The researchers selected 24 genes--all previously found to play a role in early embryos and embryonic stem cell identity--as candidate factors that might give body cells the ability to become other cell types.
The researchers found that four of those factors, known as Oct3/4, Sox2, c-Myc, and Klf4, could lend differentiated fibroblast cells taken from embryonic or adult mice the pluripotency normally reserved for embryonic stem cells.
They further reported that transplantation of the iPS cells under the skin of mice resulted in tumors containing a variety of tissues representing the three primary types found in mammalian embryos. Those primary "germ layers" in embryos eventually give rise to all an animal's tissues and organs.
Following injection into blastocysts, iPS cells also contributed to mouse embryonic development.
"The finding is an important step in controlling pluripotency, which may eventually allow the creation of pluripotent cells directly from somatic cells of patients," Yamanaka said.
While the findings could have wide applications, stem cell experts caution that the study of embryonic stem cells has much further to go.
"We still do not know whether the four factors can generate pluripotent cells from human somatic cells," Yamanaka said. Use of c-Myc, a gene implicated in many human cancers, may not be suitable for clinical applications, they added, and the process may require specific culture environments. It also remains unclear whether iPS cells can do everything that embryonic stem cells can.