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Monitoring Stem Cell Research 
marrow stem cell transplants has allowed tracking of adult stem 
cells within humans, primarily by identification of Y chromosome- 
bearing cells in female patients who had received bone marrow stem 
cells from male donors. Biopsy or postmortem samples show that 
some of the transplanted bone marrow stem cells could form liver, 
skin, and digestive tract cells,^® as well as participate in the 
generation of new neurons within the human brain.^° Bone marrow 
stem cells have also been shovm to contribute to Purkinje cells in the 
brains of adult mice'^^ and humans^^. Generation of this particular 
type of neural cell is significant in that new Purkinje cells do not 
normally appear to be generated after birth. 
Regeneration or replacement of dead or damaged cells is the 
primary goal of regenerative medicine and one of the prime 
motivations for study of stem cells. It is thus of significant interest 
that bone marrow stem cells have shown the ability to produce 
therapeutic benefit in animal models of stroke. In mice, fluorescence- 
tracked bone marrow derived stem cells expressed neuronal 
antigens and also incorporated as endothelial cells, possibly 
producing therapeutic benefit by allowing increased blood flow to 
damaged areas of the brain.^^ In rats, intravenous (IV) administration 
of rat'^ or human"^^ bone marrow stromal cells resulted in significant 
behavioral recovery after stroke. Interestingly, only a small 
percentage of the stromal stem cells appeared to incorporate into the 
damaged brain as neuronal cells (1-5% in the case of the human 
marrow stromal cells), but the levels of neurotrophin growth factors 
within the brains increased and were possibly the signal for repair of 
damaged brain tissue, perhaps by stimulation of endogenous 
neuronal precursors. It is also of interest that the marrow stromal 
cells were injected IV and not intracerebrally, indicating that the 
stem cells somehow "homed" to the site of tissue damage. Most 
studies showing adult stem cell differentiation into other tissues 
show an increased incorporation of cells, or even an absolute 
requirement for differentiation, relying on tissue damage to initiate 
the differentiation. This may indicate that without a "need” for 
replacement and repair, there is little or no activation of adult stem 
cells. The recruitment and homing of adult stem cells to damaged 
tissues are fascinating but relatively unexplained phenomena. One 
report"^® indicates that recruitment of quiescent stem cells from bone 
marrow to the circulation requires release of soluble c-kit ligand 
(stem cell factor), but the range of factors necessary for recruitment 
and homing to organs other than bone marrow is unknown at this 
time and warrants increased investigation. 
PRE -PUBLICATION VERSION 
