(B and B') Number of ECs over time (left), number of EBs over time (middle), and probabilities of EB loss through apoptosis (right graph). The simulation starts with 1000 EC cells in all figures In (B) a challenge at day 1 imposes a loss of 500 EC cells (black dashed vertical line). In (B') two successive challenges are imposed: one at day 1, reducing by half the number of ECs (as in B), and a second one at day 3, causing a loss of 200 cells. 