230
H . F I R K E T
cells doubles almost within a few hours. In the presence of amethopterin, a folic acid analogue, HeLa cells are unable to synthesize thymidine; without this metabolite, these cells cannot reproduce normal
D N A and, thus, are prevented from dividing. After addition of this
nucleoside to the nutrient medium, D N A synthesis can again occur and
6—8 h afterwards an increase of up to 70% ofthe cell number has been
observed in a very short period (Rueckert and Mueller, 1960).
Several purine and pyrimidine analogues, among which fluorodeoxyuridine is the most used, have a similar blocking action also
sharply reversed by the addition of thymidine (EidinofT and Rich,
1959, etc.). Curiously, an excess of this metabolite also blocks D N A
synthesis in Chang appendix-cell cultures. D N A production is resumed
when the medium is normalized again and there is a sharp mitotic peak
a little less than 10 h later (Xeros, 1962). Mitotic HeLa cells are not
attached so firmly to the glass as intermitotic cells. Terasima and
Tolmach (1963) make use of this difference to separate mitotic cells and
induce in their descendants a considerable amount of synchrony.
In all these cases however, synchronism is not complete. It can be
evaluated by the technique of Engelberg (1961). Cells that are in the S
stage when the block occurs, have less to accomplish in order to be ready
for the next mitosis when it is lifted. Thus, they divide before the main
group. Cells may also be rendered somewhat abnormal by these treatments. More work along these lines is obviously needed. (See also
Chapter 12.)
V I . C O N C L U S I O N S
T w o moments of cell division are particularly sensitive to interference: they occur just before prophase (end of G 2 ) and at metaphase.
The number of agents found to perturb the normal development of
division at these two moments is considerable. Within both groups very
different substances or conditions are found. So it is unlikely that, in
each case, only a single point of attack is concerned. Probably it is at
these two moments that the metabolism of the cell is most profoundly
engaged towards this supreme accomplishment, this death and rebirth
that we call division. Another important moment that can be submitted
to more specific attack is the beginning of D N A synthesis.
In this chapter many important aspects of mitosis (energy exchanges,
chemistry of the spindle, etc) have hardly been touched since Tissue
Culture has contributed little information on them. However, it is fairly
clear that most of the problems of cell division confronting the general
biologist and the pathologist have been or could be studied more
accurately in tissue or cell culture than in the animal body.
H . F I R K E T
cells doubles almost within a few hours. In the presence of amethopterin, a folic acid analogue, HeLa cells are unable to synthesize thymidine; without this metabolite, these cells cannot reproduce normal
D N A and, thus, are prevented from dividing. After addition of this
nucleoside to the nutrient medium, D N A synthesis can again occur and
6—8 h afterwards an increase of up to 70% ofthe cell number has been
observed in a very short period (Rueckert and Mueller, 1960).
Several purine and pyrimidine analogues, among which fluorodeoxyuridine is the most used, have a similar blocking action also
sharply reversed by the addition of thymidine (EidinofT and Rich,
1959, etc.). Curiously, an excess of this metabolite also blocks D N A
synthesis in Chang appendix-cell cultures. D N A production is resumed
when the medium is normalized again and there is a sharp mitotic peak
a little less than 10 h later (Xeros, 1962). Mitotic HeLa cells are not
attached so firmly to the glass as intermitotic cells. Terasima and
Tolmach (1963) make use of this difference to separate mitotic cells and
induce in their descendants a considerable amount of synchrony.
In all these cases however, synchronism is not complete. It can be
evaluated by the technique of Engelberg (1961). Cells that are in the S
stage when the block occurs, have less to accomplish in order to be ready
for the next mitosis when it is lifted. Thus, they divide before the main
group. Cells may also be rendered somewhat abnormal by these treatments. More work along these lines is obviously needed. (See also
Chapter 12.)
V I . C O N C L U S I O N S
T w o moments of cell division are particularly sensitive to interference: they occur just before prophase (end of G 2 ) and at metaphase.
The number of agents found to perturb the normal development of
division at these two moments is considerable. Within both groups very
different substances or conditions are found. So it is unlikely that, in
each case, only a single point of attack is concerned. Probably it is at
these two moments that the metabolism of the cell is most profoundly
engaged towards this supreme accomplishment, this death and rebirth
that we call division. Another important moment that can be submitted
to more specific attack is the beginning of D N A synthesis.
In this chapter many important aspects of mitosis (energy exchanges,
chemistry of the spindle, etc) have hardly been touched since Tissue
Culture has contributed little information on them. However, it is fairly
clear that most of the problems of cell division confronting the general
biologist and the pathologist have been or could be studied more
accurately in tissue or cell culture than in the animal body.