Tests for pluripotency

Researchers constantly report that they have reprogrammed somatic cells or adult stem cells into pluripotent stem cells (iPS cells). Sometimes, however, they fail to provide irrefutable proof of the pluripotency of the reprogrammed cells – even when the corresponding results have been published in prestigious scientific journals. Reliable proof of pluripotency is based on various tests:

• Proof of marker genes: Genes like Oct4, which are silent in differentiated somatic cells, are switched on in iPS cells.
• Proof of teratoma formation: If pluripotent iPS cells are injected under the skin of mice, a special form of tumor, known as a teratoma, develops. This growth contains different types of somatic cells and is similar to embryonic tumors with the formation of the three “germ layers” from which different tissue types develop.
• Proof of cell differentiation: In principle, all of the body’s cell types can be produced from iPS cells in the Petri dish. Here, too, it is essential that cells from the three germ layers be cultivated and their functionality proven.
• Proof of chimera formation: Scientists inject the iPS cells into mouse embryos and prove that they are contained in the growing organism. The iPS cells are usually marked with a fluorescence gene that makes them visible under the microscope as luminous tissue. Demonstrating the presence of iPS cells that have matured into cells of the germline is viewed as particularly important in that it proves that the cells can convey their genetic information to the next generation.

The ultimate proof of the basic pluripotency of iPS cells was provided by Chinese researchers in summer 2009 based on a particular variant of chimera formation: the researchers generated viable mice from reprogrammed somatic cells that originated almost 100 percent from the iPS cells.