Figure 2: Hierarchical cell differentiation in self-renewing tissue.

(a) A model tissue produces terminally differentiated cells through n intermediate levels of partially differentiated cells. (b) Five microscopic events can occur with a cell: (i) symmetric cell division with differentiation, (ii) asymmetric cell division, (iii) symmetric cell division without differentiation, (iv) single cell differentiation and (v) cell death. To the right of each type of event present in optimal hierarchies we give the corresponding per cell rate that is used to derive equation (2).