Thank you for your interest in AllThingsStemCell! That’s a good question. In short, there is no blastocyst-like stage — when the somatic cells are reprogrammed, they’re immediately cultured as cells in normal adherent culture. Basically, the cells to be reprogrammed are removed from their normal culture system and exposed to the genes (in a gene delivery system), then put back into a new culture system. Since human embryonic stem cells grow in an adherent culture system, for cells being reprogrammed this “new culture system” is an adherent culture system (to try and mimic the conditions the embryonic stem cells want to be happy). (See “Figure 2″ here for a picture of what the hESCs look like in normal adherent culture: http://www.allthingsstemcell.com/2009/04/human_embryonic_stem_cells/ )

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Takahashi, K., Tanabe, K., Ohnuki, M., Narita, M., Ichisaka, T., Tomoda, K., and Yamanaka, S. Induction of Pluripotent Stem Cells from Adult Human Fibroblasts. Cell. 2007. 131: 1-12.

Yu, J., Vodyanik, M. A., Smuga-Otto, K., Antosiewicz-Bourget, J., Frane, J. L., Tian, S., Nie, J., Jonsdottir, G. A., Ruotti, V., Stewart, R., Slukvin, I. I., and Thomson, J. A. Induced Pluripotent Stem Cell Lines Derived from Human Somatic Cells. Science. 2007. 318(5858): 1917-1920.

The factors were actually added to, not deleted from, the original adult cells that were reprogrammed into iPSCs. In Thomson’s paper, they list nearly 100 factors that they made vectors for originally to test (this is in supplemental table 1). But, they then tested 14 of these genes in subsets and this is how they found the core 4 that worked best to reprogram the cells. I hope this helps explain it! The original papers are the best source to look at, though sometimes they can be confusing or leave out details.

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Thanks for starting this site!