Fig. 4

Asymmetric or symmetric division in tumor development. Asymmetric division of cancer stem cells happens frequently in early stage of cancer, which not only maintains the pool of cancer stem cells, but also creates tumor heterogeneity (left). Tumors at this stage are usually well differentiated. However, the balance between asymmetric and symmetric division is broken in late stage of cancer. Increased symmetric division of cancer stem cells enables tumor to have higher proliferative capacity thus contributes to tumor progression (right). Tumors at this stage are usually poorly differentiated