Islet of Langerhans. Coloured transmission electron micrograph (TEM) of a section through two cells of the hormone-secreting islet of Langerhans in the pancreas (the membrane separating the two cells is not seen). At far left and lower right are parts of the nuclei of these cells. The cells contain membrane-bound secretory granules (circular). These contain a dense granular core of hormone (green) separated from the surrounding membrane by a clear space (yellow). The islets of Langerhans secrete insulin and glucagon hormones into the blood, which control blood sugar levels. Magnification: x8700 when printed 10cm wide.