WHAT HAPPENS IN OUR SPINE?

The spine occupies the most crucial position in the skeleton. All bones are connected with one another via this vertebral column. This series of bones runs through our backs like a tension spring, and we owe much of our mobility to it— without it, walking upright would not even be possible. The spine’s 24 vertebrae (plus the sacrum and coccyx) have another function too: Each individual vertebra (below) is hollow at its center. Together all of the vertebrae form a long tube, and the spinal cord, the body’s main neural pathway, passes through it. The spinal cord leads directly into the brain, and together these two components make up our central nervous system. The bony canal in the spine offers ideal protection for the body’s primary communication system. But it’s not impervious to harm: The spine of someone who is involved in a serious accident can be so badly injured that the nerve fibers become bruised or even torn. The result is paralysis. Scientists are working hard to discover ways to rectify such severe disabilities. There is preliminary evidence that stem cells or certain hormones can be used to “mend” the spinal cord and make it fully functional again.

WHO DEVELOPED OUR AMAZING SKELETON?

Though they may yet be perfected, by a genetic mutation, for example, our bones are already a marvel of nature. The 206 bones of our skeleton are joined together like a giant puzzle. Babies have 270 bones, but as they grow some of them fuse together. Bones make up around 14% of our body weight, and they’re among our most important organs. Every bone is a part of a larger whole. Ligaments, tendons, muscles, and cartilage join bones to the skeleton; altogether they form a unit as complex as a symphony orchestra. The conductor of this bone symphony is the spine, which holds all the parts of the skeleton together and gives the body its flexibility. Like a tension spring, the 24 articulating vertebrae are positioned along the spine and are held together by six strong ligaments, the intervertebral discs, and a multitude of muscles. The spine is what keeps us upright, and it dampens shocks better than the shock absorbers of a truck.

The basic blueprint of our skeleton is 530 million years old. This is when the first vertebrates began to appear, a group that includes mammals, fish, birds, amphibians, and reptiles. They all have a spine and a rib cage as well as a mouth at one end of the body and an anus at the other. Today more than 50,000 vertebrate species exist, all of them variations on the original primal skeleton. These range from tiny hummingbirds all the way up to gigantic blue whales. But if we adjust for body weight, hummingbirds might even have a stronger skeleton than whales. A hummingbird’s sternum is almost the size of the animal itself and protrudes from its chest like a ship’s keel. Attached to this bone are the bird’s flight muscles, which allow its small wings to beat up to 80 times per second. The bird’s tiny skeleton can easily cope with this enormous burden. On the other hand, the blue whale’s skeleton is a soft structure. Since the body weight of whales is supported by water, their bones are very soft, porous, and filled with oil. Oftentimes when one of the animals gets stranded its bones will break—a whale’s skeleton can hardly support its 200 tons of weight. But in water the bones are ideal and their oil-filled composition supplies whales with the necessary buoyancy.

ARE OUR BONES ALIVE?

Bones are far from lifeless lumps of stone: Bones grow, produce blood cells, and respond in flexible ways to stress. The fact that our bones are crisscrossed by fine blood vessels proves one thing: The bones are real living tissue and not merely minerals. The primary components of bones are calcium phosphate ( 70%) and collagen (30%). Calcium phosphate is a mineral that is also found in our teeth. It makes our bones hard, but also brittle. Collagen is a protein that has a twisted structure that’s similar to a steel cable. Collagen fibers do not tear, even when they’re burdened with 10,000 times their own weight. Inside the bones these fibers provide the proper amount of firmness. The collagen fibers surround the mineral crystals and create a stable network. The bone cells reside between these two key components. Although they make up only a very small part of the total bone mass, these cells breathe life into bones. They are constantly regenerating, keeping bones stable, and allowing them to grow.

Bone cells register exactly where and how a bone is being stressed. For example, if a couch potato were to suddenly start jogging after years of a pronounced lack of activity, then this would directly affect the bones. Every step sends pressure waves through our skeleton. The bone cells respond by reinforcing the area that is under stress with cross bracing. While a steel girder can break after

years of being overloaded, tiny tears in bones are quickly closed up again. The cells can also heal broken bones. This attribute distinguishes us from mollusks such as snails, which have no such cells in the material of their shells. If a snail’s shell were to break, the animal would be a goner.

Experiments conducted at Berlin’s Charité University Hospital showed how much stress a bone is able to withstand. Researchers burdened a bone, a steel pipe, and a comparable piece of wood until they broke. The wood gave way under a weight of 494 pounds, the steel pipe gave up at 562 pounds, and the bone lasted until 1,415 pounds of weight pressed down upon it. However, the results of such experiments are of only limited significance when it comes to the real thing. That’s because the amount of energy required to actually break a bone depends on the type of bone: “The shapes of the individual bones differ. And the compact bone matter is particularly resistant to rotational forces, while the spongy substance stabilizes bones against compression and shifting forces,” says bone expert Philippe Gillet, a scientist at Belgium’s Liège University Hospital.

WILL WE ALL GET SUPER BONES SOMEDAY?

Of course it would be even better if we all had bones like Toby Smith, which seem as though they can’t be broken. For average individuals bone density is at its highest between the ages of 20 and 30. After that bones become weaker because the body stores less calcium phosphate. This invites the question: Why haven’t we all evolved to have super bones like the Smith family? Why has nature not perfected our bones? Or is the Smith family a prototype in this direction? The first step, the mutation of a gene, has occurred in their family. However, a gene only spreads in a population when it gives its bearer an advantage. Perfection doesn’t actually matter to evolution at all. “For natural selection it is only necessary that something works—not that it works as well as possible. There is no need for people to be perfectly adapted—they only need to be adapted as well as their competitors,” says Michael Le Page, who covers biology for the Uk-based magazine New Scientist.

Our bones can bear a lot of stress. But what are the biggest challenges for them? The most severe problem affects women who’ve gone through menopause because the change in their hormonal balance contributes to a rapid decrease in bone density. Around one-third of postmenopausal women develop osteoporosis, which makes their bones extremely brittle.

Richard Lifton’s discovery of the super bone gene among members of the Smith family could be a boon for osteoporosis patients. “We’re getting better and better at understanding genetic regulation of bone density. The gene LRP5 plays a key role. In the Smith family it had mutated in a way that made it very active, which resulted in their very dense bones,” says Lifton. The geneticist and his colleagues at Yale University have also discovered something else: LRP5 increases its activity over the course of a lifetime. To compensate for this, a particular protein gradually slows down the gene. Lifton sees it as a path to a new osteoporosis therapy: “We could use drugs to disable the protein, allowing LRP5 to continue to work normally for a longer time and the bones to retain their density.” That wouldn’t give us all “super bones” like the Smiths, but for millions of older individuals, having bones that are “a bit stronger” is already more than enough— and Unbreakable can remain in the domain of superheroes.