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unnecessary first to isolate and culture stem cells before injecting 
them back into a patient to initiate tissue repair. Rather, it may be 
easier and preferable to mobilize endogenous stem cells for repair of 
damaged tissue. Initial results regarding this possibility have 
already been seen in some animal experiments, in which bone 
marrow and peripheral blood stem cells were mobilized with 
injections of growth factors and participated in repair of heart and 
stroke damage. The ability to mobilize endogenous stem cells, 
coupled with natural or perhaps induced targeted homing of the cells 
to damaged tissue, could greatly facilitate use of adult stem cells in 
simplified tissue regeneration schemes. 
GENE THERAPY APPLICATIONS WITH ADULT STEM CELLS 
Adult stem cells can provide an efficient vehicle for gene 
therapy applications, and engineered adult stem cells may allow 
increased functionality, proliferative capacity, or stimulatory 
capabihty to these cells. The feasibility of genetically engineering 
adult stem cells has been shown, for example, in the use of bone 
marrow stem cells containing stably inserted genes. The engineered 
stem cells when injected into mice could still participate in formation 
and repair of differentiated tissue, such as in lung.^®^ As another 
example, engineered stem cells containing an autoantigen, to induce 
immune tolerance of T cells to insulin-secreting cells, were shown to 
prevent onset of diabetes in a mouse model of diabetes, a strategy 
that may be useful for various human autoimmune diseases. 
Introduction of the PDX-1 gene into liver stem cells stimulated 
differentiation into insulin-producing cells which could normalize 
glucose levels when transplanted into mice with induced diabetes. 
Simply engineering cells to increase their proliferative capacity can 
have a significant effect on their utility for tissue engineering and 
repair. For example, McKee et engineered human smooth 
muscle cells by introducing human telomerase, which greatly 
increased their proliferative capacity beyond the normal lifespan of 
smooth muscle cells in culture, while allowing retention of their 
normal smooth muscle characteristics. These engineered smooth 
muscle cells were seeded onto biopolymer scaffolds and allowed to 
grow into smooth muscle layers, then seeded with human umbilical 
vein endothelial cells. The resulting engineered arterial vessels 
could be useful for transplants and bypass surgery. Similarly, human 
marrow stromal cells that were engineered with telomerase 
increased their proliferative capacity significantly, but also showed 
enhanced ability at stimulating bone formation in experimental 
animals. Genetically-engineered human adult stem cells have 
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