5.2.1 General description of the pancreas and its anatomy
The pancreas is a fish-shaped organ, about the size of a medium herring, lying under the liver (Fig. 5.1). It has a distinct head and a narrow tail, and the head end is wrapped around the small intestine. The pancreas is a complex organ since it contains both exocrine and endocrine tissues. The exocrine function of the pancreas consists of the liberation of digestive juices into the small intestine: the endocrine function consists of the production and secretion of hormones into the bloodstream, most importantly insulin and glucagon.
The vast majority of cells in the pancreas are exocrine. These cells produce an alkaline digestive juice containing a number of enzymes, particularly amylase, pancreatic lipase and the proteases, trypsin and chymotrypsin. This juice is collected into small ducts which merge to form one main pancreatic duct. This is joined by the common bile duct just before it enters the duodenum; thus, bile salts and pancreatic enzymes are liberated together into the small intestine. The digestive function of the pancreas, and its regulation, were discussed in Chapter 3 (see Table 3.2).
Scattered amongst the exocrine tissue are little groups of cells, appearing like islands under the microscope. They were first described by a German medical student, Paul Langerhans, in 1869 and are known as the Islets of Langerhans (Fig. 5.2). These are the endocrine cells. There are around a million islets in the adult pancreas, although they constitute only 1-2% of the total mass of the pancreas. Within the islets there are three types of endocrine cell: the a-cells or A cells, which secrete glucagon, the cells or B cells which secrete insulin and the 8-cells or D cells which secrete somatostatin. The 0-cells occupy about 60% of the volume of the islet. Somatostatin in the pancreas probably has a local regulatory role, affecting the secretion of insulin and glucagon, but this is not entirely clear, and it will not be considered further here. Each islet is supplied with blood by a branch of the pancreatic artery, and venous blood leaves the islet in tiny veins (venules) which merge to form the pancreatic and pancreatico-duodenal veins. As discussed in Section 4.1.1, they discharge their
contents into the hepatic portal vein, so the liver is in a unique position as regards its exposure to the pancreatic hormones.
Insulin is a peptide hormone. It consists of two peptide chains, the A and B chains, linked to each other by disulphide bonds: the A chain contains 21 amino acids, and the B chain 30 amino acids. It is synthesised within the 0-cells as a single polypeptide chain (proinsulin), and the connecting peptide or C-peptide is removed by proteolytic action before secretion (Fig. 5.3).
Clearly, for insulin to have a useful signalling function, its rate of secretion into the plasma must vary according to the metabolic or nutritional state. The most important regulator of the rate of insulin secretion is the concentration of glucose in the plasma. The (-cell is similar to the hepatocyte in that it expresses the GLUT2 transporter and the hexokinase IV isoform (glucokinase). As in the liver (Section 18.104.22.168), these give the (-cell the ability to act as a 'glucose sensor'. As the external (plasma) glucose concentration rises, so glucose flows into the cell and is phosphorylated, and then enters the glycolytic pathway. This leads to generation of ATP, which regulates events at the cell membrane (Fig. 5.4).
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