In addition to the digestive function of the pancreas, this organ also has certain specialized glandular cells that seem to be developed as specialized outgrowths of the gut endoderm and differed histologically from the rest of the pancreas tissue.
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These cells are endocrine in function and these were discovered by Langerhans, therefore, they have been known as islets of Langerhans.
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It was found that these islets were not connected to the duct system through which the digestive pancreatic juice flows; instead their secretions are freed directly into the blood circulation.
Staining studies showed that the islet cells are of two kinds, the a-cells and the α-cells. β-cells probably secrete insulin hormone and the a-cells seem to be the source of glucagon hormone.
These hormones play important roles in carbohydrate metabolism. The total islet tissue constitutes 1—3% of the whole pancreatic tissue.
The phylogeny of the islet cells was studied by Houssay, 1959; Barrington, 1962; Gorbman and Bern, 1962. These cells are found almost in all the vertebrates.
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In most higher vertebrates these are found in nests—the islets of Langerhans, in the pancreatic tissue and are concerned with carbohydrate metabolism (Barrington, 1962).
In the larval lampreys (ammocoetes), distinct follicles of Langerhans are embedded in the submucosa of the anterior intestine instead at the surface of the pancrease as in most vertebrates.
Ducts are not present, and the secretions pass directly into the blood. In some teleost fishes and in a few snakes they are grouped in several small but distinct globular mass (principal islets) in the region of the gall bladder. The islets of urodele amphibians are said not to contain.