by Prof. Michio Kaku
But even if age genes do exist and we can alter them, will we suffer the curse of Tithonus, who was doomed to live forever in a decrepit body? It is not clear that altering our age genes will reinvigorate our bodies. What is the use of living forever if we lack the mind and body to enjoy it?
A recent series of experiments show that it may one day be possible to “grow” new organs in our body to replace worn-out organs. A number of animals, such as lizards and amphibians, are able to regenerate a lost leg, arm, or tail. Mammals, unfortunately, do not posses this property, but the cells of our bodies, in principle, have, locked in their DNA, the genetic information to regenerate entire organs.
In the past, organ transplants in humans have faced a long list of problems, the most severe being rejection by our immune system. But, using bioengineering, scientists can now grow strains of a rare type of cell, called the “universal donor cells,” which do not trip our immune system into attacking them. This has made possible a promising new technology which can “grow” organ parts, as demonstrated by Joseph P. Vacanti of the Children’s Hospital in Boston and Robert S. Langer of MIT.
To grow organs, scientists first construct a complex plastic “scaffolding” which forms the outlines of the organ to be grown. Then these especially bioengineered cells are introduced into the scaffolding. As the cells grow into tissue, the scaffolding gradually dissolves, leaving healthy new tissue grown to proper specifications. What is remarkable is that the cells have the ability to grow and assume the correct position and function without a “foreman” to guide them. The “program” which enables them to assemble complete organs is apparently contained within their genes.
This technology has already been proven in growing artificial heart valves for lambs, using a biodegradable polymer, polyglycolic acid, as the scaffolding. The cells which seeded the scaffolding were taken from the animals’ blood vessels. The cells “took” to the scaffolding like children to a jungle gym.
In the past few years, this approach has been used to grow layers of human skin for use in skin grafts for burn patients, Skin cells grown on polymer substrates have been grafted onto burn patients, as well as the feet of diabetic patients, which must often be amputated for lack of circulation. This may eventually revolutionize the treatment of people with severe skin problems. As Marie Burk of Advanced Tissue Sciences says: “We can grow about six football fields from one neonatal foreskin.”
Human organs such as an ear have actually been grown inside animals as well. The scientists at MIT and the University of Massachusetts recently were able to overcome the rejection problem and (painlessly) grow a human ear inside a mouse. The scaffolding of a life-sized human ear was made of a porous, biodegradable polymer and then tucked under the skin of a specially bred mouse whose immune system was suppressed. The scaffolding was then seeded with human cartilage cells, which were then nourished by the blood of the mouse. Once the scaffolding dissolved, the mouse produced a human ear. Eventually, scientists should be able to grow this ear without the aid of the mouse. This could open up an entirely new area of “tissue engineering.”
Already other experiments have been done which show that noses can also be generated. Scientists have used computer-aided contour mapping to create the scaffolding and cartilage cells to seed the scaffolding.
Now that the technology has been shown to be effective on a small scale, the next step will be to grow entire organs, such as kidneys. Walter Gilbert predicts that within about ten years, growing organs like livers may become commonplace. One day, it may be possible to replace breasts removed in mastectomies with tissue grown from one’s own body.
Recently, a series of breakthroughs were made to grow bone, which is important since bone injuries are common among the elderly and there are more than two million serious fractures and cartilage injuries per year in the United States. Using molecular biology, scientists have isolated twenty different proteins which control bone growth. In many cases, both the genes and the proteins for bone growth have been identified. These proteins, called bone morphogenic proteins (BMP), instruct certain undifferentiated cells to become bone. In one experiment, twelve dental patients with severe bone loss in the upper jaw were successfully treated with BMP-2. (Normally, doctors would have to harvest bone from the patient’s own hip, a complicated procedure which requires surgery.)
The ultimate goal of this technology would be to grow a complex organ, such as the hand. Although this may still be decades away, it is within the realm of possibility. The step-by-step outline of such a complex process has already been mapped out.
First, the biodegradable scaffolding for the hand must be constructed, down to the microscopic details of the ligaments, muscles, and nerves. Then bioengineered cells which grow various forms of tissue would have to be introduced. As the cells grow, the scaffolding would gradually dissolve. Since blood is not yet circulating, mechanical pumps would have to provide nutrients and remove wastes during the growing process. Next, the nerve tissue would have to be grown. (Nerve cells are notoriously difficult to regenerate. However, in 1996 it was demonstrated that the severed nerve cells in mice’s spinal cords can actually regenerate across the cut.) Last, surgeons would have to connect the nerves, blood vessels, and lymph system. It is estimated that the time needed to grow such a complex organ as the hand may be as little as six months.
In the future, we may therefore expect to see a wide variety of human replacement parts becoming commercially available from now to 2020, but only those which do not involve more than just a few types of tissue or cells, such as skin, bone, valves, the ear, the nose, and perhaps even organs like livers and kidneys. Either they will be grown from scaffolding, or else from embryonic cells.
From the period 2020 to 2050, we may expect more complex organs and body parts containing a wide variety of tissue cells to be duplicated in the laboratory. These include, for example, hands, hearts, and other complex internal organs. Beyond 2050, perhaps every organ in the body will be replaceable, except the brain.
Of course, extending our life span is only one of many ancient dreams. Yet another, even more ambitious one is to control life itself, to make new organisms that have never before walked the earth. In this area, scientists are rapidly approaching the ability to create new life forms.
In summary, we may see ageing research growing in several phases. First, hormones and anti-oxidants may be able to retard the ageing process, but not stop it. Second, rapid advances in genetic research may unlock the secret of cell ageing itself. For example, in 1998, a breakthrough was made when telomerase, mentioned in Chapter 8, was shown to stop ageing in human skin cells in a petri dish. In this phase, many more age genes may be isolated and shown to control the rate of cell ageing. And lastly, human organ replacement may become standard therapy in the 21st century.
Visions; 1999: 217