Fig 1. Schematics showing key concepts of the paper.

(a) Two types of symmetric divisions: self-renewals and differentiations. Each circle represents a cell, and i denotes the ith compartment, while i + 1 denotes the (i + 1)th compartment. Panels (b) and (c) demonstrate the division chains that replenish 8 differentiated cells eliminated from the top compartment. Dead cells are denoted by X’s and arrows show divisions. Cells are arranged in horizontal layers corresponding to compartments. Only the dividing cells are shown (for example, there may be more than 4 cells in the second to top compartment in panel (b)). In (b), the dead cells are replaced by a longer division tree, and in (c) by four shorter division trees.