ABSTRACT
The beta cell is the major endocrine cell component of the islets of Langerhans, and a typical human islet will contain 3000-4000 cells. The relative proportions of the different endocrine cell types may vary in different anatomical areas of the pancreas, but on average there are 70% beta cells, 25% alpha cells and 5% D cells. The beta cell is programmed to synthesise insulin via its precursor pro-insulin, and
furthermore, it has developed detailed mechanisms to regulate the rate of secretion in response to glucose. A characteristic feature of a beta cell is the presence of a large number (up to 13000 per cell) of membrane-limited hormone-storage granules or vesicles which contain a substantial reservoir of insulin, sufficient for 24 h of secretion in the absence of new synthesis. The dense core of the storage granule contains insulin in a crystallised form, along with the connecting peptide excised during the process of conversion from the biosynthetic precursor pro-insulin. These two together make up 80-90% of the total granular protein.
