ARE TUMORS DERIVED FROM TISSUE STEM CELLS?
The maintenance of tissue function throughout the life of an organism requires replenishment of terminally differentiated cells by cells derived from a proliferative compartment. The proliferative population is in turn maintained by tissue stem cells. These undergo asymmetrical division, a mitosis that results in the generation of a daughter stem cell and a cell that will enter a number of symmetrical divisions, thus effectively amplifying the cell population that will eventually differentiate. The length of time required for the genesis of naturally occurring sporadic tumors, together with the fact that malignant tumors result from the accumulation of transmissible (genetic or epigenetic) alterations of the somatic cells, suggests that tissue stem cells may be the substrate for carcinogenesis. Inside this issue, Böcker et al (Lab Invest 2002, 82: 737–745) present evidence suggesting that tissue stem cells responsible for the maintenance of breast function can be identified in the mamm