Growth and senescence of heterogeneous human mammary epithelial cells cultures can be accurately modeled using the agent-based model. (A and B) In a model extending over several passages, cells are initially plated at 20% confluence and replated when the populations reach 80% confluence. By Passage 3, most human mammary epithelial cells (HMEC) have undergone seven divisions, and enter stasis. As a result, the time to confluence is extended, and a population doubling plateau is observed. Ultimately, the variant human mammary epithelial cells (vHMEC) take up the available area, and exponential growth resumes. (C) Changing the initial density of vHMEC leads to variable plateau periods. This dependence is fitted by a power function that allows the calculation of the initial density of vHMEC present in the primary culture based on plateau length. The results of individual simulations are plotted as points.