Immunocytochemical localization of aromatic L-amino acid decarboxylase in human, rat, and mouse bronchopulmonary and gastrointestinal endocrine cells

Aromatic L-amino acid decarboxylase (AADC) catalyzes the cellular decarboxylation of L-aromatic amino acids and is therefore involved in the synthesis of several biogenic amines. Application of the indirect immunoperoxidase method on human, rat, and mouse tissues using specific antibodies to AADC revealed all AADC-containing cells. Besides mast cells and adrenergic nerve fibers, the following cells were immunostained: neuroendocrine cells in the tracheobronchial epithelium; neuroepithelial bodies in the bronchopulmonary epithelium; Kultschitzky cells in the small intestine and appendix as well as adrenal chromaffin cells. All the latter cells belong to the so-called APUD system, the "D" in the acronym standing for the activity of the enzyme aromatic L-amino acid decarboxylase. Immunocytochemistry for AADC may become an additional tool not only to highlight APUD cells in tissue sections but also to differentiate the sites of cellular amine synthesis from those of amine storage.