236 P. L. Mollison 



cells besides haemoglobin and in this way may cause irrevers- 

 ible damage. 



Although in principle it is possible to obtain samples of 

 young and old red cells separately, comparatively little work 

 has yet been done upon differences between them. A popula- 

 tion of very young red cells can be obtained by repeatedly 

 bleeding an animal, or by allowing red cells to sediment in 

 vitro and separating the more slowly sedimenting cells. A 

 population of old red cells can be obtained from an animal in 

 which erythropoiesis has been temporarily suppressed. 



One approach is to label a population of young cells with 

 radioactive iron, and then to take samples at intervals during 

 the life-span of the labelled cells. It is comparatively easy to 

 discover whether the labelled cells differ from the recipient's 

 own cells in various respects. For example, blood samples can 

 be mixed with a series of hypotonic saline solutions and it 

 can be determined whether an undue proportion of old cells 

 have been haemolyzed, judged by radioactivity liberated 

 from labelled cells. Using such a method Cruz, Hahn, Bale and 

 Balfour (1941) showed that old red cells are more resistant to 

 hypotonic haemolysis. Stewart et al. (1950) confirmed this 

 and showed that old red cells were less resistant to mechanical 

 trauma. 



Because so little is known about age changes in red cells 

 in vivo, it is worth considering changes occurring on storage in 

 vitro and discussing whether these throw any light on the 

 normal ageing process. 



If blood is mixed with citrate and stored at + 4° C the red 

 cells slowly deteriorate. " Deterioration " is used here to mean 

 a loss of the ability of the red cells to survive normally in the 

 circulation. The rate of deterioration can be slowed by the 

 addition of dextrose (Rous and Turner, 1916), or by acidifica- 

 tion (Loutit, Mollison and Young, 1943). The rate of deteriora- 

 tion can be virtually arrested by storage in the frozen state. 

 For example, red cells stored at — 79° C for periods up to two 

 years show no progressive loss of viability with time, apart 

 from a small loss observed after a few weeks' storage (Chaplin, 



