The pancreas contains endocrine cells, which secrete hormones directly into the bloodstream, and exocrine cells, which secrete zymogen precursors of digestive enzymes into the upper small intestine. The endocrine tissue contains cell clusters known as islets of Langerhans, which contain at least four different cell types (A, D, P, and B), each specialized for synthesis of one hormone. The A cells produce glucagon; the D cells, somatostatin; and the P cells, a recently discovered pancreatic hormone. Insulin is synthesized in the B cells, which sense glucose levels and secrete insulin in response to increased levels of blood glucose.