An interview with a researcher: "Advances in the study of stem cells have fueled hopes that someday, via regenerative medicine, doctors could restore aging people's hearts, livers, brains and other organs and tissues to a more youthful state. A key to reaching this goal - to be able to provide stem cells that will differentiate into other types of cells a patient needs - appears to lie in understanding 'epigenetics,' which involves chemical marks stapled onto DNA and its surrounding protein husk by specialized enzyme complexes inside a cell's nucleus. These markings produce long-lasting changes in genes' activity levels within the cell - locking genes into an 'on' or 'off' position. Epigenetic processes enable cells to remain true to type (a neuron, for instance, never suddenly morphs into a fat cell) even though all our cells, regardless of type, share the same genetic code. But epigenetic processes also appear to play a critical role in reducing cells' vitality as they age. ... Aging seems to involve a gradual deterioration of function as cells and tissues are exposed to stresses either from outside the body, such as chemicals we ingest or irradiation from the sun, or from inside the body, such as free radicals, produced every moment when cells are making energy. These myriad insults can, among other things, alter a cell's epigenetic settings, resulting in changed patterns of gene activity that diminish the cell's overall ability to function. ... Although some aspects of cellular aging - DNA mutations, for instance - would be difficult to 'reset,' we and others have done experiments suggesting that many of the characteristics of old cells and tissues can indeed be reversed, restoring them to a more youthful state. Much of our work has focused on stem cells, and in particular on the changes that occur with age and that reduce stem cells' ability to maintain or repair tissues. Our findings fit nicely with the idea that some of the causes of aging are epigenetic in character, as opposed to actual damage to genes. Most importantly, our data suggest that cells and tissues can be rejuvenated without losing their specific characteristics - old muscle stem cells, when rejuvenated, remain muscle stem cells rather than become some more generic, undifferentiated cell."