None of us was made from scratch. Every human being develops from the fusion of two cells, an egg and a sperm, that are the descendants of other cells. The lineage of cells that joins one generation to the next — called the germline — is, in a sense, immortal.
Biologists have puzzled over the resilience of the germline for 130 years, but the phenomenon is still deeply mysterious.
Over time, a cell’s proteins become deformed and clump together. When cells divide, they pass that damage to their descendants. Over millions of years, the germline ought to become too devastated to produce healthy new life.
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On Thursday in the journal Nature, Dr. Bohnert and Cynthia Kenyon, vice president for aging research at Calico, reported the discovery of one way in which the germline stays young.
Right before an egg is fertilized, it is swept clean of deformed proteins in a dramatic burst of housecleaning.
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Combining these findings, the researchers worked out the chain of events by which the eggs rejuvenate themselves.
It begins with a chemical signal released by the sperm, which triggers drastic changes in the egg. The protein clumps within the egg “start to dance around,” said Dr. Bohnert.
The clumps come into contact with little bubbles called lysosomes, which extend fingerlike projections that pull the clumps inside. The sperm signal causes the lysosomes to become acidic. That change switches on the enzymes inside the lysosomes, allowing them to swiftly shred the clumps.