The hematopoietic stem cell niche: what are we trying to replicate?

The pluripotent nature and self-renewal capability of hematopoietic stem cells (HSCs) has resulted in these cells being seen as the ideal cell source for the on-demand production of blood and blood products. This potential to save many thousands of lives is as yet unmet, owing to our inability to reliably expand this cell source ex vivo to clinically relevant numbers. This is, to a large extent, due to the absence of a single cell surface antigen to identify them and an optimum set of in vitro conditions sufficient to expand them while maintaining 'sternness'. This review briefly summarizes the current research efforts aimed at defining the appropriate in vivo conditions to be mimicked in ex vivo culture. In particular, it focuses on the molecules known to participate in the functionality of the bone marrow stem cell niche. These niche molecules include adhesion molecules, extracellular matrix molecules, and soluble and bound growth factors. Finally, this review proposes an approach to find the optimum conditions for this expansion utilizing smart surfaces and a criterion for developing these surfaces. (c) 2008 Society of Chemical Industry.