The Skinny on Your Skeleton

It was a good day when I could finally move “Skinny” my skeleton out of my bedroom and into an actual office. Most docs have a skeleton or two in their clinic and don’t worry, we already know that you’ve secretly checked it out while you were waiting - even if you got lucky and no limbs fell off while you were playing with it. We generally think of our skeleton as our structural support beams, a scaffolding of sorts, and while it does serve that role there’s a lot more to it than just that bag of bones. Our models and posters can help you visualize what your skeleton looks like, but what you can’t appreciate on these (or on the ones Fido carries around) is how dynamic our bones really are. Our skeleton is actually in constant communication with the rest of our body systems and plays an important role in how they function. 

As was briefly mentioned in the blog about “your next body” bones are not inert dead tissue, they are very much alive and changing. In order for us to understand how busy bones are there are a few bone cells that we need to learn, so that means we need to do a little medical latin. The prefix “osteo” means bone, so whenever you see a word or diagnosis with that included, you’ll know right away it has something to do with bone or the skeleton as a whole. The second part of the word tells us a bit more about what’s going on (for example, “itis” for inflammation, “oma” for many cancers, etc) or the type of cell we are talking about. Generally speaking, “osteoblasts” are the type of bone cells that “build" bone (so important in the formation of new bone). “Osteoclasts” are responsible for bone resorption (so the breakdown/uptake of “old” bone). Finally, the mature bone cells called “osteocytes” make up the vast majority of the cells in our adult bones (about 90+ %). We used to think of osteocytes as fairly inert, but recently their role in signalling (both locally and systemically throughout the body) is being recognized more and more. All three of these cell types are involved in the process of bone remodelling (how your skeleton is replaced over time). This process of forming new bone and breaking down old bone is constantly occurring, but even more so when we are growing or after an injury to the bone. It is very important to have a good balance or “homeostasis”, in the process of bone remodelling. Too much breakdown and not enough formation and we end up with diseases such as osteoporosis. Too much formation and not enough breakdown and we can end up with disease of overgrowth of bone. So just like all of our other body systems and hormone levels, there’s a Goldilocks phenomenon with bone as well. Too little or too much is no good, we need the rate of formation and resorption to be jusssst right! 

The osteocytes are like little directors in your bones and are involved in setting the rate of the osteoblast and clast activity. They are wise, they don’t just give directions haphazardly, instead they respond to many messages such as mechanical loads on the bone, hormones, medications, and our body’s demand for calcium elsewhere - just to name a few! Exercising, especially strength training and weight-bearing exercise puts mechanical forces on the bones - both from the impact forces as well as the forces that our muscles put on them. The osteocytes sense these forces and tell osteoblasts to build more bone, improving our bone density and increasing the strength of the bone. This is just one reason why exercise, especially as we get older, is important in the maintenance of our bone health and can prevent or slow osteoporosis. Mechanical loads are not the be all end all message though. To illustrate the complexity of this, a recent study found that the participants with the most belly fat actually had much much lower bone formation and less bone volume than individuals with lower belly fat. Crazy town right? One would think that a heavier person would automatically have higher bone density because of the increased force on their bones, but the signalling molecules in abdominal fat can actually trigger the contrary! So bone remodelling is clearly about more than just the mechanical forces. 

Our gut health (specifically the microbiome) has also been found to impact our bone health. In leaky gut for example, the microbiome is off and inflammatory messengers (aka inflammatory cytokines) seem to trigger bone resorption. This can then lead to conditions like arthritis and osteoporosis. Our nervous system also affects our bone health and vice versa. The hormones released by the sympathetic nervous system such as epinephrine and cortisol can all have impacts on the balance of bone remodelling. This means, stress and psychological conditions can also affect the health of our bones. Who knew that by encouraging a bit of relaxation, treating mental health conditions, and learning to manage our stress, we are actually also helping keep our skeleton healthy?! Sleep is another factor that can affect the health of our bones. Our circadian rhythm (sleep-wake cycle) is important in our hormone fluctuations and we know by now those hormones get involved in everything! Speaking of hormones, the estrogen drop seen around menopause is a big reason why women are at a higher risk of developing osteoporosis as they get older - this means that bone remodelling is sensitive to estrogen levels as well. Whew! That’s a lot of chatter that our bone cells have to pay attention to don’t you think? - And we only just scratched the surface of that list! But our bone cells don’t just listen, they also contribute to the conversation.

Because we have found that bone produces signalling molecules (aka little messengers), we know that it must have a “systems level effect” meaning it can have impacts on our other body systems. So there’s actually a two way street in the communication between our skeleton and many of our other body systems. One, perhaps surprising, relationship is that our bones (via release of a signalling molecule called osteocalcin) can impact both the production of insulin and the sensitivity of other tissues to it. This means that our bones impact glucose metabolism and therefore our blood sugar levels. Osteocalcin has also been shown to have impacts on the sex hormones and is thought to perhaps affect fertility. In addition, via its role in the nervous system and its affects on other neurotransmitters (messengers) osteocalcin can even impact learning, memory and mood. Oh yes, and I forgot to mention…our bone marrow is of course produced and stored in our bones - but you knew about that part already ;)

So next time you think of your bones as just scaffolding and protective armour, I hope you’ll remember to give them a little more credit where credit is due- Skinny the skeleton isn’t quite so simple after all!