This review will discuss the mechanisms of repair and regeneration in various tissue types and how dysregulation of these mechanisms may lead to
cancer. Normal tissue homeostasis involves a careful balance between cell loss and cell renewal. Stem and progenitor cells perform these
biologic processes as the functional units of regeneration during both tissue homeostasis and repair. The concept of tissue stem cells capable of giving rise to all differentiated cells within a given tissue led to the concept of a cellular hierarchy in tissues and in
tumors. Thus, only a few cells may be necessary and sufficient for tissue repair or
tumor regeneration. This is known as the hierarchical model of
tumorigenesis. This report will compare this model with the stochastic model of
tumorigenesis. Under normal circumstances, the processes of tissue regeneration or homeostasis are tightly regulated by several morphogen pathways to prevent excessive or inappropriate cell growth. This review presents the recent evidence that dysregulation of these processes may provide opportunities for
carcinogenesis for the long-lived, highly proliferative tissue stem cell population. New findings of
cancer initiating tissue stem cells identified in several solid and circulating
cancers including breast, brain and hematopoietic
tumors will also be reviewed. Finally, this report reviews the cellular biology of
cancer and its relevance to the development of more effective
cancer treatment protocols.