Islets of Langerhans surrounded by exocrine cells. This clump of secretory cells forms part of the endocrine system of the body, which releases hormones into the blood. It is composed of numerous beta cells, which secrete insulin, and the less numerous alpha cells, which secrete glucagon. Insulin stimulates the uptake of glucose and amino acids from the bloodstream, whereas glucagon has the opposite effect; it stimulates the breakdown of glycogen in the tissues. This causes glucose and amino acids to be released into the blood.