Understanding the haemopoietic stem cell niche

Abstract

The concept of a bone marrow (BM) stem cell ‘niche' was first proposed by Schofield (1978), who suggested that the haemopoietic potential of residing stem cells (HSC) was due to a complex interaction of signals between HSC themselves and their immediate microenvironment. However, until recently the components and regulatory role of the ‘niche' have remained relatively theoretical, due to the inability of identifying HSC <i>in situ</i>. Many studies have now shown that HSC preferentially reside within the endosteal region at the bone and BM interface and considerable evidence suggests that HSC are regulated by adhesive interactions to the stromal cell synthesised microenvironment within this region. Our data demonstrate a discrete spatial localisation of transplanted haemopoietic cells within the BM as a result of specific, hierarchically dependent patterns of migration. Our ability to isolate defined populations of HSC from the endosteal region, allowed for a more accurate characterisation of the components and the regulatory capabilities of the HSC niche. We demonstrated that Lin^–Sca-1⁺c-kit⁺ cells (LSK) harvested from the endosteal BM region (eLSK), exhibit a superior ability to home back to the endosteal BM post-transplant as well as an increased long-term reconstitution potential compared to their central BM counterparts. Specifically, eLSK with the ability to home to the endosteal BM region within 5 hours post-transplant into non-ablated recipients, have higher long term reconstituting potential (1 in 12) compared to freshly isolated flushed LSK cells (~1 in 30 to 1 in 200). This suggests that the ability to home to the endosteal environment acts as a functional selection for primitive HSC. Recently we have extended these studies to include a series of <i>in vitro</i> and <i>in vivo</i> experiments using the highly enriched SLAM LSK phenotypes. These studies suggest that phenotypically defined primitive HSC/HPC have different functional properties depending on their location within the BM cavity, which need to be considered in assays evaluating the biological properties of HSC. Specifically, a subset of CD150⁺CD48⁺LSK, previously defined as restricted short-term HSC or HPC, isolated from the endosteal region have long-term reconstitution ability that is greater than their central BM counterpart. Recent reports in the literature suggest a role for the vasculature as a HSC niche, although the specific location and types of vasculature within the BM are not well understood. We have recently created a three dimensional model of femoral bone and blood vessels using vascular casting, histology/confocal microscopy and micro-computed tomography. This analysis demonstrated that the BM has an extensive vascular network that is continuous throughout the cavity and that any cell within the metaphyseal BM region is within 2 cell diameters of vasculature. In addition, all cells in the epiphyseal and metaphyseal BM regions are within 10 cell diameters of bone, identifying these entire regions as endosteal. Consequently, HSC should be classified as either within an endosteal niche or not, and all HSC reside within close proximity to vasculature, which itself does not create a discrete niche. In summary, we conclude that the endosteal microenvironment in which HSC/HPC reside is critical for their attraction to this region post-transplant as well as their retention within this region and the maintenance of their haemopoietic potential.