Some of the effects of aging are driven by signaling changes in important parts of our biochemistry - such as in stem cell niches, collections of cells that provide necessary support to the stem cells that maintain and repair tissue. Niches increasingly act to suppress the stem cells they contain in response to rising levels of cellular and other damage connected to aging. The stem cells themselves also suffer damage, and this evolved response is likely a way to minimize the risk of cancer at the cost of maintaining tissues, but the declining function of the stem cells so far seems to be far more a property of signals from the niche.
In the course of investigating this and similar effects, researchers have been moving blood between young and old mice. Transfusions and joining the bloodstreams of young and old mice are a way to change the signaling environment in order to see what the effects are. The outcome is that a range of measures of aging are reversed:
Experiments on mice have shown that it is possible to rejuvenate the brains of old animals by injecting them with blood from the young. ... blood from young mice reversed some of the effects of ageing in the older mice, improving learning and memory to a level comparable with much younger animals.
[Researchers] connected the circulatory systems of an old and young mouse so that their blood could mingle. This is a well-established technique used by scientists to study the immune system called heterochronic parabiosis. When [researchers] examined the old mouse after several days, [they] found several clear signs that the ageing process had slowed down. The number of stem cells in the brain, for example, had increased. More important, [they] found a 20% increase in connections between brain cells.
One of the main things that changes with ageing are these connections, there are a lot less of them as we get older. That is thought to underlie memory impairment - if you have less connections, neurons aren't communicating, all of a sudden you have [problems] in learning and memory. ... the young blood most likely reversed ageing by topping up levels of key chemical factors that tend to decline in the blood as animals age. Reintroduce these and [all] of a sudden you have all of these plasticity and learning and memory-related genes that are coming back.