Abstract
Development and homeostasis of the haematopoietic system is dependent upon stem cells that have the unique ability to both selfβrenew and to differentiate in all cell lineages of the blood. The crucial decision between haematopoietic stem cell (HSC) selfβrenewal and differentiation must be tightly controlled. Ultimately, this choice is regulated by the integration of intrinsic signals together with extrinsic cues provided by an exclusive microenvironment, the soβcalled haematopoietic niche. Although the haematopoietic system of vertebrates has been studied extensively for many decades, the specification of the HSC niche and its signals involved are poorly understood. Much of our current knowledge of how niches regulate longβterm maintenance of stem cells is derived from studies on Drosophila germ cells. Now, two recently published studies by Mandal et al.1 and Krezmien et al.2 describe the Drosophila haematopoietic niche and signal transduction pathways that are involved in the maintenance of haematopoietic precursors. Both reports emphasize several features that are important for controlling stem cell behavior and show parallels to both the vertebrate haematopoietic niche as well as the Drosophila germline stem cell niches in ovary and testis. The findings of both papers shed new light on the specific interactions between haematopoietic progenitors and their microenvironment. BioEssays 29:713β716, 2007. Β© 2007 Wiley Periodicals, Inc.