Islet of Langerhans. Colour transmission electron micrograph (TEM) of a slice through a cell of the hormone-secreting islet of Langerhans in the mammalian pancreas. At top right (blue and yellow) is part of the nucleus of this cell. The red spots in white spaces are membrane-bound secretory granules. These contain a densely granular core of hormone separated from the surrounding membrane by a clear space. The islets of Langerhans secrete hormones into the blood - mostly insulin and glucagon, which control blood sugar levels. Magnification x4,800 at 6x4.5cm size.