Transmission electron micrograph (TEM) of the ultrastructure of a beta cell in an islet of Langerhans in the pancreas. These cells synthesise and secrete insulin. Insulin is a polypeptide hormone stored in the beta cell as cytoplasmic granules with variable morphology. Granules accumulating newly synthesised insulin have a homogenous content. Other granules show a dense core indicating a semi-crystalline form of insulin. When released from the beta cell, granules discharge insulin into a blood vessel. Magnification: x22,000 when printed at a width of 10cm.