The near-miraculous properties of stem cells have intrigued medical researchers for years. With their ability to divide repeatedly and fabricate other cells, they are ideal for reconstructing or repairing tissue. Embryonic stem cells can give rise to any organ, while most adult stem cells are limited to their own origin -- neural stem cells make neurons, those from the skin form skin, and so on. Adult stem cells are constantly regenerating our blood and skin, and they also mend tissue that has been damaged by injury or disease.
Now a group of French researchers from the Pasteur Institute have discovered another awe-inspiring property: stem cells can survive without oxygen. This means that even when the body is dead, the stem cells continue living, in a state of reduced metabolism.
The team was led by Dr. Fabrice Chrétien, a histologist and neuropathologist who made a curious observation about five years ago while performing autopsies. Even after a corpse's muscle tissue had started to atrophy, he saw healthy-looking cells that resembled stem cells, with a normal nucleus and intact DNA. One day he ran a test on the corpse of a young adult, taking a muscle tissue biopsy and putting it into a container with culture. His suspicions were confirmed when the stem cells started to multiply. "Honestly, I was a little shocked," he recalled. "Shocked because I had done a biopsy on an individual who had been dead for four days, and in the box of culture the cells proliferated, becoming more numerous every day. They were alive -- and yet the person was undeniably dead. It makes you think twice about the definition of death."
In a living person, certain stem cells spend long periods of time in a quiescent state, not doing much of anything until they are activated due to stress, disease or injury. This quiescence permits them to survive and maintain their potency even under hostile conditions, whether radiation treatment for cancer or a workout at the gym. Once the onslaught has passed, they reawaken and multiply to repair the injured body part.
What Chrétien's team discovered is that they can resist anoxia, or total oxygen deprivation. He explained that inside all our cells we have little organs called mitochondria that convert oxygen into energy. When there is no oxygen, the mitochondria produce toxins that destroy the cells. "We were stupefied to see that when we removed oxygen from the environment, stem cells got rid of their mitochondria," he said. "As a result, their DNA was not damaged." The stem cells stopped breathing and went into a dormant state.
The Pasteur team has tested human muscle from the arm, leg and abdomen, as well as bone marrow from mice. They only work with adult stem cells. (Aside from the controversy of sacrificing embryos for research or medical purposes, Chrétien said that fetal cells that proliferate endlessly can lead to cancer.) They've procured viable stem cells from human bodies up to 17 days after death -- the oldest corpses they have access to -- and from mice up to 14 days post-mortem.
On a recent spring day, Chrétien's colleague, Dr. Pierre Rocheteau, walked me around the lab in a new building on the campus of the venerable Pasteur Institute. He explained that the piece of muscle tissue I saw would be "digested" by enzymes, then put through a machine that discards everything but the dormant stem cells. These would go into a plastic box with culture (glucose, serum and the like) at 37 degrees Celsius, and after three weeks, a thin, whitish layer of muscle would cover the bottom of the box.
I peered into a microscope at a sample after two weeks in culture and saw a number of little spots, all different shapes and sizes. Each stem cell had a black dot of DNA in the center. Rocheteau said they were moving a lot, but the motion was not visible to the naked eye. Then he showed me a time-lapse video of post-mortem stem cells zipping around erratically, stretching out until they split in two, multiplying exponentially, colliding and fusing. Another video displayed the final result, a strip of gently throbbing muscle.
Pasteur Institute policy forbids journalists from seeing the lab mice, but Chrétien told me the team has performed several transplants, injecting bone marrow from a 4-day-old mouse corpse into living specimens that had been irradiated to destroy their own marrow. "It worked magnificently," he said. "All the mice survived." This augurs well for his belief that one day human corpses can provide an additional source of stem cells for medical purposes, such as repairing muscle withered by muscular dystrophy or transplanting bone marrow for leukemia patients. He said that corpses can also be a useful source of stem cells for molecular screening in the pharmacological industry.
Is corpse harvesting necessary?
After meeting with Chrétien, I spoke with Dr. Vijay Gorantla, an associate professor of surgery at the University of Pittsburgh, who has been studying the possibilities of using cadaveric bone marrow to improve hand and face transplants. There is an important linguistic difference here -- Gorantla procures his marrow from brain-dead "cadavers" whose hearts are still beating, as opposed to "corpses" who are dead in every sense of the term. His team's research involves retrieving vertebral bone marrow at the same time as a donated body part and injecting it into a transplant recipient. The idea is to trick the body into accepting the hand and its foreign DNA without needing a lifetime of immunosuppressive drugs.
In the course of these experiments, he, too, was struck by the resiliency of stem cells. A colleague, Dr. Albert Donnenberg, developed a protocol for sterilizing pieces of vertebral bone with bleach or hydrogen peroxide. Despite the harshness of this chemical treatment, the stem cells maintained their counts and viability. Not only that, he found he could hold the vertebral bodies on ice for up to 72 hours before extracting the stem cells, and they were still just fine. "There's something in them that prevents them from dying or offers them this capacity to survive," Gorantla said. He was intrigued by Chrétien's findings, and imagined that one day, after further screening for infection, there could be a worldwide registry connecting patients with deceased bone marrow donors.
Other people I spoke with were more skeptical about the utility of corpses for bone marrow transplants. Dr. Willis Navarro, medical director of transplant services for the National Marrow Donor Program in Minneapolis, said that source is not a major issue. The chance of an American patient finding a living match who is willing and able to donate bone marrow is 66 to 93 percent, and umbilical cords from newborn babies can also be harvested for embryonic-like stem cells.
Chrétien believes this still isn't enough. He said an adult patient generally needs more than one umbilical cord. And in many parts of the world -- including the United States but not France, where it's illegal --parents can privately bank their own offspring's cord blood in case the child needs it later, making it unavailable to the general public.
In any case, much research remains to be done regarding sterility before any human receives injections of post-mortem cells. The slightest risk of infection from bacteria in a corpse would prove fatal for a leukemia patient with a destroyed immune system. Chrétien estimates it will take at least five more years of study before corpses can be viable sources. "And you shouldn't really wait 17 days post-mortem. We did that to prove it could be done, but it's but not ideal. I think within 48 hours after death you can have a good quantity of very effective stem cells without any problems of sterility."
Maintaining cells' "stem-ness"
In the meantime, his team's discovery also offers better ways to isolate and store stem cells. Storage can be problematic because as soon as stem cells are dissociated from tissue they start to proliferate like mad, eventually exhausting their capacity to multiply, or their "stem-ness." But when they are deprived of oxygen and kept at 4 degrees Celsius, they hibernate for up to a month. This dormancy is reversible: the cells awaken and resume their normal activity after being put in culture or transplanted into a living body.
Currently, Chrétien's team is studying the possible repercussions of their discovery on cancer treatments that consist of cutting off a tumor's blood supply and starving it of oxygen. He said it would be catastrophic if cancer stem cells didn't die with the rest of the tumor but instead went to sleep, only to wake up later and make new tumors. "We don't want an upsurge of metastasis in a few years," he explained. Though the research is in its early stages, he has found that cancer stem cells are in fact sensitive to oxygen deprivation in vitro. However, he cannot say if that is the case inside an actual person.
Indeed, it seems that not all of the body's stem cells react the same way to different aggressors. Researchers at the McKnight Brain Institute in Gainesville, Fla., led a collaboration with investigators at the Kennedy Space Center, looking at the effects of cosmic rays on the brain. Surprisingly, they found that quiescent stem cells in the brain are extremely sensitive to cosmic radiation -- a simulated mission to Mars showed up to 65 percent of them at risk of dying. According to Dr. Dennis Steindler, who directed the McKnight Brain Institute (and the study), this result contradicts the assumption that cancer tumors return after chemotherapy or radiotherapy because quiescence protects their stem cells.
Steindler said that understanding the metabolic requirements of different kinds of stem cells and how they behave under stress will provide scientists with valuable insight "extremely relevant to cancer research." This knowledge can shine a light on other diseases, too. As Chrétien noted, "We are starting to see that the quantity of oxygen varies widely in different tissues of the body, and it's not just chance -- it plays a very particular role in cell fate."
Stem cell research heralds a revolution in medical care. Cellular therapy can turn doctors into engineers of the human body, reconstructing tissue or building new organs without surgery. The fact that some stem cells have superhuman qualities makes the range of possibilities even larger. It is true that stem cells play a small role in practical medicine today. But, Chrétien predicted, "they will be enormously important tomorrow."
Photo: William Beaucardet