Structure and function
The pancreas extends retroperitoneally across the posterior abdominal wall from the second part of the duodenum to the spleen. The head is encircled by the duodenum; the body, which forms the main bulk of the organ, ends in a tail that lies in contact with the spleen. The main pancreatic duct usually joins the common bile duct to enter the duodenum as a single duct at the ampulla of Vater. The main pancreatic duct has many tributary ductules and gradually tapers towards the tail of the pancreas. Pancreas divisum is an anatomical variant in which a small proportion of the pancreas drains through an accessory duct into the duodenum.
Exocrine cells form 98% of the human pancreas. The pancreatic acinar cells form a ductal system that eventually joins into the main pancreatic duct.
Pancreatic acini synthesize digestive enzymes which are stored in secretory glands and released by exocytosis in response to stimulation by several hormones.
These receptors have been divided into two categories:
vasoactive intestinal polypeptide (VIP) and secretin that act via cyclic AMP, and another group that stimulate cellular metabolism of membrane phosphoinositides and calcium.
The main regulators of pancreatic exocrine secretion are the hormones secretin and cholecystokinin (CCK). Secretin is released when acid enters the duodenum; it stimulates pancreatic juice containing water and electrolytes, chiefly bicarbonate. CCK is released, via cholinergic pathways, when fatty acids and amino acids enter the duodenum, stimulating pancreatic enzyme secretion.
Enzymes produced are amylase, lipase, colipase, phospholipase and pro teases (trypsinogen and chymotrypsinogen).
The pro teases are secreted in the inactive form but are then activated in the duodenum by enterokinase.
The endocrine pancreas consists of hormone-producing cells arranged in nests or islets- the islets of Langerhans. They do not connect directly to the duct system. There are four main types of islet cell and these have different secretory granules in their cytoplasm:
1 t3-Cells, which are the commonest cells, produce insulin.
2 a-Cells produce glucagon.
3 D cells produce somatostatin.
4 PP cells produce pancreatic polypeptide (pp).
5 A number of other hormones, e.g. bombesin, neuropeptide Y and galanin, are present in pancreatic neurones and probably act as neurotransmitters.