Islets of Langerhans forms an endocrine part of pancreas. Paul Langerhans (1869) discovered this. Each pancreatic islet includes four types of hormone secreting cells : These are :
(i) α or A cells-constitue about 15% of pancreatic islet cells and secrete glucagon.
(ii) β or B cells-constitute about 80% of pancreatic islet cells and secrete insulin.
(iii) δ or D cellsconstitute about 5% of pancreatic islet cells and secrete somatostatin (identical to growth hormone-inhibiting secreted by the hypothalamus).
(iv) F cells-constitute the remainder of pancreatic islet cells and secrete pancreatic polypeptide.