Cancer stem cells in colorectal cancer from pathogenesis to therapy: Controversies and perspectives

Figure 2 Models of tumor development. (A) Stochastic model: every cancer cell isolated from the bulk tumor is tumorigenic and thus has the ability to proliferate extensively and initiate tumor growth. (B) Cancer stem cell (CSC) model: only a rare subpopulation of undifferentiated cells has the unique biological properties necessary for tumor initiation, maintenance, and spreading.