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Monitoring Stem Cell Research 
grown in culture are developmentally heterogeneous in that 
they contain more than one neural cell type, and the number of 
self-renewing cells is frequently low (less than five percent).^® 
Although neural stem cells are still insufficiently under- 
stood, they are already proving valuable in basic research on 
neural development. The ability to grow reproducible neural 
stem cells in vitro has facilitated identification of important 
neural stem cell growth factors and their cellular receptors. For 
example, human neural stem cells from the developing human 
brain cortex, expanded in culture in the presence of leukemia 
inhibitory factor (LIF) . allowed growth of a self-renewing neu- 
ral stem cell preparation for up to 110 population doublings. 
Withdrawal of LIF led to decreased expression of about 200 
genes, which were specifically identified through use of 
“gene chips” manufactured by Affymetrix. These genes are 
presumably involved in promoting or preserving the stem cell’s 
capacity for self-renewal in the undifferentiated state. The 
number and specificity of the molecular changes characterized 
in these experiments powerfully illustrate the usefulness of 
neural and other stem cell preparations in basic biomedical 
research. 
Human neural stem cells are also being injected into ani- 
mals to test their effects on animal models of human neuro- 
logical disease. To track the fate of the introduced human cells, 
they must first be modified or “marked" in ways that permit 
their specific detection.* Marked human neural stem cells are 
easily tracked after they are injected into experimental ani- 
mals, making it possible to determine whether they survive 
and migrate following injection. Studies of this type have pro- 
vided evidence that human neural cells can migrate exten- 
sively in the brain after injection.^® In addition, such cells can 
be injected into animal models of human diseases such as in- 
tracerebral hemorrhage and Parkinson Disease (PD) to study 
their effect on the progression of the disease.^® Although hu- 
man neural stem cells may not yet be as well characterized as 
MSCs or ESCs, they are being actively studied with the hope 
* Stem cell preparations are frequently transduced in vitro with foreign genes 
that, when expressed, produce readily visualized proteins, such as Green 
Fluorescent Protein (GFP). 
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