Aging does not progress at the same speed everywhere in our bodies, nor does it follow a regular progression over the years.
Researchers analyzed 25,306 samples from 40 types of human tissue. These samples had been collected after the deaths of 970 people aged 21 to 70. Their goal was to determine when their structure changed most rapidly.
To do this, the team used conventional microscopic images of tissues. A computer program identified a great many details in the organization of cells and other visible structures. The researchers then compared these characteristics across different age groups. This method allowed them to track the rate of change.
The results revealed several different rhythms. The coronary and tibial arteries already undergo major structural changes during the thirties. The uterus and vagina change mainly later, with an acceleration in the fifties, around menopause. Each part of the body therefore follows its own clock.
Another pattern appeared very frequently. Nine of the 14 tissues studied in detail, in addition to the ovary, showed two periods of acceleration. These generally occurred during the thirties and then around the fifties. This pattern involved the esophagus, stomach, colon, small intestine, prostate, and testes, among others.
The ovary provides a particularly clear example. Its structure changes more rapidly around the ages of 35 to 40, a period corresponding to declining fertility. A second acceleration appears around the ages of 55 to 60, around menopause.
These visible changes are also accompanied by changes in how cells function. During the acceleration phases, the researchers observed, in particular, increased activity related to inflammation. Conversely, several mechanisms associated with energy production, repair, and monitoring cell condition were less active.
The organs also do not seem to evolve entirely independently. In the same person, some tissues showed coordinated changes. This was particularly true of several parts of the digestive system and the male reproductive organs. Sex hormones could contribute to these connections, according to the team's analyses.
The researchers now have a method for comparing the rate of change between individuals. They will be able to study how certain diseases, lifestyle habits, or genetic variations are associated with faster or slower structural aging.