Islet of Langerhans. Transmission electron micrograph (TEM) of a section through cells clustered in an islet of Langerhans in the pancreas. A variety of endocrine cell types is seen representing beta cells secreting insulin, and alpha cells secreting glucagon. These hormones are stored in the many cytoplasmic granules (black spots) and released into blood vessels to regulate the supply of glucose throughout the body. Magnification: x8300, when printed 10 centimetres wide.