The haematopoietic stem cell and its clonal progeny: mechanisms regulating the hierarchy of primitive haematopoietic cells.

Transplantation and marking studies have provided an accurate description of various aspects of developmental and proliferative behaviour of totipotent haematopoietic stem cells. In particular, the remarkable ability of different clones to contribute to all lineages in a continuous, stable and long term manner is a hallmark of stem cell behaviour. Taken together, in vivo reconstitution experiments also provide the basis for a model of stem cell regulation that incorporates stochastic and non-stochastic components. More recent approaches provide optimism that many descriptive aspects of stem cell biology will soon be supported by elucidation of molecular mechanisms.