Death, from a medical and legal perspective, is generally defined as the moment when our hearts cease beating or our brains stop functioning. The biological process, on the other hand, is much more complicated.

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Thanks to new research on worms from the University College London and at Washington, scientists are learning more about the spread of death in the body.

Their findings were published this week in the journal Cell Reports.

The researchers examined the tiny roundworms C. elegans in their study and discovered the mechanisms involved when multicellular organisms die, particularly when death occurs due to old age.

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"Cell death has been widely studied but much less is known about death of whole organisms, how it happens, what triggers it, and when it begins and ends. But it's extremely important for understanding fatal diseases in humans, especially those caused by ageing," Dr. David Gems, a professor at UCL Institute of Healthy Aging who led the team of researchers, told Science Daily.

The team found that death spreads throughout the worms via a process called cellular necrosis. In this process, dying cells trigger the death of neighboring cells in a ripple effect, and shoot calcium into one another.

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Muscle cells go first, leading to what is known as rigor mortis. Death then spreads to the intestine, which in the C. elegans triggers a wave of blue fluorescence.

"The way death spreads from cell to cell by calcium is like a house burning down," lead author of the study Dr. Evgeniy Galimov, from the UCL Institute of Healthy Aging, said, according to The Independent.

"What really surprised us at first was that rigor mortis in worms begins while they are still alive," Galimov said. This is due to some significant biological differences between mammals, such as humans, and worms.

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"We realized that death from circulatory failure, as in mammals, doesn't happen in C. elegans. The worms are so small they don't need a circulatory system to get oxygen for respiration," Galimov explained.

At the same time, despite major differences between worms and humans, the researchers believe their findings have important implications for combating death and aging.

"Discovering rigor mortis in worms is exciting as it highlights a key step in the chain of events leading from healthy adulthood to death from old age. It helps us to understand death in humans, and perhaps in the future to prevent death in mortally ill patients," Gems explained.

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"This study... found that under normal aging, ATP [the molecule, adenosine triphosphate, that the body uses for energy] levels actually do not decline," Marina Ezcurra, a neuroscience lecturer at Queen Mary University of London who was not involved in the study, told Gizmodo.

"Only in animals very close to death were decreases in ATP found, suggesting that decreased ATP levels are the result of aging and pathology, rather than the other way around," Ezcurra explained.

This new research is part of a broader study examining the biology of senescence, or the process of deterioration due to age, which is the main cause of disease and death globally.

The critical next step for researchers is to gain a better understanding of how senescent changes in late life trigger waves of cellular necrosis.

Read the full study at cell.com.