THE ENDOCRINE SYSTEM Q\^ 



251. The Islet Cells of the Pancreas 



The pancreas is known to secrete two hormones, insuHn and glu- 

 cagon, in addition to a number of digestive enzymes. Scattered among 

 the acinar cells which secrete the digestive enzymes are clusters of 

 hormone-secreting cells, called islets of Langerhans, which are quite 

 different in appearance and staining properties. They have a richer 

 supply of blood vessels than the acinar cells and have no associated 

 ducts. The islet cells can be differentiated into two or more types by 

 the staining reactions of their cytoplasmic granules. The pancreas de- 

 velops as two otugrowths from the duodenum which grow together and 

 fuse in most vertebrates. The islet cells develop as buds from the pan- 

 creatic ducts and eventually lose all connection with the ducts. In 

 some bony fishes the acinar and islet tissues form spatially separate 

 organs. The pancreas of the cyclostomes is ductless and located in the 

 wall of the duodenum or in the liver. 



The human disease diabetes had been recognized for many cen- 

 turies but its cause and cure were equally unknown. A similar condition 

 was produced experimentally in dogs by von Mering and Minkowski 

 in 1889 when they surgically removed the pancreas while studying its 

 role in digestion. Many attempts were subsequently made to feed pan- 

 creas or to prepare an extract for injection into diabetics, but all were 

 unsuccessful because the proteolytic enzymes made by the pancreas de- 

 stroyed the protein hormone before it could be extracted. Finally, in 

 1922, Banting and Best prepared an extract of fetal pancreas which had 

 antidiabetic potency. The endocrine cells of the pancreas become active 

 before the exocrine ones do. The first preparation of pure crystalline 

 insulin was made in 1927 by Abel. The present commercial insulin is 

 extracted from beef, sheep or hog pancieas by an acid alcohol method 

 which rapidly inactivates the proteolytic enzymes. Insulin is a protein 

 with a molecular weight of 12,000. From the brilliant work of F. Sanger 

 in England the exact sequence of the amino acids in each of the two 

 peptide chains making up the insulin molecule is now known. One 

 chain contains 21 amino acids and the other contains 30. 



Most commercial preparations of insulin were found to contain a 

 second hormone, which increases blood sugar concentration instead 

 of decreasing it as insulin does. This hormone, now christened glu- 

 cagon, has been separated from insulin, crystallized, and found to be 

 a protein. Glucagon is secreted by the alpha cells of the islets and 

 insulin by the beta cells. 



Insulin and glucagon both take part in the regulation of carbohy- 

 drate metabolism, along with certain hormones secreted by the pitui- 

 tary, adrenal medulla and adrenal cortex. Glucagon activates the 

 enzyme phosphorylase, which is involved in the conversion of liver 

 glycogen to blood glucose, and thus raises the concentration of glucose 

 in the blood. Insulin increases the rate of conversion of blood glucose 

 to intracellular glucose-phosphate, thereby decreasing the blood glucose 

 level, increasing the storage of glycogen in liver and muscle, and in- 

 creasing the metabolism of glucose to carbon dioxide and water. A 

 deficiency of insulin decreases the utilization of sugar and the resulting 



