An argument has been made that various current day therapeutic uses of stem cells that involve intravenous administration is that "you cant just inject stem cells intravenously and you will get tissue repair". Obviously there are certain situations in which the tissue will not be repaired, however, this concept of intravenous stem cell administration is not as unrealistic as some would expect. This is most clearly illustrated in the situation of bone marrow transplant in which the donor stem cells are injected not by drilling holes into the recipient's bone but by injecting the cells intravenously. The reason why stem cells find their way into the recipient bone marrow is because the bone marrow stroma secretes at a constitutively level the chemokine SDF-1, or as some other people call it, CXCL12. Stem cells have also been injected intravenously with success for post infarct heart failure, again the idea being that SDF-1 is secreted by the target tissue, in this case it is the injured myocardium making SDF-1.
One interesting concept is whether SDF-1 is involved in homing of other stem cells. In a recent paper (Belmadani et al. The chemokine SDF-1/CXCL12 regulates the migration of melanocyte progenitors in mouse hair follicles. Differentiation 2009 Apr;77(4):395-411) the question of what causes hair follicle progenitors to home to the hair follicle was asked. You guessed it ! SDF-1 is involved.
The authors showed that melanoblasts from the neural crest express the receptor for SDF-1, CXCR-4. Expression of CXCR-4 is increased in the anagen stage of the hair follicle cycle by cells expressing the proliferation marker PCNA. The authors demonstrated a gradient of SDF-1 on the path taken by the melanoblast on the way to the hair follicle.
The authors further demonstrated the critical role of SDF-1 in migration of the melanoblast by using CXCR-4 knockout mice and Anormed/Genzyme's inhibitor of CXCR-4 AMD3100.
These data suggest the possibility that CXCR-4 and SDF-1 are involved in other progenitor cell systems besides hematopoiesis.