GENERAL ZOOLOGY 



Lumen 



Capillary 



Island of 

 Langerhans 



Fig. 4.25. The pancreas, showing an island of Langerhans, the cells of which produce the 

 hormone of the pancreas, surrounded by acini, the cells of which secrete the pancreatic juice; 

 seen in section. 



The digestive gland known as the pancreas contains groups of cells known 

 as islands of Langerhans (Fig. 4.25). These are not involved in the produc- 

 tion of pancreatic juice and its digestive enzymes (p. 72); this part of the 

 pancreas is a gland of internal secretion. Histologically, at least two kinds 

 of cells, the alpha or A cells and the more abundant beta or B cells, can be 

 distinguished in the islands. By means of a drug called alloxan, which selec- 

 tively kills the beta cells, it has been established that they are responsible for 

 the presence in the blood of a polypeptide called insulin. This compound was 

 isolated in 1922 and became available in crystalline form in 1927; the 16 

 amino acids of which it is composed and their arrangement had been deter- 

 mined by 1954. When insulin is absent, glucose is present in the blood in 

 such large amounts that it is eliminated in the urine (p. 84); this is a major 

 symptom of the disease known as diabetes mellitus. Insulin increases the rate 

 at which cells remove glucose from the blood. Continued injections of insulin 

 are effective in controlling diabetes. 



The source of another pancreatic hormone named glucagon has been local- 

 ized in the islands of Langerhans. This substance contains 15 amino acids 

 and was not separated from insulin until 1953. It is suspected, but has not 

 been proved, that the alpha cells are involved in its production. Glucagon 

 converts an inactive form of the enzyme phosphorylase to the active form in 

 the liver (see Fig. 2.9, p. 36). This results in the conversion of glycogen to 

 glucose which is released from the liver and raises the blood sugar level. The 

 amount of glucose in the blood is the regulatory factor for release of the 

 pancreatic hormones. 



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