Appendix J. 
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to differentiate to osteoblasts and chondroblasts, two closely related 
cell lineages. We could demonstrate that although a large number of 
genes are co-regulated when MAPC differentiate to these two 
lineages, specificity in differentiation can readily be detected. For 
instance a number of known and yet to be fully characterized 
transcription factor mRNAs were differentially expressed during the 
initial phases of differentiation. Studies are ongoing to further define 
the role of these genes in lineage specific differentiation. These 
studies exemplify however the power of this model system to study 
lineage specific differentiation in vitro. 
DEGREE OF PLURIPOTENCY OF MAPC: 
We have shown that transfer of 10-12 mouse MAPC into 
mouse blastocysts results in the generation of chimeric mice. When 
10-12 MAPC, expanded for 50-55 population doublings, were injected 
approximately 80% of offspring were chimeric, vdth the degree of 
chimerisms varying between 1-40% Cells found in different organs 
acquire phenotypic characteristics of the tissue. For instance MAPC 
derived cells detected in the brain of chimeric animals differentiate 
appropriately into region specific neurons, as well as astrocytes and 
oligodendrocytes More recent studies using MAPC from later 
population doublings have shown that the frequency of chimerism 
decreases when MAPC are maintained for longer time in culture, 
even though animals vsnth chimerism of more than 70% could be 
obtained (unpublished observations). These studies indicate that like 
ES cells, MAPC can give rise to most if not all somatic cell types of 
the mouse. Whether MAPC can do this without help of other cells in 
the inner cell mass, i.e. can generate a mouse by tetraploid 
complementation is not yet known. Also not yet known is whether 
MAPC contribute to the germ line when injected in the blastocyst. 
POST-NATAL CONTRffiUTION TO TISSUES: 
Neither human nor mouse MAPC injected into the muscles of 
severe combined immunodeficient (SCID) mice have led to the 
development of teratomas (unpublished observations). Likewise, we 
have not yet detected donor-derived tumor formation following IV 
injection of human or mouse MAPC in NOD-SCID animals. However, 
when mouse undifferentiated MAPC are administered IV to NOD- 
SCID mice, engraftment in the hematopoietic system as well as 
epithelia of gut, liver and lung is seen Preliminary studies using 
human MAPC suggest that a similar pattern of engraftment may 
occur, even though the level of contribution to blood, liver, gut and 
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