SPONTANEOUS NEOPLASMS IN MICE 169 



During the past ten years the staff of the laboratory has kept detailed 

 records on large colonies of mice representing a considerable number of 

 inbred stocks. One phase of the record-keeping included the collecting of 

 detailed data on the incidence of spontaneous tumors. Complete autopsies 

 were routinely performed and tissues were saved from all body regions which 

 offered any suggestion of abnormal growth. Furthermore, no tumor 

 experiments have been considered ready for publication until these tissues 

 have been studied as to their histopathology. Our collection of tissues from 

 mice which spontaneously developed tumors represents data from stocks 

 that vary widely in their tumor incidences. Data on the high tumor strains 

 show that in some stocks over 90 per cent of all breeding females living into 

 the tumor age develop some form of neoplasm, e.g., the A and C3H strains, 

 while in the low tumor strains abnormal growths are rarely found, even in 

 mice which have attained extreme senility, e.g., Mus bactrianus. 



Naturally, the number of recorded mice with spontaneous tumors is no 

 indication of the vast numbers of mice which have been employed by the 

 staff, for the stocks vary so markedly from one to another in their population 

 tumor incidence. However, inbreeding has been carried on to such a degree 

 that, on the basis of previous observations within a stock, one can predict 

 with considerable accuracy what types of tumors will probably occur, at 

 what average age they will be found and in the case of certain types of 

 growths, in what per cent of another large population these new growths will 

 develop when one employs the same stock. 



Definition and Characteristics of Tumors 



A tumor is an autonomous new growth of tissue (Fig. 88). Also, tumors 

 are atypical growths with atypical structure, apparently of independent 

 origin. They exhibit no useful function, are without limit to growth and, if 

 uninterrupted, can result in the destruction of the host. These growths arise 

 either from embryonic cell rests or from the body cells of the host. They 

 start as a localized disease involving a few cells and progressively increase in 

 size by cell division. 



Some masses of cells may grow by expansion. This will result in the 

 formation of a connective tissue capsule due to the pressure of the growth on 

 the surrounding supporting cells. Others may grow by infiltration, spread 

 along tissue spaces and lymphatics and may be found at some distance from 

 the spontaneous tumor. This is a more malignant type than the former. 

 Another form of growth combining the two above types is called interlocking. 

 The second and third types of growth are the most difficult to remove by 



