An islet, often referred to as an islet of Langerhans, is a small cluster of cells within the pancreas. These cell groupings play a central role in the endocrine system, producing and releasing hormones directly into the bloodstream. Though they make up only 1-2% of the pancreas’s volume, islets are crucial for regulating various bodily processes, especially nutrient metabolism.
Location and Cellular Composition
Islets are distributed throughout the pancreas, an organ situated behind the stomach. A healthy adult human pancreas contains approximately one million islets, each measuring about 0.2 millimeters in diameter. These structures are composed of several distinct types of hormone-producing cells: alpha, beta, and delta cells.
Beta cells are the most abundant, typically making up 65-80% of islet cells, and produce insulin. Alpha cells typically make up 15-20% and produce glucagon. Delta cells, which typically make up 3-10% of the islet, produce somatostatin. Human islets display an intermingled arrangement of alpha and beta cells, often closely associated with blood vessels.
Hormonal Roles
Insulin, produced by beta cells, lowers blood glucose levels. It signals cells throughout the body to absorb glucose from the bloodstream for energy or storage. When blood sugar levels are high, more insulin is released to facilitate glucose uptake.
Glucagon, secreted by alpha cells, performs an opposing action to insulin. Its primary role is to raise blood glucose levels when they fall too low. Glucagon signals the liver to convert its stored form of glucose, called glycogen, back into glucose and release it into the bloodstream. It can also promote glucose production from other sources, such as amino acids, a process called gluconeogenesis.
Delta cells produce somatostatin, a hormone that generally inhibits the release of other hormones, including both insulin and glucagon from adjacent cells within the islet. This local regulatory effect helps fine-tune the islet’s hormonal output, contributing to the precise control of blood glucose.
Islets and Glucose Regulation
The coordinated action of islet hormones, particularly insulin and glucagon, is essential for maintaining stable blood glucose levels, a state known as glucose homeostasis. This regulation operates through a dynamic feedback system that responds to changes in blood sugar. After a meal, carbohydrates break down into glucose, causing blood glucose levels to rise.
This signals beta cells in the islets to release insulin, prompting cells to take up glucose from the bloodstream. As glucose moves into cells, blood sugar levels decrease, and excess glucose can be stored in the liver and muscles as glycogen. Conversely, when blood sugar levels drop, such as between meals or during fasting, alpha cells release glucagon.
Glucagon instructs the liver to convert stored glycogen back into glucose, which is then released into the bloodstream, raising blood sugar levels. This continuous interplay between insulin and glucagon ensures the body maintains a steady energy supply, preventing blood glucose from becoming too high or too low. The close proximity and communication between the different cell types within the islet are important for this precise regulation.
When Islets Don’t Function Properly
Dysfunction of the pancreatic islets can have significant health consequences, most notably contributing to diabetes. In Type 1 diabetes, the body’s immune system attacks and destroys the insulin-producing beta cells within the islets. This autoimmune destruction leads to a severe lack of insulin, meaning the body cannot effectively move glucose from the blood into cells, resulting in high blood sugar levels.
Type 2 diabetes involves a different mechanism, characterized by both insulin resistance and a progressive dysfunction of the beta cells. Initially, the body’s cells become less responsive to insulin, requiring the islets to produce more insulin. Over time, beta cells may become exhausted and unable to produce sufficient insulin to overcome this resistance, leading to elevated blood sugar.
Understanding islet biology is important for developing strategies to manage and potentially cure diabetes. Research focuses on protecting existing islet cells, regenerating lost beta cells, or transplanting healthy islets into individuals with diabetes. The complex interactions within the islets and their role in glucose metabolism make them a central area of study in metabolic health.