The islet β-cell

Abstract

The β-cell is one of four major types of cells present in the islets of Langerhans, which are islands of cells distributed throughout the endocrine pancreas in most mammals. The β-cell synthesizes and secretes the hormone insulin mainly in response to glucose but also in response to several nutrients, hormones and nervous stimuli. In adult rodents the β-cell has a slow mitotic rate. Recent studies provide novel insights into the functions of the β-cell. The presence of functional insulin-like growth factor-1 and insulin receptors and components of their signaling pathway indicate an important role for insulin/IGF-1 signaling in the regulation of β-cell function. Further, the recent discovery of glucokinase (GK) and the ATP-dependent potassium channels on insulin secretory granules, the detection of AMP-protein kinase in the β-cell and the identification of a new β-cell transcription factor, mMafA, are some exciting new areas of research currently underway to further understand the complex pathways that regulate the functions of β-cells. Cell facts • The insulin-secreting β-cell is one of four major types of cells that make up the mammalian pancreatic islet. • The β-cell secretes the hormone insulin, in response to nutrients, hormones and neuronal stimuli, and therefore plays a primary role in the maintenance of glucose homeostasis. • It is estimated that adult humans have approximately 2 million islets, which make up about 2% of the pancreas by weight. In rodents, each islet is composed of 2000–4000 cells of which 70–80% are β-cells, 5% are somatostatin-producing δ-cells and 15–20% are either glucagon-producing α-cells or pancreatic polypeptide-producing PP cells depending on whether the islets are located in the splenic portion of the pancreas (body and tail in humans), which is derived from a dorsal pancreatic bud, or in the duodenal pancreas (formed from the ventral pancreatic bud: the head or uncinate process of the pancreas), respectively [Diabetologia 28 (1985) 528]. A similar pattern may be observed in humans although precise data is lacking. • The replication rate and turnover of β-cells in humans is not precisely known. In rodents, new β-cells can form in adults either by slow replication of existing β-cells or by the formation of new cells from potential ductal precursors or β-cell “stem” cells. The half-life of mouse islet cells has been estimated to be approximately 47 days with the renewal rate of glucagon-producing cells being more rapid than insulin- or somatostatin-producing cells [Pancreas 24 (2002) 153].