Stem cells are undifferentiated (non-specialized) cell types that can create identical copies of themselves (self-renew), while they can give rise to transit amplifying cells, which terminally differentiate into specialized cell types following a certain number of cell divisions. In principle, there are two categories of stem cells: First, embryonic stem cells, of which different classes mediate the development of an fertilized egg to an entire organism, and adult stem cells, which function to replenish tissue either on a regular basis (for example in the blood, skin or intestine) or following injury.
Advances in our understanding of tissue homeostasis and regeneration by adult stem cells together with the finding that cancers consist of heterogeneous cell populations (in terms of geno- and phenotype), led to the hypothesis that caners arise from stem-/transit-amplifying-cells, and/or contain stem-like cells that carry the ability to self-renew, to give rise to more differentiated progeny and importantly, to reform secondary tumors following serial transplantation into tumor-free hosts (e.g. immunodeficient mice). Many cancers, such as hematopoietic, colon, or skin cancers have been found to contain rare subpopulations of cells with biological properties similar to stem cells, named cancer stem cells (CSCs). Yet, other cancers, for example melanomas, seem to be less hierarchically organized and show a high degree of plasticity, where non-cancer stem cells can reversibly transition into a stem-like state. As these cells differ significantly from classical stem cells but share other stemness features, they are referred to as tumor-initiating cells (TICs) rather than CSCs. Regardless of this nomenclature, CSCs or TICs are considered to initiate, maintain and propagate (metastasis) cancer and are responsible for recurrences following therapy. Current cancer research focuses on identifying CSCs/TICs and understanding their biological properties as well as elucidating mechanisms that underlie plasticity. The ultimate goal is to develop drugs that target these most aggressive cancer cells to achieve sustained therapeutic responses.
Did you know that the cancer stem cell model does not exclude the stochastic, Darwinian evolution theory of cancer and both models need to be taken into account when aiming to explain cancer progression?
Did you know that the estimated number of CSC within a tumor may range from 1 out of 5 to 1 out of 100.000, depending on the method used for the analysis?