Comparative ultrastructural study of atypical adenomatous hyperplasia and adenocarcinoma of the human lung

Atypical adenomatous hyperplasia (AAH) of the human lung is considered a po ssible precursor of pulmonary adenocarcinoma. However, its true biological significance remains to be clarified. The authors studied the ultrastructur e of AAH in surgically resected lungs and compared it with that of coexisti ng adenocarcinoma in an effort to define the characteristic features of AAH . Ultrastructurally, AAH possessed oval to irregular nuclei with high nucle o-cytoplasmic ratio and large nucleoli. Development of cytoplasmic organell es was generally poorer in AAH than in adenocarcinoma. However, these diffe rences became less apparent as the degree of atypia of AAH advanced. Both l amellar bodies and electron-dense granules were found in AAH as well as in adenocarcinoma. These results suggest a close relation of AAH with adenocar cinoma of type 2 pneumocyte or Clara cell type. Further, the results of imm unohistochemical studies for surfactant apoprotein A, urine protein 1, cyto chrome P-450s, CEA, p53, c-erbB-2, Ki67, and bcl-2 well reflected the ultra structural findings. These results suggest, in accordance with previous stu dies, that AAH is a lesion closely related to adenocarcinoma. Further, AAH shares some characteristics of type 2 pneumocytes and Clara cells, implying that it might be derived from their common precursor.