Johnny T. Butler 
The oldest cells of the human body are not found only in the heart or the nervous system, as many scientists have thought for a long time.
While studying cellular aging in the human body, researcher Rafael Arrojo Drigo and his colleagues at the Salk Institute in the United States were surprised to find that some organs are made up of a mixture of young and old cells.
It is by analyzing the brain, liver and pancreas of mice that these researchers have found the presence of populations of cells and proteins whose lifespan is extremely long, some being as old as neurons.
We were very surprised to find cellular structures that are essentially as old as the organism in which they are located.
Martin Hetzer, professor at the Salk Institute
“This suggests that there is even more cellular complexity than we have imagined to date. This knowledge changes the way we look at aging organs, such as the brain, heart and pancreas,” says Hetzer.
A mosaic of ages
Most neurons in the brain do not divide in adulthood. These nerve cells therefore have a long life and their decline is associated with aging.
To date, because of technical limitations, the lifespan of cells outside the brain has been difficult to determine.
However, the technique developed by the American team applies to almost all tissues of the body. It provides important information about how non-dividing cells work and how they lose control over the quality and integrity of proteins and cell structures as they age.
“Biologists wondered how old were the cells of an organism. It was commonly accepted that neurons age, while other cells in the body are relatively young and regenerate throughout the life of the body,” says Rafael Arrojo e Drigo.
We have tried to find out if it is possible that some organs also have cells that live as long as neurons in the brain.
Rafael Arrojo e Drigo
Since researchers knew that most neurons are not replaced in a lifetime, they decided to use them as age references to compare with other non-dividing cells.
The team combined electronic isotopic tracers with a hybrid imaging method to visualize and quantify the age of cells and proteins as well as their turnover in the brain, pancreas and liver in rodents of different ages.
Thus they discovered that cells lining the blood vessels, called endothelial cells, were as old as neurons. This means that some non-neuronal cells do not reproduce or replace themselves throughout their lives.
In addition, the pancreas, an organ responsible for maintaining glycemia and the secretion of digestive enzymes, also had cells of variable age.
One region of the pancreas, known as the islet of Langerhans, even contained young and old interconnected cells.
According to the researchers, the pancreas is a striking example of a mosaic of ages, that is to say a population of identical cells that are distinguished by their lifespan.
Results from other studies suggested that the liver has the ability to regenerate in adulthood. It is for this reason that researchers chose this organ in the hope of observing relatively young liver cells.
To their surprise, the vast majority of liver cells in adult mice were as old as the animal, while cells lining the blood vessels and stellate cells, another type of liver cell, were much younger.
This new knowledge opens up new avenues of regenerative research for this organ.
