Work on heterochronic parabiosis, in which an old and young mouse have their circulatory systems joined, has led to a wide variety of investigations into which signal molecules present in the bloodstream might be important in aging. The signaling environment changes in response to rising levels of molecular damage with age, leading to alterations in cellular behavior, some of which help to compensate, and some of which cause further harm.
At the same time, there is a rising level of interest in the roles played by various forms of extracellular vesicle in intracellular communication. These membrane-wrapped packages contain a diverse set of signal molecules, and are passed promiscuously back and forth between cells. That vesicles are conveniently packaged and distinguishable by size makes it comparatively easy to harvest them from cell cultures or blood samples, and from there they can be analyzed, or perhaps used as the basis for a therapy to change the behavior of cells in old tissues.
Changing the signaling environment may produce benefits large enough to be worth chasing, as the stem cell research community has demonstrated over the past twenty years. Most first generation cell therapies work because of the signals generated
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