Hematopoietic cells and replicative senescence.

Replicative senescence describes the finite cell replicative capacity in response to chronic proliferative stimulation. A key element in this process is the shortening of the telomeres, which to a major extent is caused by the lack of expression of telomerase. Whereas this situation has been well documented for a variety of somatic cell types, the question of whether stem cells "senesce" in the course of enforced chronic sequential divisions is as yet unresolved. This article examines several distinct features of hematopoietic cells (HC) in light of their similarity to certain aspects of memory T cells. It appears that although the capacity of HC for replication is not exhausted under normal physiological conditions in vivo, under certain experimental conditions and in specific in clinical situations HC do show signs of telomere shortening. This limited potential should be taken into account both with respect to aging in vivo, and also in terms of attempts to expand these cells ex vivo for therapeutic use.