islets of Langerhans

Imagine the pancreas as a large factory mostly busy making digestive juices, with thousands of tiny specialized offices scattered throughout it. Each of those offices is an islet of Langerhans, and instead of digestive juice they release hormones straight into the bloodstream to manage blood sugar. They are named after Paul Langerhans, the medical student who first noticed these little clusters of cells under the microscope in 1869.

There are roughly one million islets in an adult pancreas, but together they make up only about one to two percent of the organ's mass. Each islet contains several cell types arranged together: beta cells (insulin), alpha cells (glucagon), delta cells (somatostatin), and PP cells (pancreatic polypeptide). Because these cells sit side by side and are richly supplied with blood, they can sense glucose and rapidly coordinate the release of opposing hormones.

An important nuance is that the islets are the endocrine part of the pancreas, while the surrounding bulk is exocrine tissue that secretes enzymes into the gut through ducts. The two systems are anatomically intertwined but functionally separate. Damage targeted at the islets, such as autoimmune destruction of beta cells, can wreck blood sugar control while leaving digestion largely intact.