Islet of Langerhans. Light micrograph of a section through an islet of Langerhans in the pancreas. The islet (centre, yellow) is composed of clumps of secretory cells. The main secretions from these cells are the hormones glucagon and insulin, which control blood sugar. These cells are endocrine, as the secretions go straight into the bloodstream. The cells surrounding the islet are packed into sac-like structures called acini (red), and secrete digestive enzymes. This part of the pancreas is exocrine, as the enzymes pass into the gut, via ducts. Magnification: x1250 when printed 10 centimetres wide.