Researchers here report that the brain is not the only organ to exhibit cells that are as long-lived as the animal containing them. A number of other organs contain at least some long-lived cells, even for tissues thought to be highly regenerative and in which tissue turnover is comparatively rapid, such as the liver. It remains to be seen as to how this new information interacts with present thinking on the damage of aging, in which there is a central role for a reduction in stem cell activity and consequent loss of new cells generated to replace old tissue populations.
Scientists once thought that neurons, or possibly heart cells, were the oldest cells in the body. Now, researchers have discovered that the mouse brain, liver, and pancreas contain populations of cells and proteins with extremely long lifespans - some as old as neurons. "We were quite surprised to find cellular structures that are essentially as old as the organism they reside in. This suggests even greater cellular complexity than we previously imagined and has intriguing implications for how we think about the aging of organs, such as the brain, heart, and pancreas."
Since the researchers knew that most neurons are not replaced during the lifespan, they used them as an "age baseline" to compare other non-dividing cells. The team combined electron isotope labeling with a hybrid imaging method (MIMS-EM) to visualize and quantify cell and protein age and turnover in the brain, pancreas and liver in young and old rodent models. To validate their method, the scientists first determined the age of the neurons, and found that - as suspected - they were as old as the organism. Yet, surprisingly, the cells that line blood vessels, called endothelial cells, were also as old as neurons. This means that some non-neuronal cells do not replicate or replace themselves throughout the lifespan.
The pancreas, an organ responsible for maintaining blood sugar levels and secreting digestive enzymes, also showed cells of varying ages. A small portion of the pancreas, known as the islets of Langerhans, appeared to the researchers as a puzzle of interconnected young and old cells. Some beta cells, which release insulin, replicated throughout the lifetime and were relatively young, while some did not divide and were long-lived, similar to neurons. Yet another type of cell, called delta cells, did not divide at all. The pancreas was a striking example of age mosaicism, i.e., a population of identical cells that are distinguished by their lifespans.
Prior studies have suggested that the liver has the capacity to regenerate during adulthood, so the researchers selected this organ expecting to observe relatively young liver cells. To their surprise, the vast majority of liver cells in healthy adult mice were found to be as old as the animal, while cells that line blood vessels, and stellate-like cells, another liver cell type, were much shorter lived. Thus, unexpectedly, the liver also demonstrated age mosaicism.