Insulin is made and secreted by beta cells found in the islets of Langerhans in the pancreas. The pancreas, nestled between the stomach, small intestine and spleen in the abdomen, secretes digestive enzymes through the pancreatic duct into the small intestine. Most of the pancreatic tissue is devoted to the production of these digestive enzymes. Islets of Langerhans are one to three per cent of the pancreas, secreting hormones that help control metabolism, and are not involved in making digestive enzymes.

In the diagram you can see three islets. Beta cells tend to be near the core of the islets, and alpha cells (pink) near the periphery.

A human has about 2 grams of islet tissue at birth, which increases to about 66 grams over age 21. The number of islets ranges from 200,000 to 1,800,000. The pancreatic cells that secrete digestive enzymes are called acinar cells, and are generically called exocrine cells, meaning they secrete their product into a duct (which in this case leads to the intestine). The pancreatic cells that secrete metabolic hormones are called islet cells, and are generically called endocrine cells, meaning they secrete their hormone product into the bloodstream. About 80 per cent of the islet cells are beta cells, endocrine cells that secrete insulin. The other 20 per cent of cells secrete other hormones. Diabetes is caused by destruction or dysfunction of the beta cells of the islets of Langerhans.

Secreting Cell Types of the Pancreas

Cell Primary Action Diabetes
Exocrine cells
acinar cells secrete digestive enzymes no change
Endocrine cells
alpha cells secrete glucagon no change
beta cells secrete insulin IDDM: destroyed
NIDDM: present, but unresponsive
D cells secrete somatostatin no change
PP cells secrete pancreatic polypeptide no change