Figure 2.

The cancer stem cell model and clonal evolution. A: The traditional cancer stem cell model postulates that a tumor-initiating cell (TIC) gives rise to distinct tumor cell types through a lineage hierarchy. Within the tumor, only the TIC and not its progeny can generate successful tumor grafts in a tumor-initiating assay (red arrows). The resulting grafts recapitulate the hierarchical organization and phenotype of the parental tumor. B: In a clonal evolution model, a tumor may contain multiple distinct tumor-initiating cells that in turn generate clonal diversity and resulting intratumor heterogeneity. These different tumor-initiating cells may differ in their tumor-propagating capabilities in serial grafting assays.