To be or not to be a stem cell: dissection of cellular and molecular components of haematopoietic stem cell niches.

Recent studies have identified multiple cell types that regulate haematopoietic stem cells (HSCs); however, proof that a specific cell type produces a specific factor important for HSC function and maintenance is largely lacking. Ding et al (2012) reported recently that conditional deletion of stem cell factor (SCF) in Leptin receptor (Lepr) expressing perivascular cells or endothelial and haematopoietic cells resulted in significant reductions in number but less profound reduction in function of HSCs. Although the long-term fate of HSCs in these models is largely unexplored and an underlying mechanism for reduction in HSCs not yet reported, these findings further implicate the vascular niche in the functional maintenance of HSCs in vivo and also raise intriguing questions for future studies in this field. The haematopoietic stem cell (HSC) niche has traditionally been considered a discrete site within the bone marrow; however, recent studies have shown that numerous cell types are critically important for HSC regulation and maintenance (Wang and Wagers, 2011). Imaging studies have shown that phenotypic HSCs can be found adjacent to osteoblasts or osteoprogenitor cells on the inner surface of trabecular bone, and genetic studies have further shown that expansion of trabecular bone, leads to expansion of HSCs (Calvi et al, 2003; Zhang et al, 2003; Lo Celso et al, 2009; Xie et al, 2009). Other studies have found that phenotypic HSCs