Selective Segregation of Damaged Molecules Is the Universal Rule of Cellular Rejuvenation, and Centriolar Retention in Stem Cells Is Its Exception

Every living thing that grows and divides faces a fundamental problem: how to stay young while making copies of itself. When a cell splits, it must decide what to keep and what to discard. If it keeps all its old, worn-out parts, those imperfections pile up over time, eventually causing the cell to fail. This is the essence of aging at a microscopic level. For decades, scientists have looked for a specific "counter" inside cells that tracks how many times they have divided, much like a mileage counter on a car. One leading idea suggested that the centriole, a small, barrel-shaped structure that helps organize the cell's internal skeleton during division, acts as this counter. The theory was that as a cell divides, the centriole accumulates invisible damage or chemical changes that eventually signal the cell to stop dividing and die. If this were true, the centriole would be a universal engine of aging for all complex life.

However, a new systematic review challenges this idea by looking at a creature that should not age if the theory were correct, yet does. The study, led by Jaba Tqemaladze, examines the tiny roundworm Caenorhabditis elegans. This worm is a biological oddity because its body is made of a fixed number of cells that stop dividing once the worm is an adult. Furthermore, during its development, the worm's body cells get rid of almost all their centrioles, keeping them only in the reproductive organs and a few sensory nerves. According to the old theory, a creature without dividing body cells and without centrioles in those cells should not experience aging driven by a division counter. Yet, the worm clearly does age, growing slower and weaker until it dies. This observation forces a rethinking of how aging works, suggesting that the centriole is not a universal timer for all life, but rather a specific mechanism that only applies to certain types of cells in certain animals.

The researchers began by treating the worm as a natural experiment to test the limits of the centriole theory. They confirmed that the adult worm's body is essentially a collection of static, non-dividing cells. Unlike humans, whose skin and gut lining constantly renew themselves through cell division, the worm's body cells do not replace themselves. Crucially, the study highlighted that the worm's body cells eliminate their centrioles early in life. The only places centrioles remain are in the germline, where eggs and sperm are made, and in the sensory neurons that have tiny hair-like projections called cilia. Despite this absence of centrioles in the aging body, the worm still follows a predictable life span, declining in health over two to three weeks. This proves that an organism can age without a centriole-based division counter, effectively ruling out the idea that centrioles are the necessary cause of aging for all living things.

In contrast, the study turns to mammals, including humans, where the situation is different. Our bodies rely on stem cells that constantly divide to repair tissues like the lining of the intestine or the blood. In these specific dividing cells, the mother centriole is not shared equally. Instead, the stem cell keeps the older, more worn-out mother centriole for itself, while the new daughter cell receives the younger, fresher one. This creates a unique scenario where the stem cell acts as a storage unit for aging. The researchers propose that this retention turns the stem cell into a local "entropy sink," a place where damage and disorder concentrate rather than being washed away. Because the stem cell keeps the old centriole through every division, that structure accumulates chemical changes and structural wear over time. This accumulation eventually limits the stem cell's ability to divide, leading to the aging of the tissue it supports.

The paper suggests that this mechanism is the exception, not the rule. In most other forms of life, such as bacteria or yeast, cells divide by selectively sending damaged molecules to one daughter cell and keeping the clean parts for the other, effectively rejuvenating the lineage. Even in the flatworm, a creature famous for its ability to regenerate entire bodies from tiny fragments, the stem cells divide without centrioles, allowing them to avoid this specific type of aging. The centriole is unique because it is a rigid structure that cannot be easily broken down and rebuilt from scratch in the same way other cellular parts can. When a stem cell keeps the old centriole, it is forced to carry the burden of that damage indefinitely. The study uses computer simulations to model this process, estimating that in humans, this accumulation of damage in stem cells reaches a critical point around eighty years of age, coinciding with the typical human life span.

The research also clarifies why our brains age differently than our skin. The neurons in the brain do not divide; they are like the body cells of the worm, permanently fixed in place. Because they do not divide, they do not retain centrioles, and they do not use a division counter. Instead, they age through a different process: the slow, diffuse buildup of protein clumps and metabolic waste that cannot be cleared out. This means that while the skin and blood age because their stem cells are trapped with old centrioles, the brain ages because its cells are stuck with unremovable molecular trash. Both paths lead to the same result—aging—but they use different mechanisms depending on whether the cells are constantly dividing or sitting still.

Ultimately, the study unifies these observations under a single principle: the selective segregation of damaged molecules. In most dividing cells, nature has a way to dump the trash into one daughter cell so the other can start fresh. The centriole breaks this rule in mammals by forcing the stem cell to keep the trash. The researchers conclude that aging is not driven by a single universal counter, but by a "renewal-gated" system. If a tissue renews itself through stem cells that keep the old centriole, it ages via that structural counter. If a tissue does not renew itself, or if it renews without centrioles, it ages through other means. This distinction explains why some animals, like the flatworm, can potentially avoid aging entirely, while others, like humans, are bound by the slow accumulation of damage in their stem cells. The paper does not claim to have solved the mystery of aging, but it provides a clear map of where the centriole fits into the puzzle, showing that it is a specific driver of aging in renewing tissues, but not the universal engine of life's decline.