\iu I LIBRA 



Synchronous divisions in mass cultures of the 



ciliate protozoon Tetrahymena pyriformis., as 



induced by temperature changes 1 



by 



ERIK ZEUTHEN and OTTO SCHERBAUM 



fyofysiologisk Laboratorium, Kobenhavns Universitet 



INTRODUCTION 



Nature has supplied us with a few cases in which neighbouring cells in a cell com- 

 munity are in phase so that they more or less simultaneously pass through the only 

 recognizable part of the cell cycle, namely cell division. Stimulating examples are 

 presented in d'Arcy Thompson's (191 7 and 1942) book, and in the review of Sonnen- 

 blick (1950) dealing with early embryonic stages in the insect egg. The fertilized 

 insect egg, like many other syncytia and plasmodia, shows a perfect synchronization 

 of nuclear division for many generations, and, as a consequence, the first cell division 

 which cuts out the blastoderm of about 2,000 cells is synchronous in the whole 

 embryo. Apparently, chemical situations identical throughout a syncytium or a 

 Plasmodium induce nuclear and cell division. In cell suspensions cell borders may 

 act as barriers to the diffusion of controlling agents; and in tissues where cells occupy 

 relatively fixed positions in a medium which is not mixed, distance alone would 

 prevent chemical interaction between any considerable number of cells. 



Whenever synchrony of division is observed in a tissue it is therefore limited to 

 groups of cells all within a short distance of each other. This observation is known to 

 many workers in histology and tissue culture. It may be significant that in spermato- 

 genic tissues where cells line a lumen in which free diffusion and even some mixing 

 may take place, we have the prettiest cases — still limited to a small mass of cells — 

 of synchronous growth and division. 



THE GOAL AND THE WORKING HYPOTHESIS 



For the reasons mentioned above we are in a difficult position when it is a question 

 of relating growth to the basic phenomena of synthesis and multiplication in the 

 cell cycle. The goal we had in mind was therefore that of establishing a synchronized 

 system in which the cell cycle could be studied on aliquots representing successive 

 stages in the cycle. Such a system would serve as a useful substitute to the few already 

 known, such as egg material and plant spores (Erickson, 1948). The introductory 



1 This work was supported by grants from Rask-0rsted Fondet and from the Eli Lilly Foundation. 



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