background
The bone marrow stroma was originally thought to function mainly as a structural framework for the hematopoietic component of the marrow. Since then it has become well established that the stroma consists of a heterogeneous population of cells, a subset of which exerts both positive and negative regulatory effects on the proliferation and differentiation of hematopoietic stem cells (HSC) in the marrow through a combination of physical and chemical signals.^1-3 The stroma also contains other (non-hematopoietic) stem cells termed mesenchymal stem cells (MSC), that are capable of both self-renewal and differentiation into mature cell t