Cell turnover and adult tissue homeostasis: from humans to planarians.

Many fully developed metazoan tissues remain in a state of flux throughout life. During physiological cell turnover, older differentiated cells are typically eliminated by apoptosis and replaced by the division progeny of adult stem cells. Independently, each of these processes has been researched extensively, yet we know very little about how cell death and stem cell division are coordinated in adult organs. Freshwater planarians are an attractive model organism for research in this area. Not only do they undergo a very high rate of somatic cell turnover throughout life, but experimental tools are now available to study this process in vivo. Together, these attributes provide an opportunity to investigate the mechanisms, functions, and regulation of cell turnover in adult tissues.