Appendix J. 
303 
culture leading to reprogramming of the genetic information and 
pluripotency [REF]. However, we have no evidence that MAPC are 
tetraploid or aneuploid early during culture, making this possibihty 
less likely. Nevertheless, studies are ongoing to rule this out. The in 
vivo studies were not set up to fully be capable of ruling out this 
possibihty. 
However, a number of findings suggest that fusion may not 
likely be the cause for the engraftment seen postnatally, nor the 
chimerism in the blastocyst injection experiment. The frequency of 
the fusion event described for the ES-BM, ES-NSC, and HSC- 
hepatocyte fusion was in general very low, i.e. 1/100,000 cells. 
Expansion of such fused cells could only be detected when drug 
selection was apphed in the in vitro systems, and withdrawal of 
NTBC (2-(2-nitro-4-trifluoro-methylbenzoyl)-l ,3-cyclohexanedione) in 
the FAH mouse model was used to select for cells expressing the 
FAH gene. The percent engraftment seen in our post-natal transplant 
models was in the range of 1% - 9%. The frequency of chimerism seen 
in blastocyst injection studies ranged between 33% and 80% when 1 
and 1 and 10-12 MAPC were injected, respectively. These 
frequencies are significantly higher than what has been described for 
fusion events with ES cells in vitro, and in the HSC-hepatocyte fusion 
studies in vivo. 
Furthermore, in contrast to what was described in the papers 
indicating that fusion may be responsible for apparent plasticity, all 
in vivo studies done with MAPC were done without selectable 
pressure, mainly in non-injured animals. Therefore, it is less likely 
that the pluripotent behavior of MAPC in vivo is due to fusion 
between the MAPC and the tissues where they engraft / contribute 
to. However, specific studies are currently being designed to formally 
rule this out. 
Primitive ES-Iike cells that persist vs. de^differentiation: 
Currently, we do not have proof that MAPC exist as such in 
vivo. Until we have positive selectable markers for MAPC, this 
question will be difficult to answer. If the cell exists in vivo, one 
might hypothesize that it is derived for instance from primordial 
germ cells that migrated aberrantly to tissues outside the gonads 
during development. It is, however, also possible that removal of 
certain (stem) cells from their in vivo environment results in 
"reprogramming” of the cell to acquire greater pluripotency. The 
studies on human MAPC suggest that such a cell that might undergo 
PRE-PUBLICATION VERSION 
