intracellular calcium ([Ca²⁺]_i) dynamics (38,49–51). Hereby, [Ca²⁺]_i imaging of numerous β-cells in acute tissue slices revealed that [Ca²⁺]_i waves originate in a group of cells and that fast [Ca²⁺]_i oscillations superimposed on a sustained plateau is the predominant type of response in β-cells during glucose stimulation (52). In addition, these studies enabled the analysis of the detailed network characteristics of β-cells within intact islets, which could be activated by glucose, modulated by N-methyl-d-aspartate receptor inhibition, and regulated by extracellular pH (53–57). Furthermore, a critical role of intracellular Ca²⁺ stores in glucose-stimulated [Ca²⁺]_i oscillations in β-cell collectives was highlighted in a recent study using mouse pancreas tissue slices (51). Interestingly, calcium imaging on a large number of α-cells in tissue slices revealed a high degree of functional heterogeneity in this cell population in the pancreas (58) and that glutamate receptor signaling in α-cells is impaired in type 1 diabetes but could be rescued with positive allosteric modulators together with a restoration of glucagon secretion (59). To assess several signal transduction parameters at the same time, calcium imaging has been combined with membrane potential recording to explore the relationship between glucose-evoked membrane potential oscillations and [Ca²⁺]_i oscillations in β-cells (49), or with membrane capacitance measurements to investigate the correlation of exocytosis and [Ca²⁺]_i in β-cells in the setting of diabetes (60).

Finally, pancreas tissue slices also enable the study of several islets simultaneously (Fig. 2). By measuring hormone release from one to several slices in response to various stimuli, the integrated secretory function of a distinct number of islets can be assessed (31,32). Moreover, high-speed live-cell imaging approaches enable assessment of cell physiology in different islets throughout a slice, allowing the investigation of islet heterogeneity within a given tissue region. Importantly, the preserved 3D organ environment of pancreas tissue slices allows in-depth histomorphometric analyses of the same samples used for functional assessments. Indeed, we recently combined morphometric and functional assays on the same and adjacent human pancreas tissue slices to dissect the contribution of β-cell mass and function in the pathogenesis of type 1 diabetes (32) and type 2 diabetes (31). These analyses facilitate the assessment and correlation of islet function with a multitude of parameters within the same tissue, including morphology, function, and cell physiology (Fig. 3).

In addition to the study of islet cell physiology within an intact tissue environment, pancreas tissue slices allow the investigation of nonendocrine pancreatic cells and tissues that modulate islet cell biology and that might play a role in diabetes pathogenesis. These different cell types, which are located within and/or outside of the islet, are preserved in tissue slices but are often lost or compromised during the isolation and resting culture period of islets (Fig. 4) (24). Among these, intraislet vascular structures, immune cells, and neurons have been shown to be conserved in pancreas tissue slices, allowing for the acquisition of novel insight into their role in islet biology (36). Functional imaging of intraislet vessels in living tissue slices revealed that glucose stimulation in β-cells leads to islet capillary dilation via adenosine-induced inhibition of pericytes that cover the islet microvasculature in the mouse pancreas (61) and that pericytes are major targets of sympathetic nerves in controlling blood vessels in the human endocrine pancreas (62). In another study of pancreas tissue slices, islet macrophages were found to monitor local concentrations of ATP released from β-cells as a measure to sense β-cell activity and thereby presumably promote a stable islet microenvironment (63). Interestingly, the pericyte coverage of human islet capillaries (61) and the expression of purinergic receptors and matrix metallopeptidase 9 in macrophages sorted from type 2 diabetes donor islets (63) were both found to be downregulated, indicating a potential role of these cells in type 2 diabetes pathogenesis. Furthermore, vagal sensory axons that innervate pancreatic islets were recently noted to express the serotonin 5-hydroxytryptamine receptor 3, thereby allowing neuronal sensing of β-cell activity by serotonin, which is co-released with insulin upon β-cell stimulation (64).

Outside of the islet, pancreas tissue slices allow for the study of the surrounding exocrine pancreas compartment and its ductal system (38,65,66). This aspect of pancreas physiology and pathophysiology has recently attained increasing interest, as interaction of the endocrine and exocrine pancreas has been implicated in the development of diabetes (67). On the one hand, exocrine acini display heterogeneous properties regarding their distance to islets; additionally, pancreas weight, which is almost entirely determined by the exocrine tissue, is already decreased in pretype 1 diabetes (68). Concomitantly, exocrine pancreas dysfunction and insufficiency are often diagnosed in patients with diabetes (69–72). On the other hand, islet function is believed to be compromised in some disorders of the exocrine pancreas (73,74). Pancreas tissue slices now allow for the investigation of both components of the pancreas simultaneously for the first time, facilitating investigations on the cross talk of the endocrine and exocrine part. Calcium imaging in intact acinar cells has revealed a well synchronized calcium activity within acini, but not between acini in mouse tissue slices (75). Simultaneous calcium imaging on both β-cells and acinar cells discovered a dual mode of action of acetylcholine, where a physiological concentration increases glucose-stimulated calcium oscillations frequency in both cell types and synchronizes acinar cells, whereas a supraphysiological concentration inhibits the synchronization between β-cells and abolishes oscillatory activity in acinar cells (76). In addition, the exocrine tissue and ductal cells have been suggested to serve as a source for regenerated islet cells via neogenesis (77). To address this hypothesis for the first time in situ, Qadir et al. (37) developed a novel long-term culture system for human and mouse pancreas tissue slices, allowing for the longitudinal study of potential neogenic processes. With this approach, they were able to show duct–to–β-cell neogenesis in mouse tissue, as well as bone morphogenetic protein-7–induced β-cell regeneration in human tissue slices (37).

The immediate surroundings of islets within the pancreas are also of particular interest in the pathogenesis of type 1 diabetes. Immune cells implicated in autoimmune processes are often located in the periphery or outside of islets and, therefore, are mostly lost during isolation (24). This hampers the study of islet immune cell interactions in disease pathogenesis. Here, pancreas tissue slices allow the study of immune cells within the pancreatic tissue and their effect on islet biology independent of their location (78). Furthermore, in advanced stages of the autoimmune attack, the tissue-preserving nature of pancreas tissue slices additionally holds an advantage to allow studies of islets that are structurally compromised and, as such, would be destroyed or unused during islet isolation. Using tissue slices prepared from the Network for Pancreatic Organ donors with Diabetes (nPOD) donor organs with and without diabetes, we were able to perform the first live-cell studies of the human pancreas in type 1 diabetes pathogenesis (32). β-Cell function, including insulin secretion and single-cell calcium live imaging of the residual β-cell mass, could be evaluated in tissue from subjects with type 1 diabetes and compared with control subjects without diabetes. This study demonstrated that tissue slices represent a novel approach for investigating structurally compromised islets in this disease as well as the potential interactions of immune cells with these islets.

Thus, pancreas tissue slices allow for investigation of human islet cell physiology and pathophysiology at different levels within the organ tissue. The platform distinguishes itself from other preparations, such as isolated islets and dispersed cells, by presenting less perturbed cells in a more intact tissue environment and, together with an almost immediate use of the viable tissue, thereby provides a closer physiological reflection of the in vivo setting. Conversely, the complexity of the tissue slice preparation impedes sample manipulation and results in a reduced experimental throughput compared with isolated islets or single cells (50,78). These biological and technical features make human pancreas tissue slices a valuable complementary platform, linking cellular and systemic studies.

Another benefit of the tissue slice preparation is the possibility to study human islet physiology even in small pancreas tissue samples, from which isolation of islets is possible, but rather tedious and of limited success (79,80). This further broadens the use of different sources of human pancreas tissue for the study of islet biology. In recent years, several efforts have been undertaken to provide researchers with an increased number of specimens of viable human pancreas for the study of organ physiology and disease pathogenesis. Thereby, pancreas tissue for research was obtained by three different approaches: 1) the procurement of organs from brain-dead or donation after cardiac death organ donors, 2) the retrieval of complete organs or pieces of tissue from patients undergoing pancreatectomy, and 3) for one study, the acquisition of tissue from pancreas biopsies of volunteers. Each of these sources provides distinct advantages for the investigation of human islet physiology and pathophysiology. However, different aspects of each tissue source also have to be considered when interpreting the obtained results (Table 1).

Pancreata procured from cadavers enable detailed investigations of the morphometry and histology of entire organs. However, postmortem changes within the organ, uncontrolled procurement conditions, and prolonged cold ischemia times might promote autolysis, and proteins or RNA are prone to destruction (17), thus limiting functional analyses of the organs. In order to overcome these limitations, several new sources of pancreas tissue became available for research purposes, such as the PanFin Network in Europe (17) or nPOD, the Human Islet Research Network (HIRN)–supported Human Pancreas Analysis Program (HPAP), and Human Atlas of Neonatal Development and Early Life-Pancreas/Immune (HANDEL-P/I) in the U.S. (81), as well as the Alberta IsletCore in Canada (20,21). In these programs of controlled and well-organized organ procurement, pancreata retrieved from organ donors represent the most comprehensive source to study diabetes in humans, providing a considerable amount of living tissue to allow for both the isolation of islets and the preparation of pancreas tissue slices. In addition, these organ procurement programs often present access to additional tissues, such as spleen, lymph nodes, blood, and more. However, potential deleterious effects of brain death seen in the liver and kidneys (82) are also debated for the pancreas. For instance, macrophage infiltration and the elevated release of inflammatory cytokines such as interleukin (IL)-1β, IL-6, tumor necrosis factor-α, and MCP-1 were recently shown to be a consequence of brain death in rats (83), but could only be confirmed for tumor necrosis factor-α and IL-6 serum levels in humans, whereas immune cell infiltration and expression of other cytokines showed no significant changes (84,85). Nevertheless, quantification of inflammatory responses and immune cell infiltration might differ in the context of type 1 diabetes and needs additional investigations. In addition, β-cells dissected from deceased donor pancreases were shown to exhibit significant upregulation of genes related to endoplasmic reticulum stress and the unfolded protein response (86).

However, effects of brain death on the human pancreas itself are hard to assess, as length and intensity of medical care prior to death are usually highly variable. Several brain death management protocols require administration of dextrose-containing fluids, insulin, or steroids that can lead to insulin resistance and contribute to terminal hyperglycemia, potentially affecting pancreatic endocrine cell function (84). Additionally, it is still debated whether prolonged hospital stay in the intensive care unit and time of brain death before retrieval increases β-cell replication in young organ donors (84,87).

Finally, organ preservation is an important parameter that might affect tissue integrity or cellular metabolism. Studies showed that warm ischemia times during organ procurement leads to a decreased islet yield at isolation and, more importantly, affects islet function compared with pancreata procured in cooled conditions (88,89). Although endocrine and exocrine tissue morphology have both been shown to be preserved in tissue procured from organ donors, these organs are typically flushed with preservative solutions and have prolonged cold ischemia times. Most of the time, this factor is unavoidable as tissues must be shipped to research laboratories. Increased cold ischemia time is well known to affect islet isolation outcomes (88,90). However, these effects can be counteracted by optimization of transport media depending on research purposes.

An alternative tissue source that can overcome some of the limitations from brain-dead organ donors originates from surgical resections. These become clinically necessary based on a localized benign or malignant neoplasm, some nonpancreatic tumors, or an exocrine disease such as chronic pancreatitis (91). As tissue usually becomes accessible shortly after resection, adverse effects imparted by hospital care and brain/cardiac death management protocols do not apply. Furthermore, warm ischemia times are usually reduced to the time period of organ clamping during surgery as tissue is cooled immediately after resection to be transferred for pathological examination. Subsequently and dependent on the proximity of the laboratory to the clinic, cold ischemia times are considerably shorter compared with donated organs. Additionally, clinically assessed parameters or even comprehensive metabolic stratification of patients, as shown by Solimena et al. (18,92), provide an unprecedented repertoire of in vivo parameters for correlation with experimentally assessed factors after resection (e.g., the in situ assessment of islet morphology and function) (93,94).

Despite these advantages, tissues obtained from surgical resections also have potential drawbacks that must be taken into account. First, with the exception of total pancreatectomies, the amount of nontumorous tissue available for research is often low, and the observations made are limited to the resected region of the pancreas. Second, although its preserved tissue integrity and microenvironment could be advantageous and valuable for exocrine-endocrine interaction studies, whether the underlying diseases of the patients may affect physiological responses within the endocrine pancreas is still being debated. For example, in cases of cystic fibrosis and chronic pancreatitis, patients showed decreased insulin secretion as well as glucose intolerance, suggesting impaired islet function due to the compromised structure of the pancreatic tissue (95). In pancreatic cancer, bidirectional effects of tumor and islets have been suggested as a possible cause of diabetes (96), but the causal relationships are still being discussed (97). Enhanced cancer growth in pancreatic ductal adenocarcinomas was shown to be associated with type 2 diabetes-related hyperglycemia in 47% of the patients (98). In addition, pancreatic ductal adenocarcinomas were shown to promote islet cell death as well as islet distortion in close proximity to the tumor (99), whereas coculture studies of cancer cells and isolated islets suggested that tumor cells cause reduced insulin secretion (100–102). However, cancer-related diabetes was suggested to occur within 2–3 years of a cancer diagnosis (103), implying that earlier manifestation of diabetes developed independent of the cancer. Additionally, the tissue, which was procured from the healthy surgical margins of the resection based on pathological examination, showed no cancer-related alterations of islet cell gene expression profiles (18). In distinct cases, localization and size of the tumor can result in papillary compression, leading to functional liver impairment and insulin resistance, which eventually leads to diabetes onset that is reversible after tumor resection and provides an interesting model for the acute compensation of liver insulin resistance with no apparent direct insult of the tumor on the islets (91,104–106). The disease-related treatment regimens of the tissue donors are also important for the study of pancreas tissue from surgical resections. Although the effects of localized tumors are potentially avoided by studying distant nontumorous tissue, resections from patients receiving chemotherapy or anticancer medication prior to surgery may not be suitable for analyses of diabetes pathogenesis due to the systemic actions of drugs. Indeed, it has been shown that various chemotherapeutic agents, including doxorubicin or l-asparaginase, as well as treatment with glucocorticoids, can directly or indirectly induce hyperglycemia (107,108).

Laparoscopically procured pancreata from tail biopsies performed as part of the Diabetes Virus Detection (DiViD) program represent an exception to the above-described methods for pancreas procurement. This approach was used to obtain tissues from six individuals with recent-onset type 1 diabetes and provided valuable insights into various aspects of the pancreatic pathogenesis in type 1 diabetes, including islet function (39), islet amyloid polypeptide deposition, (109), islet density and size distribution (110), and characteristics of insulitic islet infiltration (111). However, an unexpectedly high complication rate (in three of the six participants), including postoperative bleeding, pancreatic drainage, pain, and fever, led to the cessation of the study and, for now, speaks against the use of this tissue source on a larger scale (112).

We and others have recently shown that human transplant-grade pancreata, as well as tissue from localized surgical resections, can readily be used in combination with the tissue slice platform to study various aspects of islet biology and its pathophysiology in the natural history of type 1 and type 2 diabetes (31,32,65,113). Further investigations on slices, islets, and/or single cells using the respective tissue sources will provide essential insights into currently unanswered questions of human pancreas biology and diabetes research. Notably, the distinct sources will enable the study and shed light onto different aspects of physiology and pathology. The use of organ donor tissue with its broad donor age spectrum will facilitate, in particular, the investigation of developmental processes, intraorgan heterogeneity, and, thanks to programs like nPOD, type 1 diabetes pathogenesis. Tissue from surgical resections related to intestinal disease might be more applicable for the study of type 2 diabetes pathogenesis and the detailed correlation of pancreas function with a larger set of clinical parameters. Importantly, although most likely neither the pancreas of organ donors nor tissue from medically induced surgical resections fully reflects the in vivo setting of the human pancreas, these invaluable resources have and without a doubt will continue to move translational research a huge step closer to finding new therapeutic options. As with single cells and isolated islets, combining this platform with faster and more versatile imaging methods will allow investigators to record various cellular processes simultaneously in the near future. Tissue slices will additionally enable these recordings within multiple islets with preserved spatial information that can deepen our understanding of interislet, endocrine-exocrine, or even endocrine-immune cell cross talk in a preserved native environment. Moreover, live-cell analysis within the tissue slices will not be limited to studies of endocrine diseases but will also allow studying the impact of exocrine diseases or tumors on the different compartments within the pancreas. Finally, integrating novel techniques, such as imaging mass cytometry or spatial transcriptomics after the functional assessment of tissue slices in combination with, for example, novel clinical β-cell mass imaging techniques could be a valuable addition in the future to further synergize in vivo and in vitro knowledge in pancreas pathophysiology.

As every novel and versatile platform that opens up new possibilities, pancreas tissue slices are also associated with limitations that need to be considered when interpreting the obtained data. Compared with isolated islets, which are usually produced from the entire organ, tissue slices are prepared from small tissue blocks from a particular region (or regions) of the pancreas. Therefore, although tissue slices are advantageous in studying regional heterogeneity within and between sliced region(s), isolated islets might be better to represent a good average of all isolatable islets of the entire pancreas and, therefore, potentially display a closer correlation to the systemic metabolic data. In addition, when using tissue slices acutely to study a close to in vivo setting, it has to be considered that the quality of the donor organ or tissue, including ischemia time and disease-related treatment regimens, might have a stronger impact on the generated data than those obtained from isolated islets that are acquired after a resting and adaptation culture period in vitro. Importantly, given the different cellular and technical complexity of the preparations, isolated islets can be produced in larger numbers, are more easily cultured and shipped for longer time periods, and allow for more experimental manipulation and throughput. Therefore, isolated islets can be made accessible to a larger number of investigators and approaches around the world, enabling more comprehensive investigations of islet intrinsic biology of the same organ. Finally, although there is no selection bias for islets with any specific physical properties in the tissue slice preparation, the maximal size of islets will be limited by the chosen slice thickness (usually 120–150 µm). Thus, despite the fact that pancreas tissue slices excel in studying diabetes pathogenesis in a sustained, more physiological local environment and beyond the endocrine compartment, isolated islets will continue to be the preparation of choice for numerous approaches to study human endocrine pancreas physiology and pathophysiology.

Human pancreas tissue slices represent a novel platform to study human islet biology in health and disease. Importantly, the unique characteristics of the technique make pancreas tissue slices a welcome complementary approach to existing preparations such as isolated islets and fixed/frozen tissue sections. Slices show limitations in throughput, applicable experimental manipulations, and single-cell “omics” but possess benefits in reduced inflicted cell stress, a conserved tissue environment, and the unique opportunity to study extraislet influences on islet physiology and pathophysiology. Fortunately, different sources of human pancreas tissue have been made available to an increasing number of researchers during recent years, which make it possible to combine pancreas tissue slices with other cell platforms. Therefore, the characteristics of the human tissue source must be carefully evaluated and its implications considered when interpreting any resulting data. By further integrating innovative techniques after live-cell analyses, thereby facilitating the correlation of cellular function with morphology and omics technologies, the pancreas tissue slices platform can present new perspectives and further synergize in vivo and in vitro pancreas research. In summary, studies using pancreas tissue slices, particularly in combination with isolated islets and fixed or frozen tissue sections from the same organ/tissue, will provide unprecedented insight into the human endocrine pancreas and its pathogenesis in diabetes.

Acknowledgments. The authors wholeheartedly thank the tissue and organ donors and their families for their invaluable contribution to science. Furthermore, we thank Amanda Posgai and Sara Williams, University of Florida, for editorial assistance in the preparation of the manuscript.

Funding. The authors’ work was supported in part by the DFG Collaborative Research Centre/Transregio 127, the DFG–International Research Training Group 2251, the National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases grants R01DK123292 and R01DK131059, JRDF grant 2-SRA-2019-696-S-B, the Paul Langerhans Institute Dresden (PLID) of the Helmholtz Zentrum München at the University Clinic Carl Gustav Carus of Technische Universität Dresden, and the German Ministry for Education and Research to the German Centre for Diabetes Research. This research was also performed with the support of the Network for Pancreatic Organ donors with Diabetes (nPOD, RRID:SCR_014641), a collaborative type 1 diabetes research project sponsored by JDRF (nPOD, 5-SRA-2018-557-Q-R) and The Leona M. & Harry B. Helmsley Charitable Trust (grant 2018#PG-T1D053, G-2108-04793).

The content and views expressed are the responsibility of the authors and do not necessarily reflect the official view of nPOD. Organ procurement organizations partnering with nPOD to provide research resources are listed at https://www.jdrfnpod.org/for-partners/npod-partners/.

Duality of Interest. No potential conflicts of interest relevant to the article were reported.

Author Contributions. C.M.C., C.C., M.A.A., D.M.D., and S.S. contributed to the preparation and writing of the manuscript. S.S. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of data and the accuracy of data analysis.