Estimating cell death in G₂M using bivariate BrdUrd/DNA flow cytometry

Background: In an accompanying paper (Asmuth et al.) it was found necessary to include cell death explicitly to estimate parameters of cell proliferation. The use of bivariate flow cytometry to estimate the phase durations and the doubling times of cells labeled with thymidine analogues is well established. However, these methods of analysis do not consider the possibility of cell death. This report demonstrates that estimating cell death in G₂/M is possible. Methods: Mathematical models for the experimental quantities, the fraction of labeled undivided cells, the fraction of labeled divided cells, and the relative movement were developed. These models include the possibility that, of the cells with G₂/M DNA content, only a certain fraction will divide, with the remainder dying after some time T_R. Simulation studies were conducted to test the possibility of using simple methods to estimate phase durations and cell death rates. Results: Cell death alters the estimates of phase transit times in a rather complex manner that depends on the lifetime of the doomed cells. However, it is still possible to obtain estimates of the phase durations of cells in S and G₂/M and the death rates of cells in G₂/M. Conclusions: The methods presented herein provide a new way to characterize cell populations that includes cell death rates and common measurements of cell proliferation.