The rejuvenating effect of pregnancy on muscle regeneration.

Aging is characterized by reduced tissue regenerative capacity attributed to a diminished responsiveness of tissue-specific stem cells. With increasing age, resident precursor cells in muscle tissues show a markedly impaired propensity to proliferate in response to damage. However, exposure to factors present in the serum of young mice restores the regenerative capacity of aged precursor cells. As pregnancy represents a unique biological model of a partially shared blood system between young and old organisms, we hypothesized that pregnancy in aged mice would have a rejuvenating effect on the mother. To test this hypothesis, we assessed muscle regeneration in response to injury in young and aged pregnant and nonpregnant mice. Muscle regeneration in the aged pregnant mice was improved relative to that in age-matched nonpregnant mice. The beneficial effect of pregnancy was transient, lasting up to 2 months after delivery, and appeared to be attributable to activation of satellite cells via the Notch signaling pathway, thus supporting the possibility that pregnancy induces activation of aged dormant muscle progenitor cells.