ON THE EXISTENCE OF A Go‐PHASE IN THE CELL CYCLE

ABSTRACT The model is based on the assumption that the cell cycle contains a G o ‐phase which cells leave randomly with a constant probability per unit time, γ. After leaving the G o ‐phase, the cells enter the C‐phase which ends with cell division. The C‐phase and its constituent phases, the‘true’G 1 ‐phase, the S‐phase, the G 2 ‐phase and mitosis are assumed to have constant durations of T , T 1 T s , T 2 and T m , respectively. For renewal tissue it is assumed that the probability per unit time of being lost from the population is a constant for all cells irrespective of their position in the cycle. The labelled mitosis curve and labelling index for continuous labelling are derived in terms of γ, T , and T s . The model generates labelled mitosis curves which damp quickly and reach a constant value of twice the initial labelling index, if the mean duration of the G o ‐phase is sufficiently long. It is shown that the predicted labelled mitosis and continuous labelling curves agree reasonably well with the experimental curves for the hamster cheek pouch if T has a value of about 60 hr. Data are presented for the rat dorsal epidermis which support the assumption that there is a constant probability per unit time of a cell being released from the G o ‐phase.