APUDSystem

APUDSystem refers to the Amine Precursor Uptake and Decarboxylation concept, a historical framework in physiology and pathology that grouped certain neuroendocrine cells by a common biochemical trait. The idea posits that these cells take up amine precursors from their surroundings and convert them into biogenic amines through decarboxylation, contributing to hormone and neurotransmitter production. The APUD concept helped unify a diverse set of secretory cells into what was once called the diffuse neuroendocrine system (DNES).

Mechanistically, APUD cells are characterized by their ability to absorb amine precursors such as DOPA or 5-hydroxytryptophan and to express decarboxylase enzymes that convert these substrates into active amines like serotonin, dopamine, norepinephrine, or histamine. In many preparations, these cells also secrete peptide hormones in addition to biogenic amines. The histochemical and ultrastructural features associated with APUD cells—such as argentaffin or related staining patterns—were historically used to identify and study these cells in tissues.

Distributionally, APUD-like cells appear in multiple organs, most notably within the gastrointestinal tract (for example, enterochromaffin cells), the respiratory tract, the pancreas, and certain thyroid-related cell populations. The concept also encompassed other dispersed neurosecretory cells that shared the presumed capacity for amine precursor uptake and decarboxylation.

Legacy and contemporary view place the APUD idea within the broader evolution of neuroendocrine biology. While the term APUDSystem is largely of historical interest, it influenced early classification of neuroendocrine tumors and contributed to the understanding of the diffuse neuroendocrine system. Modern terminology emphasizes neuroendocrine differentiation and uses markers such as chromogranin A and synaptophysin, with more precise lineage and molecular characterizations guiding diagnosis and research.