Not so very many years ago it was noted that transplanting young ovaries into old mice resulted in extended life. There is still no good understanding of why this happens, and which of the numerous changes produced by this transplantation are most important in determining life span, but researchers here focus on beneficial effects for the immune system. Age-related failure of the immune system negatively impacts a wide range of important functions, including wound healing, destruction of senescent and potentially cancerous cells, and maintenance and support of neural tissues. It also leads to increased levels of chronic inflammation, a factor that contributes to the development of all of the common age-related diseases. Immune system decline is an important component of frailty in old age, so it isn't unreasonable to think that meaningful benefits will be generated by immune system restoration.
As we age, our metabolism slows and our immune system runs out of steam. Older people are more likely to have severe cold and flu symptoms, probably because they have fewer fresh immune cells left. And a slower metabolism means that glucose stays in the blood stream for longer after eating a meal. Over time, high blood sugar levels can damage organs. But experiments in mice suggest that transplanting organs from a younger individual could reverse these changes. Researchers removed the ovaries of 10 mice that were 12 months old and had gone through oestropause, a transition similar to the human menopause. They replaced these with ovaries taken from 60-day old mice - roughly equivalent to people in their early 20s in terms of ageing.
Four months later, the researchers assessed the mouse immune systems. The numbers of immune cells that respond to new infections - called naive T-cells - tend to decline with age, and had already fallen in these mice before surgery. Between the ages of 6 months (before the operation) and 16 months, the number of naive cells in these mice rose by around 67 per cent. Cell counts fell by 80 per cent in untreated mice over the same period. To test metabolism, researchers injected the mice with glucose and measured how long it took for their blood sugar levels to return to normal. The mice with young ovaries removed glucose from their blood faster than untreated mice. The findings build on the team's previous work, which found that mice transplanted with young ovaries in middle age live about 40 per cent longer than their peers, and have healthier looking hearts too. How young ovaries might exert these benefits remains something of a mystery. One theory is that the hormones produced by the eggs inside these ovaries are responsible. But when researchers killed all the eggs inside young ovaries before transplanting them into another set of older mice, they still saw the same benefits. The researchers theorize that some other kind of cell inside the ovary might be responsible for the rejuvenation.