b2815 Tissue Engineering and Nanotheranostics “9.61x6.69”
80 Tissue Engineering and Nanotheranostics
In 2005, Wang et al. demonstrated that Noggin and FGF2 are the
minimum essential factors required to sustain hESC pluripotency with
UM and no feeder cell layer was used.22,23 These results suggest FGF2
and Noggin are the two primary active factors in MEF-CM.
Activin A was identified as another active component in MEF-CM
and a product of feeder cells. Beattie et al. reported that, in feeder-
free, MEF-CM free conditions, Activin A application was able to
maintain hESCs in the undifferentiated state for more than 20 pas-
sages.^24 A similar study combining FGF2 with Activin A in feeder-
free, MEF-CM free conditions yielded equally positive results.^25
Similar results were acquired combining Nodal with FGF2, but other
members of the TGFβ-superfamily did not show the same capacity for
maintenance of pluripotency including transforming growth factor
beta 1 (TGFβ1), Cripto, and BMP4.^25
2. Induced Pluripotent Stem Cells
iPSCs are undifferentiated stem cells derived from adult fibroblasts
through the sequential application of pluripotency inducing transcription
factors. The first iPSCs were generated by Takahashi et al. from mouse
embr yonic and adult fibroblast cells in a study that used retrovirus-
mediated introduction of four transcription factors: Oct3/4, Sox2,
c-Myc, and KLF4.^26 These cells are indistinguishable from ESCs in mor-
phology, proliferation, gene expression, and the ability to generate a tera-
toma consisting of all three germ layers. Using the same four factors,
Takahashi et al. generated iPSCs from human fibroblasts the following
year.^27 Thomson et al. contemporaneously demonstrated the generation
of human iPSCs using Oct3/4, Sox2, NANOG, and LIN28.^28
Further study by Nakagawa et al. in 2008, showed that retroviral
induction of c-Myc was not necessary to form iPSCs from either mice
or human fibroblasts.^11 Although retroviral induction of c-Myc may
be unnecessary, endogenous c-Myc protein itself may still be required
for iPSC generation. It has been suggested that the induction of
Oct3/4, Sox2, and KLF4 may be enough to activate c-Myc transcrip-
tion endogenously.
With or without c-Myc, after treatment with transcription factors,
iPSCs were selected by Fbx15 expression. However, expression of
