Appropriate and inappropriate neuroendocrine products in pulmonary tumourlets.

Pulmonary tumourlets are focal aggregates of neuroendocrine cells that occur in the periphery of the lung and may be associated with chronic inflammation and scarring. Six such lesions were seen in five lungs from a series of 35 pairs of lungs studied at necropsy. All were immunoreactive for neurone-specific enolase, protein gene product 9.5, and a range of neuroendocrine products. Of the peptides found in neuroendocrine cells in normal human lungs, gastrin releasing peptide was present in all tumourlets and calcitonin in all but one; none contained leucineenkephalin. Of a series of peptide and protein hormones not present in the neuroendocrine cells of healthy human lungs, growth hormone was present in all six tumourlets and adrenocorticotrophin in two. Identical patterns of peptide expression were displayed by neuroendocrine cells in the airway associated with the tumourlets in two cases. Such cells were increased in number and abnormally clustered. Aberrant expression of peptides might accompany the morphological changes in the pulmonary neuroendocrine cells seen in diseased lungs, their florid focal proliferation occasionally resulting in the formation of a tumourlet.