Skeletons are often associated with the dead, but your bone suit is actually made up of many living organs, and it turns out they have a lot to say to the rest of the body. Far from simply providing stability, your skeleton produces blood cells, stores minerals, helps build muscle, and influences other organs and organ systems in the body.
Much like the respiratory system is made up of your lungs, diaphragm, and other organs that get air where it’s needed, your skeleton is an organ system which works together to keep you strong, mobile, and healthy. But research has also found it has functions beyond protecting the bag of bones that is the human body.
Bones are living organs
Bone is complex living tissue made up of several layers and cells that can self-deconstruct, rebuild, and regenerate. Your average long bone (think femur, radius etc.) has epiphyseal plates where bone lengthening occurs until you’re an adult, at which point these plates grow over into bone.
The outside of the bone organ is compact for strength, but inside there are cavities containing different kinds of tissue including spongy bone, fat, and marrow, the latter of which is where our red blood cells are made. This means bones are also kitted out with decent vasculature to keep the goods traveling into, as well as out of, the organ.
Unlike rigid scaffolding, bone is also being constantly broken down and remodeled with the help of bone resorbing cells (osteoclasts) and reforming cells (osteoblasts). This means bone can reshape and reform throughout life, and repair as it goes along.
How the body responds to bones
Osteocytes, a once-under-studied cell type found in fully-formed bone, were originally expected to carry out a one-dimensional role in orchestrating the remodeling of bone, but research has found that they actually influence major organs.
They’re key cells chatting with osteoclasts and osteoblasts, but their influence extends beyond bone as it also acts as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function.
Bone organs have even been found to be a regulator of male fertility as osteoblasts, our bone building cells, can induce testosterone production in the testicles.
The body also works to protect bone during pregnancy, a process which by definition involves cooking up a fair few bones. Pregnant people absorb calcium more readily than those who aren’t and produce more estrogen, a hormone that handily has a protective influence for bone.
How bones respond to the body
Your bones’ communication skills are also far from a one-way street, as research has found that bone growth can alter in response to what’s going on elsewhere in the body. Using mouse models, a study found that bone growth will slow up when leptin – the hormone that lets us know we’re well-fed – is present, meaning energy can be conserved for more pressing functions.
Building bone is an energetically expensive endeavor, so is something best reserved for when nutrients are free-flowing.
Exercise sees a mechanical interaction play out between our bones and muscles as each pulls on the other, increasing in size and strength. However, this two-way conversation has chemical underpinnings, too.
Irisin, a molecule made by muscles during exercise, promotes osteocyte health, our cells that are also in cahoots with the kidneys and pancreas. In return, osteocytes produce prostaglandin E2 in response to the repeated pull of muscles which supports muscle growth.
So, is the skeleton an organ? No, it’s a complex, chatty and receptive organ system. Clever bone suit.
[H/T: Knowable Magazine]
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