The bone marrow is akin to skin: HCELL and the biology of hematopoietic stem cell homing

The recent findings that adult stem cells are capable of generating new blood vessels and parenchymal cells within tissues they have colonized has raised immense optimism that these cells may provide functional recovery of damaged organs. The use of adult stem cells for regenerative therapy poses the challenging task of getting these cells into the requisite sites with minimum morbidity and maximum efficiency. Ideally, tissue-specific colonization could be achieved by introducing the stem cells intravascularly and exploiting the native physiologic processes governing cell trafficking. Critical to the success of this approach is the use of stem cells bearing appropriate membrane molecules that mediate homing from vascular to tissue compartments. Hematopoietic stem cells (HSC) express a novel glycoform of CD44 known as hematopoietic cell E-/L-selectin ligand (HCELL). This molecule is the most potent E-selectin ligand natively expressed on any human cell. This article reviews our current understanding of the molecular basis of HSC homing and will describe the fundamental "roll" of HCELL in opening the avenues for efficient HSC trafficking to the bone marrow, the skin and other extramedullary sites.