Shastri's and his colleagues took a new approach that has proven to be surprisingly simple. They decided to take advantage of the body's natural wound-healing response and create a special zone on the surface of a healthy bone in hopes that the body would respond by filling the space with new bone. The approach lived up to their highest expectations. Working with mature rabbits, a species with bones that are very similar to those of humans, the researchers were delighted to find that this zone, which they have dubbed the "in vivo bioreactor," filled healthy bone in about six weeks. And it did so without having to coax the bone to grow by applying the growth factors required by previous in vivo efforts. Furthermore, they found that the new bone can be detached easily before it fuses with the old bone, leaving the old bone scarred but intact.
"The new bone actually has comparable strength and mechanical properties to native bone," says Molly Stevens, currently a reader at Imperial College in the United Kingdom who did most of the research as a post-doctoral fellow at MIT, "and since the harvested bone is fresh it integrates really well at a recipient site."
Long bones in the body are covered by a thin outer layer called the periosteum. The layer is a little like scotch tape: the outside is tough and fibrous but the inside is covered with a layer of special pluripotent cells which, like marrow cells, are capable of transforming into the different types of skeletal tissue. So Shastri and his collaborators decided to create the bioreactor space just under this outer layer.
They created the space by making a tiny hole in the periosteum and injecting saline water underneath. This loosened the layer from the underlying bone and inflat