Imaging of the islet neural network

Abstract

The islets of Langerhans receive signals from the circulation and nerves to modulate hormone secretion in response to physiological cues. Although the rich islet innervation has been documented in the literature dating as far back as Paul Langerhans' discovery of islets in the pancreas, it remains a challenging task for researchers to acquire detailed islet innervation patterns in health and disease due to the dispersed nature of the islet neurovascular network. In this article, we discuss the recent development of 3-dimensional (3D) islet neurohistology, in which transparent pancreatic specimens were prepared by optical clearing to visualize the islet microstructure, vasculature and innervation with deep-tissue microscopy. Mouse islets were used as an example to illustrate how to apply this 3D imaging approach to characterize (i) the islet parasympathetic innervation, (ii) the islet sympathetic innervation and its reinnervation after transplantation under the kidney capsule and (iii) the reactive cellular response of the Schwann cell network in islet injury. While presenting and characterizing the innervation patterns, we also discuss how to apply the signals derived from transmitted light microscopy, vessel painting and immunostaining of neural markers to verify the location and source of tissue information. In summary, the systematic development of tissue labelling, clearing and imaging methods to reveal the islet neuroanatomy offers insights to help study the neural-islet regulatory mechanisms and the role of neural tissue remodelling in the development of diabetes. Keywords: 3D microscopy, graft innervation, islet graft, neurohistology, optical clearing, parasympathetic nerves, Schwann cell network, sympathetic nerves Date submitted 27 March 2014; date of final acceptance 28 May 2014 IntroductionDocumentation of the rich pancreatic islet innervation can be traced back to Paul Langerhans' thesis 'Contributions to the microscopic anatomy of the pancreas', in which he used light microscopy to reveal the different staining properties of the pancreatic tissues [1]. Numerous subsequent studies conducted using light and electron microscopy of islets of various species confirm the abundant neural tissues at the peri-islet region and their extensions into the islet core (review in Refs [2][3][4]). Besides these morphological observations, functional studies of the islet hormone secretion affected by the administration of neurotransmitters and neuropeptides have been conducted in animals and humans [2][3][4][5][6][7][8][9]. The results indicate that the neural signal pathways play an important role in the modulation of islet functions.Scattered in the pancreas, islets rely on circulatory and nervous signals to regulate islet hormone secretion in response to physiological cues. The neural pathways thus represent a natural route (or routes) to target islets for potential pharmacological intervention to influence islet functions for diabetes treatment, apart from the route from circulation. However, despite the consider...

Neuro-Insular Complexes in the Human Pancreas

“…However, according to their ultrastructural characteristics and integration with nerve fibers, these small S100-positive cells with thin processes that we detected in the human pancreas correspond to the glial (Schwann) cells observed at the periphery of islets in other mammals [3,13,14,34]. It should be noted that, in humans, S100-positive glial cells are present only in some islets in small numbers and their processes do not cover endocrine cells, as has been described in other mammals [3,13,14,32,34].…”

“…Similarly, various NIC I representing all transitional forms between pure islets with a single neuron and pure ganglia containing only a few endocrine cells have been detected in the pancreas of the cat [28]. Based on these findings, Böck [28] In addition to neurons and nerve fibers, glial cells immunopositive for S100 protein and glial fibrillary acidic protein (GFAP) have been detected at the periphery of islets in many mammals [3,13,14,32,34]. These cells have a triangular or spindle-like shape and possess long, thin leaf-like processes which cover endocrine cells, separating them from the connective tissue and from the acini.…”

“…As in histological studies, various morphological forms of the NIC were detected using electron microscopy. In many mammals, pancreatic islets are richly innervated by thin nerve fibers, which are located at the periphery of islets, forming periinsular nerve plexuses; they occasionally pass through islets separately or along capillaries [14,16,27,29,32,33]. The density of the peri-insular neural network varies between species [27].…”

Diet‐induced obesity in young mice: Consequences on the pancreatic intrinsic nervous system control of insulin secretion

“…Obesity induces endocrine pancreatic dysfunction, such as impaired pulsatility and loss of first phase insulin secretion, both under nervous control involving the pancreatic intrinsic nervous system (PINS) . PINS is made of perilobular ganglia and ganglia scattered within the parenchyma (intraparenchymal ganglia) sometimes in close proximity to the islets, whose terminal axons supply the endocrine and exocrine parts of the pancreas, and new techniques of 3D panoramic histology with tissue clearing enabled to nicely visualize the neuro‐insular network in mice and humans . Although previous human islet imaging has suggested limited neural‐islet associations, particularly a lack of parasympathetic nerves in the islets, these improved high‐definition images revealed that the sympathetic and parasympathetic nerves enter the islet core and are located in the immediate microenvironment of islet cells in humans as well as in mice .…”

Brain glucose sensing in homeostatic and hedonic regulation

“…In rodent islets there are clear vagal connections to the b cells, whereas vagal terminals are rare in human islets[53,54]. Tanycytes are glucose-sensing cells.…”