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Like skyscrapers, bodies have construction rules
STANFORD -- If you volunteer to colonize the moon or Mars, you can expect your kids born there to look different and move quite differently than you do by the time they start kindergarten.
And, if you plan to send them to boarding school on Earth, Stanford University Prof. Dennis Carter says, you may possibly need some extra cash for a wheelchair, golf cart or caretaker to help them move around.
Current scientific explanations of how we become who we are may overestimate the role of the genome inside that single cell from which we all begin, said Carter, a specialist in biomechanical engineering. He contends that it will not take generations of genetic evolution in a changed environment to substantially change your descendants.
Carter and his students have demonstrated that our bones aren't told how to grow by the genome nearly so much as they are shaped by biomechanical construction rules, which are based on our environment inside the womb and on the planet Earth.
Just as there are rules for constructing bridges and buildings that will stand up to the physical forces in their environment, so there are rules for the construction of bones - and, most likely, other tissues - within an animal species.
Carter and his students have been able to write those rules, in the form of mathematical formulas called algorithms, for bones in a variety of species, living and extinct. The rules can be expected to drift over millions of years through genetic selection or evolution. Run in a computer program, the rules help predict the effects of different environments on bone development.
"These algorithms are a brilliant mechanism for survival, and they could evolve differently for each gravity field," he said. The bones of all Earthlings would change on Mars, but by far the most change would occur in those conceived and born there.
Human skeletal construction begins in the womb, where the local cellular environment pushes and pulls developing cartilage that will eventually ossify into bone, he said. The researchers have found that bone orients itself towards principal stresses and becomes most dense where those stresses are greatest in order to resist them. The hip joint, for instance, is a ball and socket upon which the pelvis puts tremendous tensile and compressive stresses, which, in turn, determine where the bone is dense and where it is porous.
"Cartilage begins ossifying into bone at about seven weeks after conception, when the muscles first begin to fire," Carter said.
The process of bone remodeling to handle changing loads or pressures continues throughout life, with the most change taking place before birth and in childhood. Learning to walk in Earth's gravity requires building femurs about twice as dense as on Mars.
In any gravity, an active, slightly pudgy childhood is best for building strong bones, Carter says.
Adult couch potatoes lose bone mass, and working out on a stationary bicycle won't help, Carter and his former student Robert Whalen found in research conducted for NASA.
"As far as loading the skeleton goes, it does virtually nothing," Carter said. "Weightlifting is far better."
Carter's research has many practical applications in medicine, but he enjoys having fun with it too. Recently he teamed up with graduate student Marjolein van der Meulen and paleontologist Kevin Padian of the University of California-Berkeley to see if they could end an old argument about whether extinct winged lizards known as pterosaurs were active flyers or gliders.
The pterosaur's bones turned out to be thinner than those of any living mammals, Carter said. It takes much lower loads, or stresses, to model the pterosaur's femurs than its wings.
"They look like very active flyers because they have these wimpy little leg bones compared to the large wing bones," he said.
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