220
H . F I R K E T
of apparently healthy cells do not synthesize D N A (or at least do not
take up any labelled precursor of this substance) for periods of up to
several days. This prevents their division. They seem not to take part
in the cell cycle at all. This is found both in explant cultures (Firket,
1958a) and in cell cultures (Stanners and Till, 1960) where the nutritional environment and crowding are more uniform.
It will be shown that it is mainly one part of the cycle that contributes to the variability sometimes within the same culture. Even twin
cells, not far from each other, may exhibit differences of 2-8 h in the
duration of the intermitotic period (Jacoby, 1937, 1949; Hsu, 1960;
Sisken and Kinosita, 1961a). One can only guess at the reasons for these
differences among genetically identical cells in the same medium.
B. M O R P H O L O G I C A L C H A N G E S
No spectacular modifications in the morphology of the cells are
visible during the intermitotic period, but quantitative changes can be
observed. Bucher and Kloti (1955) have followed the increase in
volume of the nuclei of fibroblasts. Rapid after division, it is slow and
irregular throughout the remaining part of interphase. A small but
constant contraction precedes prophase by about 1 h. Nucleolar changes
are parallel. Variations of volume could be only a sign of modifications
in water content. But measurements of concentration with the interference microscope complete this picture. Nuclear and cytoplasmic
dry weight changes are parallel. The post-telophase rise in volume is
due to a gain in mass and a simultaneous drop of concentration. In
HeLa cells, the maximum imbibition is obtained 8-10 h after division.
The preprosphasic contraction is purely a loss of water with no decrease
of the dry weight (Sandritter, Schiemer, Kraus and Dorrien, 1960).
Mitochondrial modifications are not dissimilar. The number of
mitochondria, or rather the amount of mitochondrial substance increase
for several hours after mitosis. This stops when the total amount is
readjusted to the mean level characteristic of intermitotic fibroblasts.
The chondriome then remains quantitatively constant until some time
before the next division, when there is a small, but unmistakable,
decrease (Frederic, 1958).
C. M E T A B O L I C C H A N G E S (DNA C Y C L E )
As could be expected, the most important changes during the cell
cycle are not morphological, but chemical. In all materials studied,
from Viciafaba root tip (Howard and Pelc, 1953; Woodard, Rasch and
Swift, 1961) and Protozoa (Prescott, 1960) to various mammalian cell
types in culture (Painter and Drew. 1959; Stanners and Till, 1960),
H . F I R K E T
of apparently healthy cells do not synthesize D N A (or at least do not
take up any labelled precursor of this substance) for periods of up to
several days. This prevents their division. They seem not to take part
in the cell cycle at all. This is found both in explant cultures (Firket,
1958a) and in cell cultures (Stanners and Till, 1960) where the nutritional environment and crowding are more uniform.
It will be shown that it is mainly one part of the cycle that contributes to the variability sometimes within the same culture. Even twin
cells, not far from each other, may exhibit differences of 2-8 h in the
duration of the intermitotic period (Jacoby, 1937, 1949; Hsu, 1960;
Sisken and Kinosita, 1961a). One can only guess at the reasons for these
differences among genetically identical cells in the same medium.
B. M O R P H O L O G I C A L C H A N G E S
No spectacular modifications in the morphology of the cells are
visible during the intermitotic period, but quantitative changes can be
observed. Bucher and Kloti (1955) have followed the increase in
volume of the nuclei of fibroblasts. Rapid after division, it is slow and
irregular throughout the remaining part of interphase. A small but
constant contraction precedes prophase by about 1 h. Nucleolar changes
are parallel. Variations of volume could be only a sign of modifications
in water content. But measurements of concentration with the interference microscope complete this picture. Nuclear and cytoplasmic
dry weight changes are parallel. The post-telophase rise in volume is
due to a gain in mass and a simultaneous drop of concentration. In
HeLa cells, the maximum imbibition is obtained 8-10 h after division.
The preprosphasic contraction is purely a loss of water with no decrease
of the dry weight (Sandritter, Schiemer, Kraus and Dorrien, 1960).
Mitochondrial modifications are not dissimilar. The number of
mitochondria, or rather the amount of mitochondrial substance increase
for several hours after mitosis. This stops when the total amount is
readjusted to the mean level characteristic of intermitotic fibroblasts.
The chondriome then remains quantitatively constant until some time
before the next division, when there is a small, but unmistakable,
decrease (Frederic, 1958).
C. M E T A B O L I C C H A N G E S (DNA C Y C L E )
As could be expected, the most important changes during the cell
cycle are not morphological, but chemical. In all materials studied,
from Viciafaba root tip (Howard and Pelc, 1953; Woodard, Rasch and
Swift, 1961) and Protozoa (Prescott, 1960) to various mammalian cell
types in culture (Painter and Drew. 1959; Stanners and Till, 1960),