[The question of the functional significance and origin of pulmonary "tumorlets"]

In 1955, the term "tumorlets" was proposed to denote unusual microscopic clusters of epithelial cells occasionally detectable in the lung. The endocrinal nature of these formations has been proved, but there are conflicting opinions on the essence of the process (hyperplasia, dysplasia, and neoplasia). Pulmonary proliferation of endocrinal cells, which was similar to that of tumorlets, was found in 2 of 4 observations. The analysis of the results of the studies performed by the authors and of data available in the literature makes it possible to conclude that the proliferates of pulmonary endocrinal cells are, in terms of their functional significance, heterogeneous. Some of them appear as preneoplastic hyperplasia of endocrinal cells. The individual clusters acquiring independent blood circulation (stroma) may be regarded as true tumor-microcarcinoids. The authors also define the transitional forms between hyperplasia and neoplasia.