Figure 2.

Cell behavior in IFE. Analysis of large clonal lineage-tracing studies reveals mouse tail epidermis containing two populations of proliferating cells, stem cells (green), and progenitor cells (blue), along with differentiated basal cells (red) that have exited the cell cycle and are waiting to stratify. Epidermal maintenance is achieved by the self-renewing population of progenitors. As a differentiating cell leaves the basal layer through stratification, a neighboring progenitor divides. Progenitor division occurs once a week on average, and results in two progenitors, two differentiated cells, or one cell of each type as shown. The outcome of a given progenitor division is random, but the probabilities are balanced, so homeostasis is achieved across the progenitor population. Stem cells (green) are clustered around the hair follicles and under the edges of the overlying scales, and only divide every 3 months, generating stem or progenitor daughters with the probabilities shown. Stem cells make a negligible contribution to tissue maintenance, but proliferate following wounding.