APUD systemAmine Precursor Uptake and Decarboxylation system. A morphologic and functional subgroup of the endocrine system, encompassing: the C cells of the thyroid and ultimobranchial body; type-I cells of the carotid body and paraganglia; norepinephrine and epinephrine-producing cells of the adrenal medulla; melanoblasts; and the pineal gland and posterior pituitary cells. APUD cells take up tryptophan, converting it to 5-HT (serotonin), which is converted to mono-amine oxidase (MAO) 5HIAA.
APUD tissue is visualised with silver stains (Fontana-Masson), an argentaffin stain, or Bodian, an argyrophilic stain; bichromate; diazo salts; formaldehyde-induced autofluorescence; immunoperoxidase; and EM (Dense-core neurosecretory granules).
The term APUD system is little used in the working parlance and has been replaced by neuroendocrine system.