80 Discussion 



keeping with your results is the fact that if, in addition to the 

 reduction in proteins, the total intake of food is reduced, amyloidosis 

 in this strain can be made to disappear almost completely. In the 

 A strain of mice there are three different entities which tend to kill 

 these animals: (1) high incidence of mammary tumours (but only 

 in breeding females), (2) high incidence of lung tumours, and (3) the 

 deposition of amyloid. So far as I know the appearance of the other 

 two conditions cannot be easily retarded. 



To what extent were your rats inbred in these experiments, Dr. 

 Berg ? Were they genetically homogeneous or were you dealing 

 with a mixed colony ? 



Berg: They were not homogeneous but were closely related. We 

 have two lines of rats that have been randomly inbred for 65 

 generations. These derive from a small group of Sprague-Dawley 

 rats acquired in 1945. Obviously, if the rats were more homogeneous 

 the curves would be steeper than those we have shown. 



Qruneherg: That being so, additional information might be ob- 

 tained by using highly inbred strains of mice, which have very dif- 

 ferent lifespans. Some strains die early because they have an early 

 onset of manunary tumours, such as the C3H strain; and the I 

 strain has a short lifespan because it develops lesions in the stomach. 

 Other strains differ in lifespan for reasons which are not yet fully 

 understood. In addition one can use homogeneous but not inbred 

 material (first generation hybrids) and segregating material (Fg 

 generations) and so on. It should also be possible to subdivide the 

 causes of mortality further by systematic changes in the environ- 

 ment, particularly change in diet, as you discussed, but also by dif- 

 ferences in crowding, temperature, etc. If this were done it would 

 probably turn out that there is no unique life expectation of the 

 species ; presumably each genotype and each type of environmental 

 situation has its own expectation of life, and in man the survival 

 curve is presumably a superimposition of a whole family of curves. 



Muhlbock: Amyloidosis is one of the major diseases in mice. In 

 different inbred strains there are differences in incidence of amyloido- 

 sis. In some strains there is a very high incidence at 18 months of 

 age, whereas in other strains it is nil, or very low. Therefore one 

 should investigate a number of different strains before generalizing 

 and saying that for that species this is the age of onset of the disease. 



Berg: You are quite right. Our results apply only to our particular 

 Sprague-Dawley strain, maintained under our conditions. 



Sacher: Amyloidosis has been under investigation in our laboratory 

 by Dr. S. Lesher (1957. J. nat. Cancer Inst, 19, 1119). He finds a 

 high incidence in the A strain and in F^ hybrids with an A-strain 



