97 The Pancreas
Overview of Pancreatic Islets
Pancreatic islets, also called the islets of Langerhans, are regions of the pancreas that contain its hormone-producing endocrine cells.
Learning Objectives
Differentiate among the types of pancreatic islet cells
Key Takeaways
Key Points
- The pancreatic islets are small islands of cells that produce hormones that regulate blood glucose levels. Hormones produced in the pancreatic islets are secreted directly into the bloodstream by five different types of cells.
- The alpha cells produce glucagon, and make up 15–20% of total islet cells. The beta cells produce insulin and amylin, and make up 65–80% of the total islet cells. The delta cells produce somatostatin, and make up 3–10% of the total islet cells.
- The gamma cells produce pancreatic polypeptide, and make up 3–5% of the total islet cells. The epsilon cells produce ghrelin, and make up less than 1% of the total islet cells.
- The feedback system of the pancreatic islets is paracrine, and is based on the activation and inhibition of the islet cells by the endocrine hormones produced in the islets.
Key Terms
- endocrine: Produces internal secretions that are transported around the body by the bloodstream.
- paracrine: Describes a hormone or other secretion released from endocrine cells into the surrounding tissue rather than into the bloodstream.
- exocrine: Produces external secretions that are released through a duct.
The pancreas serves two functions, endocrine and exocrine. The exocrine function of the pancreas is involved in digestion, and these associated structures are known as the pancreatic acini.
The pancreatic acini are clusters of cells that produce digestive enzymes and secretions and make up the bulk of the pancreas. The endocrine function of the pancreas helps maintain blood glucose levels, and the structures involved are known as the pancreatic islets, or the islets of Langerhans.
Pancreatic islets or islets of Langerhans: The islets of Langerhans are the regions of the pancreas that contain its endocrine (hormone-producing) cells.
The pancreatic islets are small islands of cells that produce hormones that regulate blood glucose levels. Hormones produced in the pancreatic islets are secreted directly into the blood flow by five different types of cells.
Pancreatic tissue: The small cells in the middle are beta cells, and the surrounding larger cells are alpha, delta, gamma, and epsilon cells.
The endocrine cell subsets are:
- Alpha cells that produce glucagon and make up 15–20% of total islet cells. Glucagon is a hormone that raises blood glucose levels by stimulating the liver to convert its glycogen into glucose.
- Beta cells that produce insulin and amylin and make up 65–80% of the total islet cells. Insulin lowers blood glucose levels by stimulating cells to take up glucose out of the blood stream. Amylin slows gastric emptying, preventing spikes in blood glucose levels.
- Delta cells that produce somatostatin and make up 3–10% of the total islet cells. Somatostatin is a hormone that suppresses the release of the other hormones made in the pancreas.
- Gamma cells that produce pancreatic polypeptide and make up 3–5% of the total islet cells. Pancreatic polypeptide regulates both the endocrine and exocrine pancreatic secretions.
- Epsilon cells that produce ghrelin and make up less than 1% of the total islet cells. Ghrelin is a protein that stimulates hunger.
The feedback system of the pancreatic islets is paracrine—it is based on the activation and inhibition of the islet cells by the endocrine hormones produced in the islets. Insulin activates beta cells and inhibits alpha cells, while glucagon activates alpha cells, which activates beta cells and delta cells. Somatostatin inhibits the activity of alpha cells and beta cells.
Types of Cells in the Pancreas
The islets of Langerhans are the regions of the pancreas that contain many hormone-producing endocrine cells.
Learning Objectives
Distinguish between the cell types of the pancreas
Key Takeaways
Key Points
- The pancreas reveals two different types of parenchymal tissue: exocrine acini ducts and the endocrine islets of Langerhans.
- The hormones produced in the islets of Langerhans are insulin, glucagon, somatostatin, pancreatic polypeptide, and ghrelin.
- The pancreatic hormones are secreted by alpha, beta, delta, gamma, and epsilon cells.
Key Terms
- somatostatin: A polypeptide hormone, secreted by the pancreas, that inhibits the production of certain other hormones.
- insulin: A polypeptide hormone that regulates carbohydrate metabolism.
- glucagon: A hormone, produced by the pancreas, that opposes the action of insulin by stimulating the production of sugar.
Pancreatic Cells
The pancreas is a glandular organ that belongs to both the digestive and the endocrine systems of vertebrates. It is an endocrine gland that produces several important hormones, including insulin, glucagon, somatostatin, and pancreatic polypeptide.
It is also a digestive, exocrine organ, that secretes pancreatic juice that contains digestive enzymes to assist with digestion and the absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.
Under a microscope, stained sections of the pancreas reveal two different types of parenchymal tissue. The light-stained clusters of cells are called islets of Langerhans, which produce hormones that underlie the endocrine functions of the pancreas.
The dark-stained cells form acini, connected to ducts. Acinar cells belong to the exocrine pancreas and secrete digestive enzymes into the gut via a system of ducts.
Islets of Langerhans
Islets of Langerhans: A porcine islet of Langerhans. On the left is a brightfield image created using hematoxylin stain; nuclei are dark circles and the acinar pancreatic tissue is darker than the islet tissue. The right image is the same section stained by immunofluorescence against insulin, indicating beta cells.
The pancreatic islets are small islands of cells that produce hormones that regulate blood glucose levels. Hormones produced in the pancreatic islets are secreted directly into the blood flow by five different types of cells. The endocrine cell subsets are:
- Alpha cells that produce glucagon, and make up 15–20% of total islet cells. Glucagon is a hormone that raises blood glucose levels by stimulating the liver to convert its glycogen into glucose.
- Beta cells that produce insulin and amylin, and make up 65–80% of the total islet cells. Insulin lowers blood glucose levels by stimulating cells to take up glucose out of the blood stream. Amylin slows gastric emptying, preventing spikes in blood glucose levels.
- Delta cells that produce somatostatin, and make up 3–10% of the total islet cells. Somatostatin is a hormone that suppresses the release of the other hormones made in the pancreas.
- Gamma cells that produce pancreatic polypeptide, and make up 3–5% of the total islet cells. Pancreatic polypeptide regulates both the endocrine and exocrine pancreatic secretions.
- Epsilon cells that produce ghrelin, and make up less than 1% of the total islet cells. Ghrelin is a protein that stimulates hunger.
The islets of Langerhans can influence each other through paracrine and autocrine communication. The paracrine feedback system is based on the following correlations:
- The insulin hormone activates beta cells and inhibits alpha cells.
- The hormone glucagon activates alpha cells which then activate beta cells and delta cells.
- Somatostatin hormone inhibits alpha cells and beta cells.
Insulin Secretion and Regulation of Glucagon
Glucagon is a peptide hormone that works in conjunction with insulin to maintain a stable blood glucose level.
Learning Objectives
Summarize the relationship between insulin secretion and glucagon regulation in blood glucose homeostasis
Key Takeaways
Key Points
- Glucagon and insulin are peptide hormones secreted by the pancreas that play a key role in maintaining a stable blood blood glucose level.
- Glucagon is produced by alpha cells in the pancreas and acts to raise blood sugar levels.
- Insulin is produced by beta cells in the pancreas and acts to lower blood sugar levels.
Key Terms
- insulin: A polypeptide hormone that regulates carbohydrate metabolism.
- glycogen: A polysaccharide that is the main form of carbohydrate storage in animals and also converts to glucose as needed.
- glucagon: A hormone, produced by the pancreas, that opposes the action of insulin by stimulating the production of sugar.
Glucagon and insulin are peptide hormones secreted by the pancreas that play a key role in maintaining a stable blood glucose level. The blood glucose level is carefully monitored by cells within the pancreas that respond by secreting key hormones.
Glucagon
Glucagon staining: This is an image from a microscope stained for glucagon.
Glucagon is produced by alpha cells in the pancreas and elevates the concentration of glucose in the blood by promoting gluconeogenesis and glycogenolysis. Glucose is stored in the liver in the form of the polysaccharide glycogen, which is a glucan.
Liver cells have glucagon receptors and when glucagon binds to the liver cells they convert glycogen into individual glucose molecules and release them into the bloodstream—this process is known as glycogenolysis. As these stores become depleted, glucagon then encourages the liver and kidney to synthesize additional glucose by gluconeogenesis. Glucagon also turns off glycolysis in the liver, causing glycolytic intermediates to be shuttled to gluconeogenesis that can induce lipolysis to produce glucose from fat.
Insulin
Insulin is produced by beta cells in the pancreas and acts to oppose the functions of glucagon. It’s main role is to promote the conversion of circulating glucose into glycogen via glycogenesis in the liver and muscle cells.
Insulin also inhibits
gluconeogenesis and promotes the storage of glucose in fat through lipid synthesis and also by inhibiting lipolysis.
In Disease
When control of insulin levels fails, diabetes mellitus can result. As a consequence, insulin is used medically to treat some forms of diabetes mellitus.
Patients with type 1 diabetes depend on external insulin (most commonly injected subcutaneously) for their survival because the hormone is no longer produced internally.
Patients with type 2 diabetes are often insulin resistant and, because of such resistance, they may suffer from a relative insulin deficiency. Some patients with type 2 diabetes may eventually require insulin if other medications fail to control blood glucose levels adequately.
