In 1869, a German medical student, Paul Langerhans, described clumps of clear cells intermingled among the other cells of the pancreas. Langerhans admitted that he did not know the function of these special cells.
These clumps of cells, now known as the islets of Langerhans, are only a small part (about 1%) of the mass of the pancreas, and are distributed throughout the pancreas. Although there are about a million islets in a healthy adult human pancreas, their combined mass is only 1 to 1.5 grams.
It was not until 1893 when it was first suggested that these cells might produce an internal secretion, and when they were named in Dr. Langerhans' honor as "les ilots de Langerhans" (the little islands of Langerhans). It was the naming of these clumps of cells as "islets" which led, years later, to the primary hormone secreted by these cells being called insulin. Sometimes, the "islets" are referred to as "islands;" whether referred to as islets or islands, they are the same thing: clumps of hormone-secreting cells in the pancreas.
After identification that the islets have a function to produce hormones, which is an entirely different role than the rest of the pancreas has (which is to produce digestive enzymes to help digest foods), the two portions of the pancreas were given names to help clarify which portion of the pancreas was under discussion. The portion of the pancreas that produces digestive juices is termed the exocrine pancreas, and the islet portion, that produces hormones, is called the endocrine pancreas.
Several hormones are made in the islets of Langerhans
The islets are comprised of several different types of cells, each secreting a specific peptide hormone. Best known are the beta cells, which make insulin and another hormone, amylin. Other cell types in the islets include the alpha cells, which make glucagon; the delta cells, which make somatostatin; epsilon cells, which make ghrelin; and PP cells, which make pancreatic polypeptide.
Glucagon, made in the alpha cells, is released when blood glucose levels fall too low, causing the liver to convert stored glycogen into glucose and release it into the bloodstream, thereby raising blood glucose levels.
Insulin, made in the beta cells, has as its major role the storage of glucose in tissues as glycogen, resulting in lowering of blood glucose levels.
Amylin, also made in the beta cells, slows gastric emptying and has other effects. It works together with insulin to lower blood glucose levels.
Somatostatin, made in the delta cells, inhibits the release of growth hormone and gastrointestinal hormones, lowers the rate of gastric emptying, and suppresses insulin release.
Ghrelin, made in the epsilon cells, is considered part of the hunger hormone systems.
Pancreatic polypeptide, made in the PP cells, regulates the release of pancreatic secretions.
Transplanting the islets
For many years, it has been thought that transplanting the islets of Langerhans might control diabetes, especially type 1 diabetes. This was initially proven in rodent experiments, and later when whole pancreas or partial pancreas transplants were done in humans. But transplanting the entire (or partial) pancreas is major surgery, and involves problems such as where the exocrine pancreatic secretions (the digestive juices that have nothing to do with the islet-cell functions) would go. In 1999, a new tactic for providing immunosuppression to transplant recipients made islet transplantation a reality; the new combination of immunsuppressing medications is called the "Edmonton transplant protocol."
The NIH points out that a major obstacle to more widespread use of islet transplantation is the shortage of islets: although organs from about 7,000 deceased donors become available each year in the US, fewer than half of the donated pancreases are suitable for either whole organ pancreas transplantation or for the harvesting of islets. Research to find alternative sources of islets, or to grow islets in laboratories, is underway.
Also see:
Paul Langerhans (1847-1888): a centenary tribute
http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1291675&blobtype=pdf
from the Journal of the Royal Society of Medicine
Islets of Langerhans
http://en.wikipedia.org/wiki/Islets_of_Langerhans
at Wikipedia
Islet transplantation in patients with Type 1 diabetes mellitus
http://www.diabetesmonitor.com/isletsum.htm
at the Diabetes Monitor
Pancreatic islet transplantation
http://www.diabetesmonitor.com/b22.htm
at the Diabetes Monitor
