It was one of their medical observations: that human bone is one of the few tissues that can re-grow after injury. Hippocrates knew that and hoped that power could be harnessed for healing. Now, 2400 years later, reports from commercial and university laboratories suggest that scientists have begun to do just that: to grow bones and cartilage virtually at will. "This is exciting because we are mimicking the natural process of development," said Dr. A. Hari Reddi, a professor of biology and orthopedics at the John Hopkins medical center in Baltimore, who has worked on bone growth for more than thirty years. "We are following the same steps that occur in the first week after conception." The success is one of several in the new field of tissue engineering, the growing of spare parts for the body. The new power to grow human tissues and organs is a result of years of basic research followed by rapid progress in molecular biology and genetic engineering. Among the tissues now grown successfully, at least in the laboratory, are skins, bone cartilage, liver, kidney and teeth.
The new work on bones is among the most advanced, and researchers say that the new treatment will soon be available for a variety of conditions in which the body needs to grow new bones but cannot. The key to the recent success is the family of molecules known as BMPs, for bone morphogenic proteins. They are made when an injury occurs and set off the formation of new bone and cartilage by homing in on certain immature or unspecialized cells, and inducing them to proliferate and become one of several specialized tissues, like bone and cartilage. All this was learned over the last few decades, as scientists labored to find the magical molecules that would produce natural bone growth. They pulverized bones and removed the calcium from the resulting powder, working with the remaining material to isolate the factor that was causing bone growth.
"But the work for many years went like a snail," Reddi said. Then in recent years, with the new techniques of molecular biology, scientists were able to isolate both the proteins responsible for bone growth and the genes responsible for producing them. Roughly, 20 protein molecules have been identified that could induce bone growth. Each of the molecules also seems to have the power to stimulate other to begin growing. Reddi says that he and other scientists had found that the genes that made the BMPs were both ancient and general. Even fruit flies, which have no bones, use them to set off growth of specialized tissues like wings. "These are not just bone signals but are general signals to initiate differentiation in many tissues," he said, referring to a wide variety of tissues ranging from kidneys to brain to gonads. "What we are working with, is the body's own signaling molecules that cells tell to go ahead, 'you be bone' or 'you be muscle'," said Dr. Charles Cohen, chief scientist at Creative Biomolecules, one of the companies working on making products from bone proteins.
"There are two steps," he said. "The BMP signal to the cells says, 'Go!'" he said. "Then information the cells get from the neighborhood where they live tells them to be bone or cartilage. "Over the last five or six years, dozens of papers have shown that researchers can reliably stimulate natural bone growth in mice, rabbits, dogs and monkeys. Now the first tests from human experiments are coming in, and they show success as well, researchers say. Two small studies in humans were presented at scientific meetings last month by representatives of the Genetic Institute Inc., in Cambridge, Massachusetts. One was a study at four universities in which twelve dental patients with bone loss in their upper jaws underwent oral surgery in which BMP-2 and a sponge made of artificially produced collagen, a central component of skin and bone, were implanted in the area where was none, and all went on to get implants. The standard treatment for all these cases would have involved surgery of the mouth and also surgery of the harvest bone from the hip for implantation in the mouth. Such procedures are frequently successful, but they are expensive and lengthy and simply cutting down on surgery reduces risk. "We are talking about an outpatient procedure versus the current treatment which involves hospital stay and surgery," said Dr. Gerald Riedel, at the bone protein project.

Q.1. What are the major advantages of being able to grow bone over conventional methods like surgery?