SPOXTANEOUS NEOPLASMS IN MICE 209 



4. Carcinoma simplex. 



5. Carcinosarcoma. 



Workers have disagreed as to the degree of malignancy of lung tumors. 

 Among our tumor slides there are a large number of cases of spontaneous 

 tumors of the lung, many of which show characteristics definitely demon- 

 strating the malignant nature of lung tumors. 



Adenoma. — Some small tumors are classed as adenomas because of their 

 cell arrangement and comparative inactivity, but small size alone is not a 

 true indication of the mass being benign. We have not observed a well 

 developed capsule, possibly because of the looseness of the lung architecture. 

 Adenomas appear fairly frequently as subserous, pearly white, slightly 

 elevated nodules, often one half to two millimeters in diameter. On section 

 they may be lenticulate to round. Their histopatholog>' shows rather 

 closely packed, poorly staining, polyhedral cells whose arrangement as 

 twisting, branching tubules with blunt ends is suggestive of immature, 

 uninflated air cells. Between these poorly defined structures is a network 

 of thin-walled, capillar}'-like blood vessels. The polyhedral cells are not 

 markedly different from many of the lining cells of normal pulmonary alveoli. 

 They have centrally placed, rounded or oval, somewhat hypochromatic 

 nuclei with one or more nucleoli, and abundant pale cytoplasm filled with 

 fine eosinophilic granules. The tumor cells differ from the normal in that 

 some are twice as large as the normal cells, some have lobulated nuclei and 

 others show two nuclei within a single cell. Not all of these tumors are at 

 the surface but are most likely to be observed there in gross dissection. 



Some other tumors no larger than the above may differ from them chiefly 

 in that the parenchyma cells are more closely packed, have more eosinophilic 

 cytoplasmic granules, and exhibit a preponderance of large irregular cells. 

 These have hypochromatic nuclei which often appear as two distinct nuclei 

 within a single cell. It is not uncommon to see nuclei with multiple lobules, 

 and sometimes a dozen or more closely clustered, rounded nuclear masses are 

 seen within a single tumor cell. This indicates amitotic division. Mitotic 

 figures are also occasionally seen. 



The outlines of these two types of tumors dift'er in that the latter may be 

 more irregular. In this type there are some foci of infiltration into normal 

 alveoli, while other foci show compressed, collapsed alveoli resulting from 

 the pressure of the tumor growth by expansion. Invasion of the smaller nor- 

 mal bronchi can sometimes be seen. We call this type an adenocarcinoma. 



Adenocarcinoma. — It is a common belief that this tumor originates from 

 the lining cells of the alveoli but it may also originate from the bronchi. 



