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The islets of Langerhans are microscopic structures 50–250 μm in diameter. They are scattered throughout the pancreas, with a maximum density in the tail. The islets appear to have a great reserve capacity; islet dysfunction is not a major problem even after 90% of the pancreas is removed in a distal pancreatectomy. The islets are not connected to the exocrine duct system; the hormonal products are secreted directly into the bloodstream.
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Microscopically, the islets are composed of small uniform cells with round nuclei and scant cytoplasm. Routine microscopy does not permit differentiation of the various types of cells contained within the islets; this requires immunohistochemistry (Table 46-1 and Figure 46-1).
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The most important hormone secreted by the pancreas is insulin. The B (beta) cells of the islets are the only source of insulin in the body, and failure of secretion of adequate amounts of insulin results in diabetes mellitus.
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Glucagon, secreted by the A (alpha) cells, also plays a role in glucose metabolism. The role of glucagon is a less vital one, and absence of glucagon has not been shown to cause clinical disease. The physiologic functions of pancreatic polypeptide (PP) and vasoactive intestinal polypeptide (VIP) have not been elucidated, and the amount of somatostatin and gastrin normally secreted by the pancreatic islets is thought to be too small to be of any physiologic significance. However, excessive secretion of any of these hormones by pathologic islets causes specific clinical syndromes.
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Assessment of islet structure is very difficult because of their small size and scattered distribution in the pancreas. Only large islet cell neoplasms are distinguishable on computerized tomography. The main tests of islet function are serum assays of the various hormones secreted by the islets.
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Diabetes mellitus is a chronic disease characterized by relative or absolute deficiency of insulin, resulting in glucose intolerance. It occurs in 4–5 million persons in the United States (approximately 2% of the population).
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Normal Insulin Metabolism
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Insulin is a polypeptide composed of an A chain, with 21 amino acids, and a B chain, with 30 amino acids (Figure 46-2). It is released from the B ...