Like other somatic stem cells, hematopoietic stem cells (HSC) contain the capacity to self-renew and to give rise to committed progenitor cells that are able to replenish all hematopoietic cell types. To keep a constant level of HSC, the decision whether their progeny maintain the stem cell fate or become committed to differentiation needs to be highly controlled. In this context it became evident that HSC niches fulfill important functions in keeping the level of HSC more or less constant. Before discovering such niches, it was widely assumed that HSC divide asymmetrically to give birth to a daughter cell maintaining the stem cell fate and to another one which is committed to differentiation. Here, I summarize some of the experimental data being compatible with the model of asymmetric cell division and review some of our latest findings, which demonstrate the occurrence of asymmetric cell divisions within the HSC and hematopoietic progenitor cell compartment. Since cell polarity is an essential prerequisite for asymmetrically dividing as well as for migrating cells, I will also discuss some aspects of cell polarity of primitive hematopoietic cells.
(c) 2007 S. Karger AG, Basel.