Cancer stem cells, a family expressed by all cancers, are necessary for the origin and critical for the growth of all cancers. In the latter tissue the cancer stem cells are concentrated and operative in small areas called niches, analogous to the stromas of non-cancer tissues.  In addition to cancer stem cells, the niches contain normal cancer cells and various types of cooperative non-cancer cells. Most important among the latter are the mesechymal stem cells, analogous to those expressed by non-cancer tissues, that are important, and in some cases necessary for the development of several cancers. Various effects dependent on the already mentioned, two types of stem cells are induced not directly by them, but via their secreted extracellular vesicles, the exosomes and ectosomes. Additional cancer-cooperative cells, concentrated within the niches, include fibroblasts and immune cells that participate in the development of some cancer-relevant processes including angiogenesis. Cancer stem cells govern many critical processes of cancer life, from initiation and progression, including generation and distribution of metastases, therapy resistance, and cancer-relapse, as well as additional severe tissue problems such as efflux of anti-cancer drugs, autophagy, and evasion of immune surveillance. Without their stem cells specific cancers cannot develop. In case of defect, the cancer stem cells are induced by transformation of either normal cancer cells or mesenchymal stem cells. The present knowledge about cancer stem cells and their key effects are of critical importance in the development of therapeutic processes addressed against these cells.

Mesenchymal stem cells, Cancer stem cells, Extracellular vesicles, Niches, Cancer differentiation, Cancer initiation, Progression and relapse, Metastasis, Cancer microenvironment, Therapy targets