The endosteal 'osteoblastic' niche and its role in hematopoietic stem cell homing and mobilization.

The concept of hematopoietic stem cell (HSC) niche was formulated in 1978, but HSC niches remained unidentified for the following two decades largely owing to technical limitations. Sophisticated live microscopy techniques and genetic manipulations have identified the endosteal region of the bone marrow (BM) as a preferential site of residence for the most potent HSC - able to reconstitute in serial transplants - with osteoblasts and their progenitors as critical cellular elements of these endosteal niches. This article reviews the path to the discovery of these endosteal niches (often called 'osteoblastic' niches) for HSC, what cell types contribute to these niches with their known physical and biochemical features. In the past decade, a first wave of research uncovered many mechanisms responsible for HSC homing to, and mobilization from, the whole BM tissue. However, the recent discovery of endosteal HSC niches has initiated a second wave of research focusing on the mechanisms by which most primitive HSC lodge into and migrate out of their endosteal niches. The second part of this article reviews the current knowledge of the mechanisms of HSC lodgment into, retention in and mobilization from osteoblastic niches.