Total Generation Time Research Articles

Life Cycle Analysis of Mammalian Cells: II. Cells from the Chinese Hamster Ovary Grown in Suspension Culture

A method for life cycle analysis in mammalian cells which utilizes the collection function has been applied to the Chinese hamster ovary grown in suspension. The following durations were found for the various parts of the life cycle: S, 4.13 hours; G1, 4.71 hours; G2, 2.81 hours; mitosis, 0.81 hours. The cell has a total generation time of 12.4 hours as opposed to 20.1 hours for the S3 HeLa cell. However, the relative lengths of each phase of the life cycle are identical within experimental uncertainty in the two cells.

Patterns of epidermal cell division: I. Genetic behavior of the G 1-cell population

Fig. 2 provides a visual synopsis of the background information and of the main findings in the present report. The concept [5, 6] of separate G 1- and G 2-epidermal cell populations depicted in Fig. 2 was confirmed and extended. It now appears that G 2-population cells can hold up in the G 2 period of the cell cycle for as long as 5 days before entering mitosis. Experiments are in progress to determine the maximum length of time these epidermal cells can remain in the G 2 phase. The answer to the main question in the present study is illustrated in the lower diagrams of Fig. 2. G 1-population epidermal cells are genetically discrete—in the sense that they maintain and propagate the G 1-pattern of cell division from one generation to the next. The results in the present study indicate that the epidermis also maintains a continuous supply of G 2-population epidermal cells. Their origin and genetic behavior are under investigation. It was also noted that thymidine label remains metabolically available in the blood for a considerable period of time. In one experiment, 14C-thymidine was injected at least 45 min prior to stimulation of DNA synthesis and yet a full complement of appropriate epidermal cells were 14C-labeled. This observation supports several recent views on this question [3, 4, 7]. Finally, it was shown that proliferating epidermal cells do not necessarily leave the basal layer, differentiate and slough off after division [8, 9, 10], but that they are capable of repeated divisions. This was demonstrated by initiating and tracing two successive divisions in the same epidermal cells. The total cell generation time, from one mitosis to the next, can take place within 3 days and the G 1 phase of the cycle can be reduced to less than 24 hr.

Mitotic cycle of the kala-azar parasite, Leishmania donovani.

SUMMARY: The growth rate and the time schedule of the mitotic cycle of the lepto-monads of Leishmania donovani, the Kala-azar parasite, were studied. The numbers of cells in the different division stages were estimated by photomicroscopy after Feulgen staining. The observed time intervals of resting, prophase, metaphase, anaphase, telophase and binucleate condition were 15.2, 1.1, 3.9, 1.1, 0.9 and 1.8 hr., respectively; the total generation time was 24 hr.

Cell population kinetics in the ear epidermis of mice

Data on fast-growing cell populations in the mouse have suggested that the “doubling sequence” consisting of the S, G 2, and mitotic phases is of the constant duration of approximately 9 hr; differences in generation times seem to be largely due to different durations of the G 1 phase. This hypothesis was tested in a cell population which grows at a moderately slow rate, viz., the epidermis of the ear, which has a generation time of about 3 weeks. The durations of the various phases of proliferation and maturation were estimated by labeling with tritiated thymidine and obtaining the following data. fraction of cells labeled soon after labeling, fraction of mitoses labeled at various times after labeling, fraction of cells in the basal layer, and distribution of labeled cells. The following estimates were obtained: total generation time, about 24 days; S phase, about 30 hr; G 2 plus early prophase, about 6.5 hr; mid-portion of mitosis, about 3.8 hr; G 1 phase, more than 22 days; lifetime of mature cell, 20 1 2 days. Since the total doubling time is four times as long as in fast-growing cells, the hypothesis tested is disproved. On the other hand, the fraction of the generation time occupied by G 1 is much greater than in fast-growing tissues.

DNA synthesis in individual L-strain mouse cells

The time relationship between DNA synthesis and mitosis has been determined for L-strain mouse cells cultivated in vitro. DNA synthesis was detected autoradiographically by following the uptake of [ 3H]thymidine into the cell nucleus. DNA synthesis in this cell system was found to take place in an approximately linear fashion over a single period of six to seven hours, ending three to four hours before mitosis, for a total generation time of twenty hours. A mathematical treatment for exponentially multiplying cultures is presented.