Artificial Scaffolds and Mesenchymal Stem Cells for Hard Tissues - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 159

Fig. 1 The differentiation potential of ESCs. ESCs have the capacity for self-renewal. They can
originate tissue-specific SCs from the three germinal layers, ectoderm, endoderm and mesoderm,
and naturally have the ability to differentiate into all tissue-specific cell lineages
New reports have demonstrated that the lower the differentiation stage of the
original cell type, the less the amount of transcription factors needed for the
reprogramming, hinting to a ranking in the necessity of the four factors with Sox2
and Oct4 being the major ones. Consequently, iPS can be achieved with only one
factor, Oct4, using precursor cells as source [ 31 - 33 ]. The first reprogramming
strategies involved retroviral transfection or a multi-protein expression vector
combined with the piggyBac transposon system for the delivery of the necessary
transcription factor genes [ 34 , 35 ]. However, both types of pluripotent cells, ESCs
and iPS, are prone to cause cancer, as shown after transplantation into SCID mice,
where they form teratomas [ 36 ]. This tumor risk is even higher for iPS
reprogrammed with retroviruses, since these viruses integrate randomly, thus