What comes to mind when you think of bones? Maybe you picture a spooky skeleton or a gruesome crime scene. But bones are far from being just a static part of the body that we see in movies or museums. In fact, bones are essential to our daily lives, allowing us to move, jump, dance, and even write this very sentence. Our bones are alive, and they constantly adapt to our needs and lifestyles.
Think about it: every time you walk, run, or lift something heavy, your bones are working hard to support you and keep you going. They are not just passive pillars that hold up the body, but active participants in our lives. They also play a key role in protecting our vital organs, storing minerals, providing attachment points for muscles, housing the stem cells that make our blood, and so much more.
Bone design reflects every role they play. So they are not just simple structures, but complex systems of cells, tissues, and minerals that work together in a remarkable way. It is an excellent example of biological hierarchy where different levels depend on each other to give bones their overall strength and resilience.
At the highest levels, the skeletal system includes bone, tendons, ligaments, and cartilage. Together, they form the general scaffolding of the body and all the types of joints that allow us to move. Each aspect of the skeletal system is incredibly complex in its form and function. When that balance is disrupted, we can develop diseases such as osteoarthritis.
But let's focus on bone itself. Nearly every aspect of the bone structure is related to one of its many functions and being as tough as possible.
For example, have you ever noticed that bones are rounded, even in the places where they jut out a bit where muscles attach? That's not just a coincidence - it's actually because stress tends to concentrate at sharp points. So by being rounded, bones are able to distribute stress more evenly during physical activity.
Now bones also include nerves, blood vessels, bone material, sub-structures, and bone marrow in the center. Blood vessels and nerves are mainly housed in the Haversian and Volkmann canals, which connect the entire bone and marrow space to the rest of the body. Haversian canals go along the length of the bone while Volkmann canals go across the bone. These nutrient and signaling highways let bone talk with every other organ in the body and even our gut microbiome!
Bone itself has two types: spongy bone and compact bone. Our spongy bone acts like an archway or flying buttress on cathedrals, redirecting the load bones experience to other areas, such as the compact bone. Compact bone is our main structural element, serving as our bone's shell as well as acting as a mineral depot. If we are running short on calcium, for example, we can dissolve a little bone to free calcium to use elsewhere in the body. This is incredibly handy for handling breastfeeding.
If we zoom in further, we see other substructures, called osteons. Osteons are set up with these concentric circles of lamellae like tree rings. Not all animals have osteons, such as most mice, but humans do. The outermost ring of osteons is called a cement line. The purpose and composition of cement lines in bone have been the subject of ongoing research. Studies indicate that cement lines have a different composition than bone, with less collagen, calcium, and phosphate, and more sulfur, other proteins, and smaller mineral crystals. This unique composition makes them weaker but gives them a more malleable quality (ductile).
So why do we need a soft, squishy layer in our bones? Research suggests cement lines make our bones less prone to breaking. Imagine dropping something malleable like wet clay versus a glass plate. The glass plate will break and the clay will not. This is similar to a crack going through a bone. A crack will go around squishy cement lines because it is easier to go through the stiffer bone material. Plus going around the squishy parts requires more energy before fully breaking. Additionally, cement lines may help keep bones healthy by transmitting mechanical information to bone cells through this deformation.
Lamellae of osteons do the opposite. Lamellae are sheets of collagen fibers at various angles. The purpose of those angles changing is to give more strength to the bone in different directions. Collagen fibers are larger collections of collagen fibrils. Those fibrils are made up of many collagen molecules that are mineralized with hydroxyapatite crystals. Hydroxyapatite crystals are nature's smallest crystals. These tiny crystals reinforce our collagen so that it's harder and stronger. Even though the mineral crystals make the bone harder and stronger, it does not necessarily mean harder to break! We need both the strength of minerals and the flexibility of collagen and other proteins to keep our bones tough (i.e. resistant to breaking). If we have too much mineral, our bones break easier like that glass plate.
But it's not just the physical structure of bones that makes them so remarkable. It's also the way they constantly change and adapt to our lifestyle and health. Our bone cells play a significant role in keeping our bones changing throughout our lives. Osteocytes, osteoblasts, and osteoclasts are the main three types of cells. Their functions mold our bone structures. Osteocytes makeup 90% of the cells in our bones, are one of the longest-lived cells in the body, and sense mechanical stresses. Osteocytes sense fluid flow caused by stresses on our bones, which may be amplified by other structures like cement lines. When they sense stress, they alter their function to tell osteoblasts to build bone. This is why some tennis players have thicker bones on their dominant hand. When we aren't putting much stress on our bones, like astronauts, we can lose it because the osteocyte function is saying we do not need the bone. They then tell osteoclasts to consume bone if we have no use for it. Our cells can also sense microcracks and other minor damage in the bone structure and fix them before they create bigger problems. Through these abilities, our bone cells keep our bones healthy and moving well enough for most of our lives.
But the unique abilities of adaptation and how bone form follows function makes bone very difficult to mimic in biomedical applications, such as joint replacements or tissue engineering. It is not hard to make a prosthetic to replace a joint or to get bone to grow. It is hard to capture complex mechanical properties, structures, and multiple functions correctly. So the next time you think of bones, don't just think of death and decay, but take a moment to appreciate them. Think of life, movement, and resilience. Think of the incredible biological marvel that is bone.
References
Currey JD. Bones: Structure and Mechanics [Book]. Princeton University Press; 2002. 436 p. ISBN: 0-691-09096-3
Zimmermann EA, Ritchie RO. Bone as a Structural Material. Adv Healthc Mater [Internet]. 2015 Jun 24;4(9):1287–304. Available from: http://dx.doi.org/10.1002/adhm.201500070
Nobakhti S, Limbert G, Thurner PJ. Cement lines and interlamellar areas in compact bone as strain amplifiers - contributors to elasticity, fracture toughness and mechanotransduction. J Mech Behav Biomed Mater [Internet]. 2014 Jan;29:235–51. Available from: http://dx.doi.org/10.1016/j.jmbbm.2013.09.011
Grünewald TA, Johannes A, Wittig NK, Palle J, Rack A, Burghammer M, et al. Bone mineral properties and 3D orientation of human lamellar bone around cement lines and the Haversian system. IUCrJ [Internet]. 2023 Mar 1;10(Pt 2):189–98. Available from: http://dx.doi.org/10.1107/S2052252523000866
Bonewald LF. The amazing osteocyte. J Bone Miner Res [Internet]. 2011 Feb;26(2):229–38. Available from: http://dx.doi.org/10.1002/jbmr.320
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