Tissue injury is known to cause stem cells to exit the bone
marrow, where they normally reside, and enter into circulation, apparently en
route to attempt to repair the area of injury. This concept has been
demonstrated in patients with heart attacks, in that following damage to the
heart muscle, an increased number of stem cells is observed in circulation, as
is described in this video
http://www.youtube.com/watch?v=NqEggEYilh0. The same holds true in patients
with stroke, in that after a stroke, there is an association between higher
number of endothelial progenitor cells (a type of stem cell that gives rise to
blood vessels), and positive neurological outcome (Dunac et al. Neurological
and functional recovery in human stroke are associated with peripheral blood
CD34+ cell mobilization. J Neurol. 2007 Mar;254(3):327-32). This is one of
the reasons why patients take nutritional supplements such as Stem-Kine that
increase the numbers of stem cells in circulation.
In a recent study from the Department of Genetics, of the
Southwest Foundation for Biomedical Research, in San Antonio, Texas researchers
attempted to dissect specifics of how tissue damage increases the number of
circulating stem cells. Since heart attacks and strokes occur in different
degrees of severity in people, the scientists used a reproducible model of
tissue injury in baboons. They blocked circulation to part of the leg by tying
off the femoral artery. As a comparator approach, they injected other baboons
with the drug granulocyte colony stimulating factor (G-CSF) which is currently
used by hematologists to "harvest" stem cells from the blood of stem cell
donors.
In baboons receiving G-CSF and the group inflicted with
circulation blockade, the increase in stem cells in circulation peaked at day
3. The stem cells expressing CD34 were twice as high in the circulation of
animals that received G-CSF as compared to the animals with ligated femoral
artery. In contrast, another type of stem cell, the CD133+/KDR+/CXCR4+/CD31+
cell, which represents endothelial progenitor cells, was detected at higher
levels in ligated animals as compared to G-CSF treated animals. When these
cells were grown in tissue culture plates, they resembled functional blood
vessel cells called "endothelium"
This study suggests that different types of stem cells are
"told" by conditions in the body to leave the bone marrow and to go into
circulation. Given that ligation of an artery is expected to cause damaged to
the endothelium, it is conceivable that the release of endothelial progenitor
cells is occurring in order for the body to attempt to heal injured tissue. If
this concept is correct, it will be interesting to see if the stem cells that
increase in circulation in patients with a heart attack have a propensity to
become heart cells when placed in tissue culture. The other interesting point
raised by this study is whether chemicals can be administered that would assist
the body in increasing the number of the proper type of stem cell in circulation
after injury.