Can you grow a human bone outside the human body? The answer may soon be yes, but before we can understand how that's possible, we need to look at how bones grow naturally inside the body. Most bones start in a growing fetus as a soft, flexible cartilage. Bone-forming cells replace the cartilage with a spongy mineral lattice made of elements like calcium and phosphate. This lattice gets harder, as osteoblasts, which are specialized bone-forming cells, deposit more mineral, giving bones their strength. While the lattice itself is not made of living cells, networks of blood vessels, nerves, and other living tissues grow through special channels and passages. And over the course of development, a legion of osteoblasts reinforce the skeleton that protects our organs, allows us to move, produces blood cells, and more. But this initial building process alone is not enough to make bones strong and functional.

If you took a bone built this way, attached muscles to it, and tried to use it to lift a heavy weight, the bone would probably snap under the strain. This doesn't usually happen to us because our cells are constantly reinforcing and building bone wherever they're used, a principle we refer to as Wolff's Law. However, bone materials are a limited resource, and this new, reinforcing bone can be formed only if there is enough material present. Fortunately, osteoblasts, the builders, have a counterpart called osteoclasts, the recyclers. Osteoclasts break down the unneeded mineral lattice using acids and enzymes so that osteoblasts can then add more material. One of the main reasons astronauts must exercise constantly in orbit is due to the lack of skeletal strain in free fall. As projected by Wolff's Law, that makes osteoclasts more active than osteoblasts, resulting in a loss of bone mass and strength.

When bones do break, your body has an amazing ability to reconstruct the injured bone as if the break had never happened. Certain situations, like cancer removal, traumatic accidents, and genetic defects, exceed the body's natural ability for repair. Historical solutions have included filling in the resulting holes with metal, animal bones, or pieces of bone from human donors, but none of these are optimal as they can cause infections or be rejected by the immune system, and they can't carry out most of the functions of healthy bones. An ideal solution would be to grow a bone made from the patient's own cells that's customized to the exact shape of the hole, and that's exactly what scientists are currently trying to do. Here's how it works. First, doctors extract stem cells from a patient's fat tissue and take CT scans to determine the exact dimensions of the missing bone. They then model the exact shape of the hole, either with 3D printers,

or by carving decellularized cow bones. Those are the bones where all of the cells have been stripped away, leaving only the sponge-like mineral lattice. They then add the patient's stem cells to this lattice and place it in a bioreactor, a device that will simulate all of the conditions found inside the body. Temperature, humidity, acidity, and nutrient composition all need to be just right for the stem cells to differentiate into osteoblasts and other cells, colonize the mineral lattice, and remodel it with living tissue. But there's one thing missing. Remember Wolff's Law? An artificial bone needs to experience real stress, or else it will come out weak and brittle, so the bioreactor constantly pumps fluids around the bone, and the pressure tells the osteoblasts to add bone density. Put all of this together, and within three weeks, the now living bone is ready to come out of the bioreactor.

and to be implanted into the patient's body. While it isn't yet certain that this method will work for humans, lab-grown bones have already been successfully implanted in pigs and other animals, and human trials may begin as early as 2016.

How to grow a bone - Nina Tandon

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