This image shows a section of the pancreas, which is composed of masses of serous acini (exocrine secretory units) arranged in many small lobules.  These acinar cells produce digestive enzymes that are drained by ducts that feed into a centrally located pancreatic duct,  which fuses with the bile duct just before it enters the duodenum.   Scattered  among the acini are  the more lightly-staining islets of Langerhans, which contain the glucagon-synthesizing alpha cells and more numerous insulin-producing beta cells.  These islets constitute the endocrine portion of the pancreas.