The pancreas is a gland about 6 inches long that is shaped like a thin pear lying on its side. The wider end of the pancreas is called the head, the middle section is called the body, and the narrow end is called the tail. The pancreas lies behind the stomach and in front of the spine.

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. In standard histological sections of the pancreas, islets are seen as relatively pale-staining groups of cells embedded in a sea of darker-staining exocrine tissue. The image to the right shows three islets in the pancreas of a horse.

There are two kinds of cells in the pancreas:

 
• Endocrine pancreas cells make several kinds of hormones (chemicals that control the actions of certain cells or organs in the body), such as insulin to control blood sugar. They cluster together in many small groups (islets) throughout the pancreas. Endocrine pancreas cells are also called islet cells or islets of Langerhans. Malignant islet cell tumors are a rare type of pancreatic cancer.
 
• Exocrine pancreas cells make enzymes that are released into the small intestine to help the body digest food. Most of the pancreas is made of ducts with small sacs at the end of the ducts, which are lined with exocrine cells.
An islet cell tumor may also be called a pancreatic endocrine tumor (PET), pancreatic neuroendocrine tumor, islet cell carcinoma, or pancreatic carcinoid. Islet cell tumors may or may not cause symptoms Islet cells make and release hormones into the blood. Islet cell tumors may be functional (the hormones that are released cause symptoms) or nonfunctional (the hormones that are released do not cause symptoms) tumors:   * Functional tumors make one or more hormones, such as gastrin, insulin, and glucagon, that cause symptoms. Most functional tumors are benign (not cancer).   * Nonfunctional tumors make hormones that do not cause symptoms. Symptoms are caused by the tumor as it spreads and grows. Most nonfunctional tumors are malignant (cancer). There are different kinds of functional islet cell tumors Islet cells make different kinds of hormones such as gastrin, insulin, and glucagon. Types of functional islet cell tumors include the following:
 
• Gastrinoma: A tumor that forms in cells that make gastrin. Gastrin is a hormone that causes the stomach to release an acid that helps digest food. Both gastrin and stomach acid are increased by gastrinomas. When increased stomach acid, stomach ulcers, and diarrhea are caused by a tumor that makes gastrin, it is called Zollinger-Ellison syndrome. A gastrinoma usually forms in the head of the pancreas and sometimes forms in the small intestine. Most gastrinomas are malignant (cancer).
 
• Insulinoma: A tumor that forms in cells that make insulin. Insulin is a hormone that controls the amount of glucose (sugar) in the blood. It moves glucose into the cells, where it can be used by the body for energy. Insulinomas are usually slow-growing tumors that rarely spread. An insulinoma forms in the head, body, or tail of the pancreas. Insulinomas are usually benign (not cancer).
 
• Glucagonoma: A tumor that forms in cells that make glucagon. Glucagon is a hormone that increases the amount of glucose in the blood. It causes the liver to break down glycogen. Too much glucagon causes hyperglycemia (high blood sugar). Glucagonomas are often malignant. A glucagonoma usually forms in the tail of the pancreas. Most glucagonomas are malignant (cancer).
 
• Other types of tumors: There are other rare types of functional islet cell tumors that make hormones, including hormones that control the balance of sugar, salt, and water in the body. These tumors include:       o VIPomas, which make vasoactive intestinal peptide. VIPoma may also be called Verner-Morrison syndrome.       o Somatostatinomas, which make somatostatin.

    These other types of tumors are grouped together because they are treated in much the same way.


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National Cancer Institute

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