The pancreas is an elongated organ nestled next to the first part of the small intestine. Its gross anatomy and the structure of pancreatic exocrine tissue and ducts are discussed in the context of the digestive system. The endocrine pancreas refers to those cells within the pancreas that synthesize and secrete hormones. The endocrine portion of the pancreas takes the form of many small clusters of cells called islets of Langerhans or, more simply, islets.
Humans have roughly one million islets.
The image to the right shows three islets in the pancreas of a horse. Islets are richly vascularized, allowing their secreted hormones ready access to the circulation. Additionally, they are innervated by parasympathetic and sympathetic neurons, and nervous signals clearly modulate secretion of insulin and glucagon.