9. The pancreas is mixed exocrine- endocrine gland that produces digestive enzymes and hormones. The enzymes are stored and released by cells of the exocrine portion, arranged in acini. The hormones are synthesized in clusters of endocrine epithelial cells known as islets of Langerhans.

11. The exocrine portion : It is a compound acinar gland, similar in structure to the parotid gland. In histological sections, a distinction between the two glands can be made based on the absence of striated ducts and the presence of the islets of Langerhans in the pancreas. Another characteristic detail is that in the pancreas the initial portions of intercalated ducts penetrate the lumen of the acini.

12. Nuclei, surrounded by a pale cytoplasm, belong to centroacinar cells that constitute the intraacinar portion of the intercalated duct. These cells are found only in pancreatic acini. Intercalated ducts are tributaries of larger intralobular ducts that,in turn, form larger interlobular ducts lined by columnar epithelium, located within the connective tissue septa. There are no striated ducts in the pancreatic duct system.

14. The exocrine pancreatic acinus is composed of several serous cells surrounding a lumen.These cells are highly polarized,with a spherical nucleus, and are typical protein –secreting cells. A thin capsule of connective tissue covers the pancreas and sends septa into it, separating the pancreatic lobules. The acini are surrounded by a delicate sheath of reticular fibers.The pancreas also has a rich capillary network, essential for the secretory process.

15. The number of zymogen granules (S) present in each cell varies according to the digestive phase and attains its maximum in animals that have fasted.

17. The exocrine pancreas secretes 1500- 3000 mL of isosmotic alkaline fluid per day containing water, ions,and several proteases which are: 1.Trypsinogen 1,2,3. 2.Chymotrypsinogen. 3.Prolastases 1,2. 4. Protease E. 5.Kallikreinogen. 6.Procarboxypeptidases A1,A2,B1,B2.

18. In addition, Amylase, Lipase(trigleceride lipases, colipase, carboxyle ester hydrolase), Phospholipase A2,and nucleases (deoxyribonuclease and ribonucleases). The majority of the enzymes are stored as proenzymes in the secretory granules of acinar cells, being activated in the lumen of small intestine after secretion.

19. Enterokinase, an intestinal enzyme, cleaves trypsinogen to form trypsin, which then activate the other proteolytic enzymes in a cascade. This is very important for the protection of the pancreas as well as the synthesis of protease inhibitors by the acinar cells. In condition called acute necrotizing pancreatitis, the proenzymes may be activated and digest the whole pancreas, leading to very serious complications. The causes are trauma, infections, gallstones etc.

20. Pancreatic secretion is controlled mainly through two hormones – secretin and cholecystokinin – that are produced by enteroendocrine cells of the intestinal mucosa (duodenum, jejunum). Stimulation of the vagus nerve (parasympathetic stimulation) will also produce Pancreatic secretion.

21. Gastric acid(or pH< 4.5) in the intestinal lumen is a strong stimulus for secretin release. Secretin causes acinar ad duct cells to add water and bicarbonate to the fluid, promoting the secretion of an abundant alkaline fluid rich in electrolytes and poor in enzymatic activity. This fluid neutralizes the acidic chyme (partially digested food ) so that pancreatic enzymes can function at their optimal neutral pH range. The release of cholecystokinin is triggered by the presence of long-chain fatty acids, gastric acids, and certain essential aminoacids in the intestinal lumen. Cholecystokinin promotes secretion of a less abundant but enzyme rich fluid acting mainly in the extrusion of zymogen granule. The integrated action of both these hormones provides for a heavy secretion of enzyme-rich pancreatic juice.

22. In conditions of extreme malnutrition such as kwashiorkor, pancreatic acinar cells and other active protein – secreting cells atrophy and lose much of their rough endoplasmic reticulum. The production of digestive enzymes is also hindered.

23. ISELETS OF LAGERHANS The islets of langerhans are multihormonal endocrine microorgans; they appear as rounded clusters of cells embedded within the exocrine pancreatic tissue. Although most islets are 100-200 mm in diameter and contain several hundred cells,small groups of endocrine cells are also found interspersed among the pancreatic exocrine cells. There may be more than 1 million islets in the human pancreas, with a slight tendency for islets to be more abundant in the tail of the pancreas. A fine capsule of reticular fibers surrounds each islets, separating it from the adjacent pancreatic tissue. Each islets consists of lightly stained polygonal or rounded cells, arranged in cords separated by a network of blood capillaries.

24. Routine stains allow the recognition of acidophils () and basophils ()

25. Using immunocytochemical methods, four types of cells – A,B,D, and F-have been recognized in the iselets. The ultrastructure of these cells resembles that of cells synthesizing polypeptides. The secretory granules of cells of the iselets vary according to the species studied.In human, the A cells have regular granules with a dense core surrounded by a clear region bounded by a membrane.The B (Insuline –producing )cells have irregular granules with a core formed of irregular crystals of insulin in complex with zinc.

27. The relative quantities of the four cell types found in islets are not uniform ; they vary considerably with the iselet's location in the pancreas. A: 20% Usually in periphery Glucagon Acts on several tissues to make energy stored in glycogen and fat ; increases blood glucose content. B: 70% Central region InsulinActs on several tissues to cause entry of glucose into cells and decreases blood glucose content D F

40. EM detail of cytoplasm of pancreatic acinar cell loaded with lamellae of rough endoplasmic reticulum, typical of a protein-secreting cell. The cell nucleus is at upper left.