Pancreatic islet cells. Transmission electron micrograph (TEM) of a section through pancreas tissue, showing cells (mostly in the lower frame) 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. A blood vessel is at upper left, with a lymphocyte (type of white blood cell) in the lumen. Islet cells contain many dense cytoplasmic granules for storage of inactive forms of the hormones. Magnification: x3000 when printed at 10 centimetres tall.
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