SENESCENCE IN PROTOZOA 



3-1 Individual Cells 



Much theoretical study was devoted during the last century 

 to the 'immortality' of protozoans, and their insusceptibility to 

 senescence, following the concepts put forward by Weismann. 

 It was considered that in unicellular organisms generally, and 

 in populations of metazoan cells undergoing division without 

 differentiation, the product of a cell's division is always a pair 

 of daughter cells having the same age status, and destined each 

 to lose its identity in another division. This theory makes very 

 important assumptions about the nature of the copying pro- 

 cesses which underlie cell division. In the majority of cases to 

 which it was applied, the assumptions are probably correct, 

 although there seem to have been no direct experiments 

 designed to show whether, in a given protozoan population, 

 the diagram of lineages shows any tendency to segregate the 

 deaths of individual cells towards its edges, as in a metazoan 

 genealogy. 



Weismann had been impressed (1882) that in protozoa there 

 is no death because there is no corpse. 'Natural death' of indivi- 

 duals (often apparently from strictly endogenous causes) does 

 occur in protozoa, as Jennings (1945) has shown (see below); 

 and the assumption that there is no unrenewable matter at cell 

 division is not universally true; in many forms, especially those 

 producing swarm spores, there may be a substantial corpse, at 

 least as tangible as the rejected parental shell of the dividing 

 radiolarian. This is less often demonstrably the case in somatic 

 cells, and the analogy between strictly acellular organisms and 

 tissue cells cannot now be whole-heartedly maintained: it is 

 still generally held, however, that the outcome of a protozoan 



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