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Monitoring Stem Cell Research 
one another.^® Using dual antibody staining and fluorescence- 
activated cell sorting, Gronthos and colleagues^^ isolated hu- 
man MSCs in almost pure form and expanded them substan- 
tially in vitro. Thus, human MSC preparations isolated in dif- 
ferent laboratories by different methods may have similar but 
not identical properties. 
A molecular analysis of genes expressed in a single-cell- 
derived colony of MSCs provided evidence for the activity of 
genes also turned on in bone, cartilage, adipose, muscle, he- 
matopoiesis-supporting stromal , endothelial , and neuronal 
cells. These results are surprising in that MSCs derived from 
a single cell appear to be expressing genes associated with 
multiple major cell lineages . It is possible that different cells 
within the colony had already entered into distinct differentia- 
tion pathways, resulting in a developmentally heterogeneous 
population composed of several different cell types. 
Mesenchymal stem cells are important for research and 
therapy for several reasons. First, because they can be differ- 
entiated in vitro into multiple cell types, they make possible 
detailed research on the molecular events underlying differen- 
tiation into bone,^® cartilage, and fat cell lineages. Second, they 
have recently been shown to support the in vitro growth of 
human embryonic stem cells. Thus, they could replace the 
mouse feeder cells used previously, obviating the need to sat- 
isfy FDA requirements for xenotransplantation , should the 
ESCs or their derivatives ever be used in human clinical re- 
search or transplantation therapy. Third, clinical studies are 
already underway in which MSCs are co-transplanted with 
autologous hematopoietic stem cells into cancer patients to 
replace their blood cell-forming system, destroyed by radiation 
or high dose chemotherapy.^® It is believed that the MSCs will 
support the repopulation of the bone marrow by the injected 
hematopoietic stem cells. 
In addition, injecting allogeneic MSCs (MSCs from a geneti- 
cally different human donor) may also prove valuable in modu- 
lating the immune system to make it more accepting of foreign 
tissue grafts [see Itescu review, reference (5)]. Finally, MSCs 
have the potential for cell-replacement therapies in injuries 
involving bone, tendon, or cartilage and possibly other dis- 
eases. They are, in fact, already being tested as experimental 
PRE-PUBLICATION VERSION 
