Mitotic coincidence of chick embryo hepatocytes in vivo and the transition probability model of the cell cycle

The initial accumulation of carbamoylphosphate synthetase in embryonic rat hepatocytes, and the cell cycle

The hormone-induced expression of the hepatocyte-specific enzyme carbamoylphosphate synthetase can take place in each phase of the cell cycle and is not restricted to the G1 or the G0 phase. To arrive at this conclusion, the cell cycle parameters of embryonic day 14 rat hepatocytes in vitro were determined by autoradiography after labeling with (3H)-TdR or with (3H)- and (14C)-TdR. An S-phase of approximately 14 h, a G2 + M-phase of 8 h, a G1-phase of 8-13 h and a total cell cycle of 30-35 h were measured. Freshly isolated embryonic hepatocytes have exponential growth parameter values, but shift to a steady state growth under culture conditions in the presence of hormones (glucocorticosteroids, thyroid hormones and cyclic AMP). The length of the S-phase and of the total cell cycle remain constant during the culture time. The time course of accumulation of carbamoylphosphate synthetase protein in embryonic hepatocytes is identical in all phases of the cell cycle. It is suggested that hormones, in particular glucocorticosteroids, simultaneously and independently regulate growth mode and gene expression in developing hepatocytes. The nucleotide-analogue 5-bromodeoxyuridine inhibits the hormone-induced expression of carbamoylphosphate synthetase only in cells that are exposed to the drug during early S-phase, indicating replication of the carbamoylphosphate synthetase gene in that part of the cell cycle.

A general method for evaluating the cell cycle time in mammalian epidermis

A method is proposed for the evaluation of the cell cycle time of in vivo steady state populations. It requires only an accurate estimation of either the labelling index or the mitotic index and an approximate evaluation of the period between the initiation of the S phase and the completion of mitosis. The method proposed takes into account the fact that the cell cycle is non-deterministic, the angle at which cells divide and the effect of cell extrusion from the basal layer, plus the fact that the cells can only be extruded when in certain phases of the cell cycle. Limitations of other methods recently reported are discussed.