PROBLEMS CONCERNING THE TUMOR VIRUSES 325 



Ponten's (1956) investigations are pertinent to the rate of Rous virus 

 production. This investigator did not use tissue cultures but found that 

 ascites tumors of Rous sarcoma origin showed virus activity in direct 

 proportion to the growth rate of tumor cells as well as to the elapsed time. 



Epstein (1956), working along the same lines, found a correlation between 

 the biological activity of the Rous virus and the number of cells containing 

 visible virus particles. His main objective was to ascertain whether biological 

 assay would support the detection of virus particles by means of the electron 

 microscope, because such controls were lacking in previous studies by 

 electron microscopists. Examination of the cells from 13 ascites tumors 

 derived from the Rous sarcoma revealed that the incidence of cells containing 

 visible virus particles ranged between 1 in 50 and 1 in 3000 in 9 tumors, with 

 no detectable particles in 4 tumors. The particles were located in, or in the 

 walls of, cytoplasmic vacuoles of tumor cells, with an average of 100 particles 

 per cell. He tested 5 tumors for virus activity and obtained a correlation 

 between the incidence of cells containing particles and the tumor-producing 

 ability of tumor extracts. This permitted the conclusion that the particles 

 were visible forms of the Rous virus. 



While, at first glance, Epstein's observation that only 168 of 27,637 tumor 

 cells contained particles could be interpreted as not confirming the work of 

 Rubin, there is, as pointed out by Epstein, no disagreement if only a small 

 proportion of tumor cells contained infectious virus particles, whereas the 

 remainder contained a nondetectable stage in the development of the virus. 

 This interpretation is also in harmony with the suggestion of Carr (1947), who 

 calculated that not all the virus in a Rous sarcoma could be infectious. 



Haguenau et al. (1958) approached the problem of cell- virus relationships by 

 fulfilling three criteria they considered essential: (1) the inoculation of cell- 

 free material of known potency to induce the tumors, (2) dilutions of the cell- 

 free material to establish quantitative relationships between the inocula and 

 resultant tumors, (3) determinations of the activities of extracts from the 

 resultant tumors. With these criteria in mind, they induced tumors with 

 variable amounts of virus, prepared thin sections of the tumors and examined 

 them with the electron microscope, and performed bio-assays of the induced 

 tumors. In addition, electron micrographs were made of pellets obtained from 

 virus suspensions of the induced tumors. The results of the combined bio- 

 logical and electron microscopic studies revealed statistically significant 

 differences between tumors evoked with large and small amounts of virus in 

 (1) the number of cells containing particles, (2) the number of particles per 

 cell. Within the tumors induced with large concentrations of virus, there was 

 a correlation between the number of particles and the activity of the tumor 

 extracts. Pellets from tumors induced with large amounts of virus contained 

 many more particles than those induced with small amounts. 



