Islets

An islet is a very small island or landmass, smaller than a conventional island, or a minor landform that rises above water. The term is used primarily in geographic contexts and in biology to denote small clusters of tissue. In geography, islets form along coastlines, in river deltas, or within archipelagoes, and they may be composed of rock, sand, coral, or peat. They are often exposed to marine erosion, storms, and sea-level rise, and many are uninhabited, though some host settlements or serve as tourist attractions. Islets can provide important breeding sites for seabirds and other wildlife and may support unique plant communities; their ecological integrity is often sensitive to disturbance and invasive species.

In anatomy, the term islet refers to the islets of Langerhans, tiny clusters of endocrine cells embedded in the pancreas. These islets contain several cell types, including insulin-secreting beta cells, glucagon-secreting alpha cells, and somatostatin-secreting delta cells, among others. They regulate blood glucose and coordinate metabolic responses to meals. The islets are richly vascularized and receive autonomic input to modulate hormone release. The term was introduced by Paul Langerhans in the nineteenth century.

Because the same word is used in different fields, context is important: geographic islets are landforms, whereas pancreatic islets are glandular tissue involved in endocrine function.