A mathematical model for stochastic fate specification describes and predicts germ cell population dynamics in culture. (A) Histogram of cell cycle length in groups of retrospectively identified SSCs (n = 110), differentiating mortal cells (n = 76, synchronized sibling divisions are counted as one), and all cells (n = 186). No significant difference was observed between the SSCs and the differentiating cells. Mean cell cycle length for all cells was 40.0 ± 8.2 h. (B and C) The mathematical model (red line) fits the measured germ cell growth (B, black dots), and the measured germ cell survival curve (C, black dots; mean ± SEM, n = 84 in three independent experiments). (D) The mathematical model predicts a constant SSC content of 17.9% over time (red bars). Dark gray bars indicate the measured SSC content at the indicated time points (mean ± SEM, n > 4).