Islet of Langerhans. Transmission electron micrograph (TEM) of a section through cells clustered in an islet of Langerhans in the pancreas. A variety of endocrine cell types are seen, including beta cells, which secrete insulin, alpha cells, which secrete glucagon and delta cells that secrete somatostain. These hormones are stored in the many cytoplasmic granules (black spots) and released into blood vessels to regulate the supply of glucose throughout the body.