6. C E L L D I V I S I O N
229
division. O n the contrary, negative results may sometimes be more
illuminating: fluoro-acetate blocks the tricarboxylic cycle, but has no
effect on the process of cell division or the growth of fibroblasts (Allsop
and Fell, 1950). Thus, this cell can find the energy for division elsewhere. Mitosis can also occur in cells markedly infected by viruses
(Wheelock and Tamm, 1959; Fastier, 1960). This will probably soon
be translated into chemical terms.
Less definable conditions also affect the frequency of division, for
instance the localization of the cell in the culture or wounding of the
growth area. As similar problems are found in the animal or human
body, and cultures being simpler systems, it might be useful to take up
again the study of what are, to the tissue culturist, classical facts: in
explant cultures, the percentage of mitosis is highest, not at the extreme
border where cells are scattered, nor in the inner part of the migration
zone where the population is dense, but in an intermediate ring area
(Willmer, 1933) where the incorporation of thymidine (Firket, unpublished) and the production of alkaline phosphatase (Chevremont
and Firket, 1949) are also most active. It is also classical that when a
sector of such a culture is removed, cell division and migration fill up
the gap and no more. The analogy with wound healing in the body is
striking and our understanding of the phenomenon no better. It seems
however that some chemical analysis of the micro-environment of the
cells is now becoming possible and should be undertaken.
V . S Y N C H R O N I S M OF D I V I S I O N S IN C U L T U R E S
Variations of metabolism during the cell cycle and many of the
problems just mentioned could be more easily studied if all the cells of a
culture could be induced to divide at the same time. Such synchrony is
naturally occurring—at least temporarily—in various cleaving eggs
and can be obtained in Protozoa by several procedures, mainly temperature shocks. Considerable information has resulted in recent years
from the study of these systems (Zeuthen, 1958). In spite of various
attempts, vertebrate cells in culture are less responsible to the experimental inducement of synchrony. Waves of mitoses have been produced
by sudden improvement of a deficient medium (Jacoby et al., 1937;
Jacoby, 1937, 1949) or a sudden return to 37°C after a period of cooling
(Spear, 1928; S. Chevremont et al., 1957).
Several promising techniques have been developed recently. Newton
and Wildy (1959) give a cold shock (4°C) for 1 h to HeLa cultures and
then bring them back to 37°C. Mitoses in progress are abortive and no
increase of cell number is found for about 17 h. Then, the number of
229
division. O n the contrary, negative results may sometimes be more
illuminating: fluoro-acetate blocks the tricarboxylic cycle, but has no
effect on the process of cell division or the growth of fibroblasts (Allsop
and Fell, 1950). Thus, this cell can find the energy for division elsewhere. Mitosis can also occur in cells markedly infected by viruses
(Wheelock and Tamm, 1959; Fastier, 1960). This will probably soon
be translated into chemical terms.
Less definable conditions also affect the frequency of division, for
instance the localization of the cell in the culture or wounding of the
growth area. As similar problems are found in the animal or human
body, and cultures being simpler systems, it might be useful to take up
again the study of what are, to the tissue culturist, classical facts: in
explant cultures, the percentage of mitosis is highest, not at the extreme
border where cells are scattered, nor in the inner part of the migration
zone where the population is dense, but in an intermediate ring area
(Willmer, 1933) where the incorporation of thymidine (Firket, unpublished) and the production of alkaline phosphatase (Chevremont
and Firket, 1949) are also most active. It is also classical that when a
sector of such a culture is removed, cell division and migration fill up
the gap and no more. The analogy with wound healing in the body is
striking and our understanding of the phenomenon no better. It seems
however that some chemical analysis of the micro-environment of the
cells is now becoming possible and should be undertaken.
V . S Y N C H R O N I S M OF D I V I S I O N S IN C U L T U R E S
Variations of metabolism during the cell cycle and many of the
problems just mentioned could be more easily studied if all the cells of a
culture could be induced to divide at the same time. Such synchrony is
naturally occurring—at least temporarily—in various cleaving eggs
and can be obtained in Protozoa by several procedures, mainly temperature shocks. Considerable information has resulted in recent years
from the study of these systems (Zeuthen, 1958). In spite of various
attempts, vertebrate cells in culture are less responsible to the experimental inducement of synchrony. Waves of mitoses have been produced
by sudden improvement of a deficient medium (Jacoby et al., 1937;
Jacoby, 1937, 1949) or a sudden return to 37°C after a period of cooling
(Spear, 1928; S. Chevremont et al., 1957).
Several promising techniques have been developed recently. Newton
and Wildy (1959) give a cold shock (4°C) for 1 h to HeLa cultures and
then bring them back to 37°C. Mitoses in progress are abortive and no
increase of cell number is found for about 17 h. Then, the number of