**2. Structure and function of the pancreas**

The pancreas is an unpaired gland of the alimentary tract with two different but complementary functions: the production of digestive enzymes and alkaline fluid in exocrine cells that help with the breakdown of energy-rich nutrients and the synthesis by endocrine cells of hormones needed to control the storage and usage of energy-rich nutrients [16]. In humans, the pancreas is a well-defined organ with three major parts: the head, the body, and the tail, extending from the duodenum to the spleen [17, 18]. In mice, the pancreas is not as well-defined as in humans but is rather diffusely distributed in a dendritic manner while it is still composed of three major parts: the duodenal lobe, the largest splenic lobe, and the smallest gastric lobe [19]. A fibrous capsule surrounds the pancreas, and the connective tissue extending into the gland divides the parenchyma into larger lobes and smaller lobules [18, 20]. Each lobule is composed of acini that consists of pyramid-shaped acinar cells [21]. The exocrine part accounts for

#### *Application of Transmission Electron Microscopy to Detect Changes in Pancreas Physiology DOI: http://dx.doi.org/10.5772/intechopen.104807*

96–99% and the endocrine islets of Langerhans for the remaining 1–4% of total pancreas parenchyma [22, 23]. In mice, the islets are mainly interlobular, while in humans they are usually intralobular, mostly on the edge of lobules [24, 25]. Despite some differences [26], islets of Langerhans from mice and men possess many structural and functional similarities [27, 28]. They are round to oval, vary in size, and range from a few to several thousand endocrine cells. Importantly, the islet size distributions are similar in humans and mice (and also many other species) [22]. At least five different types of endocrine cells can be found in islets in both species [27]. Most numerous are the beta cells that synthesize and secrete insulin. Alpha cells secrete glucagon, while delta cells and PP cells release somatostatin and pancreatic polypeptide, respectively. Finally, epsilon cells are the least abundant, synthesizing and secreting ghrelin [23–25].

As mentioned above, the exocrine pancreas plays a crucial role in the enzymatic digestion of carbohydrates, proteins, and lipids and secretes a bicarbonate-rich fluid [29–31]. The enzymes are stored in an inactive proenzyme form in the so-called zymogene granules located at the apical membrane of acinar cells and are released in response to stimulation by neurohormonal secretagogues, such as acetylcholine and cholecystokinin, via exocytosis. The fusion of granules with the apical plasma membrane releases the enzymes first into the acinar lumen, from where they pass via the ductal tree into the small intestine [32]. In contrast, islets release their hormones into the numerous islet blood capillaries that drain into the portal venous system. In the liver and other key target tissues, such as the skeletal muscle and adipose tissue, insulin acts as a key anabolic hormone, promoting glycogenesis, glycolysis, lipogenesis, and proteinogenesis, and suppressing gluconeogenesis, lipolysis, and proteolysis [33–35]. On the other hand, glucagon acts mainly in the liver, where it promotes glycogenolysis and gluconeogenesis, and inhibits glycolysis and glycogenesis [36, 37].
