APUD

APUD stands for Amine Precursor Uptake and Decarboxylation, a historical term used to describe a heterogeneous group of neuroendocrine cells dispersed throughout the body that take up amine precursors from the bloodstream and convert them into biogenic amines. These cells are part of the diffuse neuroendocrine system (DNES) and secrete a variety of amines and neuropeptides that can act as local mediators or circulating hormones.

APUD cells are found in several organ systems, most notably in the mucosa of the gastrointestinal and respiratory tracts, including enterochromaffin (Kulchitsky) cells and enterochromaffin-like cells, as well as in other neuroendocrine tissues such as the pancreas. Embryologic origins vary by site, but many APUD cells arise from neural crest cells, with other populations deriving from endodermal lineages.

The products of APUD cells include biogenic amines such as serotonin, histamine, dopamine, and norepinephrine, along with a range of peptide hormones and related mediators. Because APUD cells can produce serotonin and other amines, tumors arising from these cells—neuroendocrine tumors—are sometimes referred to as carcinoid tumors. Carcinoid syndrome may occur when secreted serotonin reaches systemic circulation, typically via the liver.

Today the APUD concept is largely superseded by the broader DNES and neuroendocrine tumor framework. Modern diagnosis relies on immunohistochemical markers such as chromogranin A and synaptophysin to identify neuroendocrine differentiation, reflecting a shift from the historical APUD terminology to a more unified classification.