Synchronous divisions in mass cultures of the ciliate protozoon Tetrahymena pyriformis 



Another advantage of Tetrahymena is that the cell cycle is so short (2J-2I hours at 

 the optimum temperature of 28-29 C.) that several cycles can be studied in succes- 

 sion in the course of a day. Also the cell is reasonably big, being 50-70 microns in 

 length. Some will consider it a serious limitation that the cytology is relatively little 

 known and furthermore that it is atypical of cells in general. As in other ciliates 

 there is an amitotic macronuclear and a mitotic micronuclear system. Our cells (the 

 Lwoff strain), like other strains of Tetrahymena which have been grown in pure culture 

 for a long time, do not possess a micronucleus and are therefore amitotic and asexual. 

 Recently, however, mating types of Tetrahymena, all of them having a micronucleus 

 of course, were isolated by Elliott and Gruchy (1952) and were further described by 

 Elliott and Hayes (1953). 



Chromosomes have been demonstrated in the dividing micronucleus of Tetra- 

 hymena, as in other ciliate micronuclei (Elliott and Hayes, 1953; Faure-Fremiet, 

 1953; Sonneborn, 1949). In the macronucleus of ciliates a ribonucleic-deoxyribonu- 

 cleic acid system is present, as in other kinds of cells, but morphologically it is organ- 

 ized in a very special way. In the macronucleus of the ciliates clearcut volume changes 

 (Popoff, 1908) and structural changes, simultaneous with the division of the micro- 

 nucleus and thus preceding cell division, can be revealed by the use of nucleic acid 

 stains (Faure-Fremiet, 1953; Sonneborn, 1949). 



CHARACTERIZATION OF STAGES 



The successive stages in the cell cycle may also be characterized by other means in 

 addition to the standard cytological ones, for instance by possible differences in the 

 capacity of the system for growth. A single stationary phase Tetrahymena cell was 

 introduced into a microrespirometer and the rate of respiration was accepted as a 

 measure of the respiring mass present at any time. The cell synthesized and multi- 

 plied in the respirometer. The results of a few runs (Zeuthen, 1953a) are shown in 

 Figure 1. From the onset of furrowing in one division until 10-20 minutes before the 

 next division there is a linear increase in mass ( ~ respiratory rate) . In the account 

 which follows this period will be called the 'synthetic phase'. Then follows a period 

 lasting 10-20 minutes with no further increase in mass. This will be called the 'pre- 

 division period'. When division of the cell begins the macronucleus is already some- 

 what stretched. A study of the literature on ciliates in general and on Tetrahymena in 

 particular makes it exceedingly likely that the 'pre-division period' covers or partly 

 coincides with the period of macronuclear swelling and reorganization referred to 

 above. At the end of the pre-division period, or at the beginning of division, synthesis 

 is resumed, and at twice the rate before 'pre-division'. Thus, on the basis of the curves 

 given in Figure 1 , and for purely practical reasons, we have divided the cell cycle in 

 Tetrahymena, growing on our media, into three phases: cell division coincides with 

 the early part of the synthetic phase, and in the pre-division period there is a block 

 to further rise in respiration. In support of this way of dividing the ciliate cell 

 cycle into physiological phases, reference is made to a number of papers in which 

 an attempt is made to follow cell volume from one division to the next. The situation 

 is reviewed by Richards (1941) and by Wichterman (1953). In the present writer's 

 opinion the overall picture given in their papers is very suggestive of that observed 



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