Henrietta Lacks, a Virginia tobacco farmer, died in 1951, but her cells are still alive today. Before Lacks succumbed to cervical cancer at age thirty-one, her doctor at Johns Hopkins excised part of her tumor and sent it down the hall to the hospital’s Tissue Culture Laboratory. There, Lacks’s cells were placed in a mixture of human placental blood, beef embryo extract, and fresh chicken plasma in the hopes that they might survive outside the human body.
And survive they did. Henrietta Lacks’s cervical cells proved to be the first immortal human cell line; rather than die off as most cell cultures do, Lacks’s cells thrived and even reproduced in the lab. Today, Lacks’s cells, dubbed HeLa cells, live in laboratories all over the world, nurtured in a variety of growth media and nestled in bioreactors and held at a steady 37°C. It is estimated that some 50 million metric tons of HeLa cells are alive today, derived from a woman who stood just over five feet tall.
Although animal tissue had been successfully cultured before, HeLa was the first human cell line to reproduce successfully and indefinitely in the lab, and its contribution to modern medicine is immeasurable. HeLa cells were key to the development of the polio vaccine. They’ve played an invaluable role in gene mapping and in vitro fertilization. Bits of Henrietta Lacks have even been sent into space. Cultivating that first HeLa culture was also the first step on the path to nurturing human tissue, blood, and eventually entire organs outside the human body.
Keeping Cells Alive Without a Body
So what do you feed your human cell cultures? There are now dozens of human cell lines available for purchase—not to mention stem cell lines—and an entire industry has sprung up around their care. Gone are the days when researchers would place their cells in chicken plasma and hope for the best. Researchers have had decades to tinker with the menu, and each cell line has its favorite mix of foods. Most start out with a base of Eagle’s Minimum Essential Medium, the most popular growth medium on the market, which supplies cells with their daily recommended dose of amino acids, salts, glucose, and B vitamins. That usually gets topped off with a helping of animal-derived serum, often from a fetal calf, which provides essential factors for cellular growth. However, for the vegetarian researcher, serum-free alternatives are available. Add a dash of antibiotics, and you’ve got a recipe for well-fed cells free of bacterial contamination.
But human tissue cannot live on growth medium alone. In addition to food, researchers must also provide their cell and tissue cultures with shelter and plenty of exercise. To that end, bioreactors are the YMCAs of the laboratory. Cell and tissue cultures want to live at the same temperature, humidity, gas levels, and pH that they would find inside the human body, and bioreactors offer all those comforts of home. They also keep cells safely in the dark, away from the deleterious effects of light. But what makes bioreactors more than just souped-up Petri dishes is the mechanical stimulation they provide to their cellular tenants. Regular exercise helps cells form their extracellular matrices, the part of animal tissues that provides support for the cells and platforms for growth and healing. Bioreactors rake and churn cells into stronger, healthier cultures. It’s thanks to these carefully crafted, closely regulated environments that researchers have cultivated living, implantable heart, bone, skin, kidney, liver, cartilage, skeletal muscle, and other types of tissue.
The Future of Blood
Blood banking predates HeLa cultures by more than three decades, and has more modest aims. The emphasis has long been on staving off blood cell death rather than nurturing live blood cultures. In fact, when the Red Cross stores blood for transfusion, it separates the platelets from the red blood cells from the plasma, treating each component according to its particular needs. Platelets can be kept at room temperate for just five days, and they must be stored in agitators that provide constant side-to-side motion to help platelets retain their adhesive properties. Red blood cells can be frozen, but because freezing the cells is so costly, they are usually refrigerated at 6°C for up to 42 days. Plasma, on the other hand, can be more readily frozen for up to a year. Some medical facilities will store whole blood, refrigerating the blood after treating it with an anticoagulant solution.
But, in the last few years, we’ve seen major breakthroughs that could lead scientists to culture infusible blood cells in the lab. In 2008, a company called Advanced Cell Technology produced the first functional lab-grown red blood cells. Red blood cells live a mere 120 days even inside the human circulatory system, so any living, growing culture of blood would need a constantly refreshed supply of red blood cells. In the human body, red blood cells grow in bone marrow, and the ACT researchers were able to form red blood cells from embryonic stem cells by growing them on stromal cells from bone marrow. Without those stromal cells, which function as loose connective tissue, lab-grown red blood cells retain their nuclei and risk becoming cancerous. Over the last few years, Arteriocyte, a DARPA contractor, has made progress “pharming” blood using a similar method, taking hematopoietic stem cells, which are harvested from adult bone marrow, and placing them in a growth factor that imitates the conditions of bone marrow. The technology could mean a ready supply of pathogen-free blood, constantly refreshed by new blood cells.
Breathing Life Into Disembodied Organs
The ability to keep blood alive in a lab has far-reaching consequences, but few medical advances are as visually dramatic as a disembodied organ kept alive and pumping in the lab. Transplanting an organ from one body into another has required no small amount of resurrection. Once removed from a donor, organs are packed in cold saline to prevent decay and eventually brought back to life inside the recipient’s body. But while livers, pancreases, and intestines can remain viable outside the body for up to a day, hearts and lungs lose their viability after just four to six hours, making each transplant a race against the clock and each excised organ a quickly decaying lump of flesh.
However, if our organs maintain their normal functions while outside the body—if our hearts still pump blood, if our lungs breathe, if our kidneys filter out waste—their odds for viability improve. Andover, Massachusetts-based TransMedics has developed the Organ Care System, a squat device that vaguely resembles a trash compactor but reproduces many of our body’s own functions. Like the bioreactor, the OCS keeps excised organs in a sterile environment, at a carefully regulated temperature and humidity, while nutrient-rich blood flows through the organ. And just as the bioreactor keeps cells active and moving, the OCS keeps organs pumping. It’s startling to watch a lung breathe—pumping air in and out—underneath plastic, to see a telltale heart beat inside its clear box. This past spring, transplant researchers at Toronto General Hospital revealed that damaged donor lungs can actually heal during their time on a ventilator. At present, organs can spend only a few hours inside these mechanical bodies, but perhaps someday those machines could keep our lonely, bodiless hearts beating indefinitely.
Keeping extremities such as hands and feet alive, however, is a much more difficult proposition. Severed limbs have been reattached to their rightful owners, but only a few hours after their removal. To keep an extremity alive long-term, a scientist would need to maintain the circulation in all those tiny blood vessels, or else the extremity would wither and decay. When a girl in Zhengzhou lost her hand in a tractor accident last year, physicians grafted the severed hand onto her leg to keep it alive until they could reattach it to her arm. There is no machine that could have done it for her.
But we’ve come a long way from cultivating a handful of cancer cells in a lab, and every day scientists learn more about the particular wants and needs of our tissues and organs. It may be just a matter of time before we can walk into a laboratory and see our own hands waving back at us.
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