Pancreatic islet cells. Coloured transmission electron micrograph (TEM) of a section through pancreas tissue, showing cells known as Islets of Langerhans. These cells occur in distinct ovoid clusters numbering about 1 million in humans. The cells synthesize insulin and glucagon and other peptide hormones which are released directly into blood vessels. Islet cells contain many dense cytoplasmic granules (red) for storage of inactive forms of the hormones. Magnification: x3000 when printed at 10 centimetres tall