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," says study author and Salk Institute chief science officer Martin Hetzer.

"This suggests even greater cellular complexity than we previously imagined and has intriguing implications for how we think about the ageing of organs, such as the brain, heart and pancreas."

Most neurons in the brain do not divide during adulthood and thus experience a long lifespan and age-related decline. Yet, largely due to technical limitations, the lifespan of cells outside of the brain was difficult to determine.

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. 

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. 

Does it surprise you that so many cells in your body may be as old as you are?