Pulmonary blastoma. An ultrastructural study of a case and its transplanted tumor in athymic nude mice

: A pulmonary blastoma in a 52-year-old Japanese man was serially transplanted into athymic nude mice. This report emphasizes the characteristics of the neoplasm seen on light and electron microscopic examination. The original tumor consisted of branching tubular structures and primitive mesenchymal components with a few smooth-muscle cell bundles, and resembled fetal lung tissue. Electron microscopic examination revealed that the immature stromal cells resembled epithelial cells. After serial passages in nude mice, the stromal element of the tumor disappeared, and the epithelial element showed squamous metaplasia and occasional mucus production. Endocrine-type granules were observed in a few tumor cells. Histogenetic considerations, including a suggestion that the tumor was derived from entodermal cells, are presented.