  Recent studies at individual cell resolution have revealed phenotypic
heterogeneity in nominally clonal tumor cell populations. The heterogeneity
affects cell growth behaviors, which can result in departure from the idealized
uniform exponential growth of the cell population. Here we measured the
stochastic time courses of growth of an ensemble of populations of HL60
leukemia cells in cultures, starting with distinct initial cell numbers to
capture a departure from the {uniform exponential growth model for the initial
growth (``take-off'')}. Despite being derived from the same cell clone, we
observed significant variations in the early growth patterns of individual
cultures with statistically significant differences in growth dynamics, which
could be explained by the presence of inter-converting subpopulations with
different growth rates, and which could last for many generations. Based on the
hypothesis of existence of multiple subpopulations, we developed a branching
process model that was consistent with the experimental observations.
