Fig. 1.

Scheme of the hierarchical stem cell model in healthy and malignant tissue. a Asymmetric cell division of a stem cell (SC; depicted as dark blue cells) in normal tissue results in the generation of a daughter stem cell as well as committed and dividing progenitor cells that can give rise to terminally differentiated cells (shown as light blue cells) of the given tissue. b Genetic and/or epigenetic alterations can transform stem cells and/or progenitor cells, leading to the escape from intracellular and extracellular control mechanisms that restrain aberrant cell proliferation and uncontrolled tissue growth. Constant self-renewal and the production of heterogeneous malignant progeny is considered a hall mark of cancer stem cells (CSC). The CSC model in malignant tissue represents a hierarchical organization, where rare self-renewing and long-lived CSC give rise to the tumor mass consisting of heterogeneous cancer cells with variable degree of differentiation and proliferative capacity (orange cells). CSC are more resistant to radiation- and chemotherapy calling for targeted approaches that eliminate CSC in multi-modal treatment strategies [134]