To some longevity acolytes, stem cells promise the secret to eternal youth. For a hefty fee, you can pay a startup to extract your own stem cells and cryogenically freeze them, in the hope that they can one day be used in a treatment to help extend your life.

Other firms let you bank stem cells from your babys umbilical cord and placenta after childbirth, if youre convinced the high cost represents an insurance policy against future illness. Or you can follow the example of Sandra Bullock and Cate Blanchett and opt for an anti-ageing cream made with stem cells derived from the severed foreskins of newborn babies in South Korea.

Stem cells are the parent cells which give rise to other cells in our bodies. Since scientists first isolated human embryonic stem cells in a lab and grew them over 20 years ago, they have been mooted as a source of great hope for regenerative medical treatments, including for age-related degenerative conditions such as Parkinsons, Alzheimers, heart disease and stroke.

But apart from a few small-scale examples, the only stem cell-based medical treatment practised in clinics uses haematopoietic stem cells found in the blood and bone marrow which only produce blood cells for transplants in blood cancer patients. These cells are taken from a patients sibling or an unrelated donor, before being infused into a patients blood, or theyre taken from a patients own blood before being reinfused. The procedure has been used to treat blood malignancies for almost half a century, and recently multiple sclerosis too. So how likely is it that the predictions about stem cells' longevity-enhancing powers will become a reality?

In September 2019, Google banned ads for unproven or experimental medical techniques such as most stem cell therapy, citing a rise in bad actors attempting to take advantage of individuals by offering untested, deceptive treatments [that can often] lead to dangerous health outcomes. The decision was welcomed by the International Society for Stem Cell Research, which emphasised that most stem cell interventions remain experimental. Selling treatments before well-regulated clinical trials have been done, the body said, [threatens public] confidence in biomedical research and undermines the development of legitimate new therapies.

Its easy to see how less scrupulous companies can exploit the allure of stem cells, which seem to occupy a place in our collective consciousness as a kind of magical elixir. High hopes for stem cell-based therapies have grown since 2006, when the Japanese biologist Shinya Yamanaka created a new technology to reprogram adult cells, such as skin cells, into a similar state to embryonic stem cells, which are pluripotent, meaning they can develop into any tissue in the body. The Nobel prize-winning breakthrough was hailed as a major step in the study of stem cells without the need for controversial embryo research, and towards the use of these human induced pluripotent stem cells to regenerate damaged or diseased organs or effectively grow new spare parts which could treat the life-limiting and life-shortening illnesses associated with ageing.

Gerontologist Aubrey de Grey, whose Strategies for Engineered Negligible Senescence (SENS) research foundation aims to eliminate ageing-related diseases, thinks the chances well soon have stem cell based therapies are high. For anything that's in clinical trials, you're talking about maybe five years before it's available to the general public, he says, citing stem cell treatments for Parkinsons disease, currently being tested in phase two clinical trials, as one of the developments he thinks is likely to come soonest.

However, given that these trials involve a relatively small number of participants and most clinical trials ultimately fail, his predictions might be overly optimistic. Often described as a maverick, De Grey believes that humans can live forever and there is a 50 per cent chance medical advances of which stem cell therapies will play an important part will make this a reality within the next 17 years. Though living forever, he says, is not the ultimate goal but a rather large side effect of medicine which will successfully prevent or repair the damage that comes with ageing.

For New Jersey-based Robert Hariri, who co-founded Human Longevity Inc, which set its sights more modestly on making 100 the new 60, stem cells derived from placentas present especially exciting opportunities. A biomedical scientist, surgeon and entrepreneur, Hariri says his current venture Celularity which is focused on engineering placental cells, including stem cells, to create drugs for cancer and other conditions is not as concerned about the actual age number, but about preserving human performance as we age and treating the degenerative diseases that rob us of our quality of life.

Many of those working in the field, however, remain cautious in their optimism. Researchers have highlighted the potential risks of giving pluripotent cells to patients, whether they are induced or embryonic, as these cells can develop cancer-causing mutations as they grow.

Davide Danovi, a scientist at Kings College Londons Centre for Stem Cells & Regenerative Medicine, says the path to stem cell-based therapy is very long and full of hurdles. The supply chain involves challenges, he says. On the one hand, allogeneic treatments those with stem cells derived from one individual and expanded into big batches to create cells to treat many individuals have the advantage of being similar to the traditional pharmaceutical business models. The product is clear, its something that comes in a vial and can be scaled up and mass produced, Danovi says. But this treatment can present a greater risk of rejection from the patient, as opposed to the more bespoke autologous option which is more expensive and time-consuming as it involves extracting a patients own stem cells before reprogramming them.

Danovi is most excited by the potential of stem cells to treat age-related macular degeneration. In 2017 Japanese scientist Masayo Takahash led a team that administered transplants of artificially grown retinal cells created from induced pluripotent stem cells taken from donors to five patients with the eye condition, which can cause blindness, and theyre reported to be doing well. The eye, he says seems to be a place where immunity plays less of a role relative to other issues, so you can host cells which come from another individual with fewer problems [of rejection]. But, with other organs such as the liver, he says there are major conceptual problems with creating enough tissue. Its like the clean meat burger - you're talking about a production that is, in many cases, not easy to reach with the current technology.

Hariri believes placentas will solve some of the production challenges crucially, theyre an abundant commodity, with the vast majority thrown out after childbirth. His interest was sparked 20 years ago when his oldest daughter was in the womb: When I saw her first ultrasound in the first trimester, the placenta had already developed into a relatively sizable organ, even though she was just a peanut-sized embryo. Id been taught that the placenta was nothing more than an interface, but [if that was the case], you would expect that it would grow at the same rate as the embryo. His curiosity piqued, he began to see the placenta not as an interface but as a biological factory, where stem cells could be expanded and differentiated to participate in the development of that foetus. That intrigued me and I started to collect placentas and just, you know, basically disassemble them.

Placentas have numerous benefits, he says they dont carry the same ethical controversy as embryonic stem cells, for one thing. Scientists working on embryonic stem cells have to destroy an early embryo, and that option yields them a dozen cells, which have to be culture-expanded in the laboratory into billions of cells. In contrast, the placenta houses, billions and hundreds of billions of cells, which can be expanded as well, but you're starting out with a dramatically larger starting material.

Increasingly, scientists in the anti-ageing sphere are focusing on an approach that seems like the opposite of planting fresh stem cells into our bodies. Experts such as Ilaria Bellantuono at Sheffield Universitys Healthy Lifespan Institute are working towards creating senolytics medication that could kill off our senescent cells, the zombie cells that accumulate in tissues as we age and cause chronic inflammation. I think stem cells are very good for specific disease, where the environment is still young, Bellantuono says, but the data in animal models tells us that senolytics are actually able to delay the onset and reduce the severity of multiple diseases at the same time for example, there is evidence for osteoarthritis, osteoporosis, cardiovascular disease, Alzheimer's, Parkinson's, and diabetes. She explains that while human trials are still in their early stages, senolytics are likely to be more cost-effective than stem cell therapy and the status quo of older patients taking multiple pills for multiple diseases, which can interact with each other. Besides, she adds, they may actually work in tandem with stem-cell based therapies in the future, with senolytics creating a more hospitable environment in tissues to allow stem cells to do their work.

And as for the so-called penis facial? Its far from the only ultra-expensive stem cell skincare making bold anti-ageing claims but youre probably better off saving your money, as you are with the experimental medical treatments on offer. Stem cells are definitely exciting but theyre not the key to eternal youth. At least, not yet.

Robert Harari will be one of the speakers at WIRED Health in London on March 25, 2020. For more details, and to book your ticket, click here

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Are stem cells really the key to making humans live longer? - Wired.co.uk