Pancreas anatomy. Illustration of the pancreatic hormone insulin (orange dots) being secreted in the pancreas. Insulin plays a vital role in the metabolism of sugar in the blood. The pancreas is at upper left, with the expanded view at right showing the structure of an islet of Langerhans. This consists of A-cells, B-cells, D-cells and acinar cells. The islet is composed of clumps of secretory cells that produce the hormones glucagon and insulin. These cells are endocrine, as the secretions go straight into the bloodstream (blood vessels shown, red). The cells surrounding the islet form structures called acini (green), secreting digestive enzymes.