Senescent cells are key drivers of the ageing process — and a new generation of drugs is seeking to banish them from our body.

Key points: As we age we have an increased number of senescent or "sleeping" cells that don't divide but still have effects on the body

As we age we have an increased number of senescent or "sleeping" cells that don't divide but still have effects on the body Senescent cells are thought to drive ageing, so some scientists are developing drugs to eliminate these cells

Senescent cells are thought to drive ageing, so some scientists are developing drugs to eliminate these cells But others say there are ethical concerns around trying to stop ageing and age-related disease

A normal, healthy cell can divide in a controlled manner. In contrast, senescent cells have lost their ability to divide.

They are not dead though. These cells hang around in the body and stay metabolically active, according to Keefe Chan, cell senescence expert at the Peter MacCallum Cancer Centre.

"They're essentially like sleeping cells," Dr Chan said.

As we age, our bodies accumulate senescent cells, which then drive many age-related processes and diseases.

Scientists are now focusing on using our understanding of cell senescence to create drugs to increase years of healthy life and even dramatically increase the human lifespan.

What makes senescent cells different?

Senescent cells look squashed in comparison to normal cells, most of which are round and smooth. ( ABC: Catalyst )

As well as not dividing, sleeping cells look very different to regular cells.

"They are very large and flat and have what we term a 'fried egg' morphology," Dr Chan said.

Some cells enter a sleeping state irreversibly — they will never divide again. However, some cells can wake up and revert to their original state.

What can happen to cells? Division: Cells in our body divide for growth and renewal. This usually happens through a process called mitosis .

Cells in our body divide for growth and renewal. This usually happens through a process called . Apoptosis: The regulated and "pre-planned" death of a cell. This is essential for normal growth and function and allows for the right amount of cells in the body. Different cell types have different "lifespans", meaning that after a certain number of days, they will undergo their pre-planned death. Apoptosis is orderly, and waste products from the dead cells are removed from the body.

The regulated and "pre-planned" death of a cell. This is essential for normal growth and function and allows for the right amount of cells in the body. Different cell types have different "lifespans", meaning that after a certain number of days, they will undergo their pre-planned death. Apoptosis is orderly, and waste products from the dead cells are removed from the body. Necrosis: Cell death caused by exposure, extreme conditions or toxins. Widespread necrosis can lead to serious disease.

Cell death caused by exposure, extreme conditions or toxins. Widespread necrosis can lead to serious disease. Senescence: Cells that can no longer divide, but have not died. They remain in the body, but do not perform their previous functions.

A major reason our bodies evolved to have senescent cells is to suppress cancers, Dr Chan explained.

Genetic mutations triggered by things such as UV exposure can sometimes cause cells to replicate uncontrollably — and uncontrolled cell growth is cancer.

Cells are often able to detect these mutations and in response go to sleep to stop them dividing.

The moles on our skin are hotspots for senescent cells for this reason. When the body senses genetic mutations caused by UV exposure, scientists believe cells then become senescent to shut down cancer growth.

Senescent cells also have a role in wound healing, embryonic development and recruiting immune cells to clear infection.

Moles are hotspots for senescent cells. By becoming senescent, sun-damaged cells are less likely to become cancerous. ( Getty Images: SCIENCE PHOTO LIBRARY )

Sleeping cells and ageing

While these sleeping cells have their uses, they also cause harm to the body.

Judith Campisi of the Buck Institute for Research on Aging found that senescent cells are major drivers of the ageing process and many age-related diseases.

As we age, senescent cells increase in the body. The build-up of these cells causes tissue damage and degradation, which can eventually lead to organ failure, Professor Campisi said.

Professor Campisi has studied the negative effects of senescent cells in the body. ( ABC: Catalyst )

Senescent cells, while unable to divide, continue to pump out chemical signals.

"One of the things those signals do is it causes the immune system to begin to react, and that is called inflammation," Professor Campisi said.

Prolonged and heightened inflammation is implicated in many age-related diseases, including Alzheimer's, arthritis, cardiovascular problems and more.

For most of human history, people didn't reach an advanced enough age to suffer many of the age-related diseases driven by senescent cells, Professor Campisi said.

In the last decade there has been an explosion of research aiming to stop or reverse ageing by targeting and eliminating senescent cells with drugs known as senolytics.

"There's no reason you can't enjoy life at 95 as much as you could at 25," Professor Campisi said.

Targeted treatments for cancer and cognitive decline

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Targeted senolytics that aim to treat specific diseases are far closer to becoming a reality, according to Dr Chan.

After cancer treatment, cancer can sometimes return and metastasise.

Chemo does not always kill all cancer cells, rather, it can make some cells senescent. These sleeping cells can then "reawaken" as treatment-resistant cancer.

"These cells have the capacity to repopulate an entire tumour just from a single cell," Dr Chan said.

Dr Chan and his team are trying to develop senolytics that could be applied after cancer to eliminate these sleeper cells and make it less likely that cancers will return.

Studies of senolytics in mice have shown their potential to slow neurodegenerative diseases that lead to cognitive decline such as Alzheimer's disease, and reduce post-traumatic osteoarthritis.

The science is only just now coming to a place where some senolytic drugs are being tested on humans.

The fountain of youth?

The science of senolytics has also caught the attention of groups trying to achieve dramatic increases in longevity, and even immortality, Dr Chan said.

"It's the thought that if you eliminate the senescent cells you will stay young forever — that's the blue-sky thinking," he said.

Some researchers are working towards drugs which would allow people to extend youth, perhaps indefinitely. ( Unsplash: Sean Stratton )

In mice, the application of senolytics has been found to dramatically increase the lifespan and hold off the onset of age-related disease.

But don't expect to be popping a pill to keep give you eternal — or at least prolonged — youth any time soon.

"We are a long way from there yet. Eliminating senescent cells can have good and bad effects," Dr Chan said.

One of the problems, he said, is that the drugs struggle to identify just senescent cells and can accidently kill normal cells.

But is getting old really a bad thing?

The premise that ageing and death are necessarily "bad" needs to be challenged, according to Matt Beard, a philosopher and ethicist at the Ethics Centre.

"These movements see mortality as a problem that needs to be solved rather than a reality that needs to be accepted," Dr Beard said.

"Recognising mortality and living in light of that mortality enables us to live meaningfully."

Some ethicists suggest that we need revalue ageing, rather than develop drugs to stop it. ( Jose Luis Pelaez Inc )

Movements to extend life and create immortal people tend to coalesce around the Silicon Valley start-up culture, he observed.

It is concerning, Dr Beard said, that tech elites are spending so much capital on trying to extend their own lives while many people are dying from preventable diseases around the world.

"The epicentre of the tech movement seems to be preoccupied with questions that service primarily them and those who are close to them — rather than the world at large."

However, Dr Beard isn't wholly sceptical of research into senolytics.

"Health is a good thing, so if sleeping cells are driving painful or debilitating illness, there is a moral imperative to investigate."