Islet of Langerhans. Transmission Electron Micrograph (TEM) of a slice through three cells of the hormone-secreting islet of Langerhans in the mammalian pancreas. The mottled ovals are the cells' nuclei (membranes between different cells are not clearly shown). The islets of Langerhans secrete hormones into the blood, mostly insulin and glucagon, which control blood sugar levels. The hormones are packaged in small, membrane-bound spaces called secretory granules. These contain a densely granular core (black dots) separated from the membrane by a clear halo (white). Magnification x8000 at 7x9 inch size.