Scientists have transplanted neural cells derived from a monkey's skin into its brain and watched the cells develop into several types of mature brain cells. [After] six months, the cells looked entirely normal, and were only detectable because they initially were tagged with a fluorescent protein. Because the cells were derived from adult cells in each monkey's skin, the experiment is a proof-of-principle for the concept of personalized medicine, where treatments are designed for each individual.
And since the skin cells were not "foreign" tissue, there were no signs of immune rejection - potentially a major problem with cell transplants. "When you look at the brain, you cannot tell that it is a graft. Structurally the host brain looks like a normal brain; the graft can only be seen under the fluorescent microscope."
The transplanted cells came from induced pluripotent stem cells (iPS cells), which can, like embryonic stem cells, develop into virtually any cell in the body. iPS cells, however, derive from adult cells rather than embryos. In the lab, the iPS cells were converted into neural progenitor cells. These intermediate-stage cells can further specialize into the neurons that carry nerve signals, and the glial cells that perform many support and nutritional functions. This final stage of maturation occurred inside the monkey.