Welcome to Episode 2 of the Bone Coach Podcast. Kevin Ellis here. For today’s episode, I want to introduce you to our bones. These amazing, living structures that do so much to ensure our health.
I’m going to give you an overview of our bones, their functions, the types of bone found in our bodies, the different ratios of those bones and why that’s important, what the foundation of our bones is made of, what cells are involved in the bone remodeling process and why bone remodeling is important.
If you’re not well-versed in bone health, that’s okay. This episode is all about helping you get a basic understanding.
A lot of these terms may be new to you, but I’m sharing them because I think it’s important for you to understand, at least at a basic level, some of the major components of bone health.
I mentioned in the very first episode, where I shared my personal story about being diagnosed with osteoporosis in my early 30’s, is that one of the key pieces that helped me shift my mindset from one of worry and fear and overwhelm to one of empowerment and determination was knowledge and education.
That’s really what I’m doing in these first few episodes is introducing you to new words, and terms, and information so that you can start to build that knowledge base that will help you feel empowered about being your own health advocate….AND you’ll have a better understanding of the terms used in later episodes when I’m interviewing bone health experts.
Before we get started I also want to let you know you can find all of the detailed show notes over at bonecoach.com/podcast/understand-your-bones
And remember, the last thing I want at this point is for you to feel overwhelmed, so don’t try to remember everything I’m sharing here, I just want you to be familiar with some of this info about our bones. You can always listen again at a later time.
So let’s go ahead and dive right in...
Quick Overview of Our Bones
How Many?
The adult human skeleton is composed of 206 bones. When we’re born, we have about 270 bones. But as we become adults some of those bones fuse together as we grow. 1
Functions
Those bones serve a variety of different functions. 2
Support
They hold us up: our skin, our tissues, our muscles, our organs, our bodies. They provides a rigid framework for other soft tissues of the musculoskeletal system to attach to such as muscles, tendons, ligaments.
Movement
Your muscles specifically attach to your bones via tendons. When your muscles contract, your bones act as a lever while your joints form a pivot point. This interaction between bones and muscles is what helps us move.
Protection
They protect our internal organs from trauma like the rib cage does for the heart and lungs, like the skull does for the brain.
Blood cell (RBC, WBC, and platelets) production and maintenance
Red blood cells, white blood cells, and platelets are formed within your bone marrow through a process called hematopoiesis. This is a perfect example of how bone is living tissue. 3
Nutrient storage (calcium, phosphorus)
Calcium and phosphorus, are stored within your bones. When your body needs more of these resources, they can be released back into your bloodstream for use. Iron in the form of ferritin can be found in the bone matrix. Even specific growth factors, including insulin-like growth factor or IGF-1, are housed in bone and then released periodically.
What Types?
Architecturally, there are two different subtypes of bone. Cortical and trabecular.
Cortical (Compact)
Cortical bone, also called compact bone, forms the hard outer exterior of bone. It’s dense and solid and surrounds the marrow space.
To help you visualize this, think of a white PVC pipe, the kind of piping used in a lot of homes for plumbing. This PVC pipe has a thick outer exterior and a hollowed out inside. That thick outer exterior is like cortical bone. The hollowed out inside is like the marrow space.
This kind of bone is strong, much stronger than the other type of bone we’ll talk about next. Cortical bone is resistant to bending and compression and is much more dense.
About 80% of the bone in our body is that dense cortical bone.
You’ll find this type of bone mostly in the shaft of long bones like the femur (which is that large bone right in the middle of your thigh) and in your forearm. There’s also a good amount in the hip. 4 5
Trabecular (Cancellous, spongy)
Then we have trabecular bone, also called cancellous or spongy bone. This is the central part of bone, made up of a network of columns, plates and rods in the bone marrow compartment. Think of trabecular bone like a honeycomb-like structure inside your bones.
Then we have trabecular bone, also called cancellous bone.
This is the central part of bone, made up of a network of columns, plates and rods in the bone marrow compartment.
So, let’s go back to the example we used for cortical bone which is a PVC pipe.
You have the thick outer exterior of the pipe, which is the cortical bone. Then you have the hollowed out inside which is the marrow space. Now fill in that marrow space with a honeycomb.
That’s trabecular bone.
Trabecular bone is like a honeycomb-like structure inside your bones.
This type of bone makes up only 20% of total bone but the surface area is much greater, about 10 times greater, than that of cortical bone.
You’ll find more trabecular bone in areas that are more subject to compression and are under continuous stress from motion and weight-bearing activity, like the vertebrae and the ends of long bones.
Trabecular makes up only 20% of total bone but has ten times the surface/volume ratio of cortical bone. It’s surface area is much greater.
It also responds eight times faster to changes in load making it far more dynamic. That’s why you’ll find more trabecular bone in areas that are more subject to compression and are under continuous stress from motion and weight-bearing activity, like the vertebrae and the ends of long bones.
Bone Ratio
These bones (cortical and trabecular) are found in different ratios in different locations of the body.
We know the human skeleton is composed of 80% cortical bone and 20% trabecular bone overall.
In the spine, your vertebrae are predominantly trabecular bone. They are about 75% trabecular bone and 25% cortical bone. The wrist has a similar ratio.
The femoral neck, on the other hand, is only 25% trabecular bone and is 75% of that dense cortical bone.
The femoral head is about 50/50.
Why does this matter?
Because the different bone types decline at different rates, and it can shed some light on why people are more prone to fracture in certain areas first.
There’s even evidence that bone mass in the trabecular and cortical bone compartments is regulated by different cell types and external cues.
Having bone loss in a specific area with more of a certain type of bone (either cortical or trabecular) could provide insight into an underlying condition that could be contributing…
For example, hyperparathyroidism, a condition where too much parathyroid hormone (PTH) is produced resulting in calcium being released from the bones, could cause a more significant decrease in cortical bone (specifically in the distal forearm) as opposed to trabecular bone (which you would find more of in the spine). 6
In contrast, if there’s a marked decrease in estrogen for women, like what happens with menopause, other hormone issues, or nutritional deficiencies, you may see areas with more trabecular bone decrease more rapidly, because trabecular is typically the first to go. 7
Now let’s talk about the foundation of bone...what is bone made of...
I’m simplifying here, but if you feel like this section is still a bit too technical, don’t worry, I’ll do a short summary and visualization at the end of it.
Bone Foundation
The foundation of bone is a, tough, fibrous, braided structure, made mostly of specialized collagen protein. We call this the matrix. 8
This matrix is home to all the necessary components needed to keep bones alive and healthy: enzymes, proteins, and cell-signaling molecules called cytokines.
This matrix is also where minerals are laid and where mineral reserves are stored.
The two primary minerals in this matrix are calcium and phosphate, and they join together to form what’s called hydroxyapatite.
This mineral complex binds to the collagen fibers making them strong and rigid.
So, our bones are made of collagen protein. That’s what gives bone its flexibility.
Adding minerals to that collagen protein gives bones their stiffness.
Without the addition of minerals to that collagen, your bones would be like rubber bands. Conversely, if you just had minerals with no collagen, your bones would be brittle like chalk. 9
It’s the balance between the two of these, collagen and minerals, that provides bone with its ability to be both flexible and stiff.
And that’s really what makes up this tough, fibrous structure, the matrix, that’s the foundation of our bones.
And if you’re like me, that probably leads you to think about:
“How are bones even formed”
“How are they broken down (another word for this is called resorption)”
“Who is even involved in this process?”
...That’s what we’re going to talk about right now.
This matrix is also the place upon which minerals are laid and where mineral reserves are stored.
Calcium and phosphate are the primary minerals in this matrix. They join together to form what’s called hydroxyapatite.
This mineral complex binds to the collagen fibers making them strong and rigid.
And that’s what makes up this tough, fibrous collagen protein structure that’s really the foundation of our bones.
Then that probably leads you to think about:
“How are bones even formed”
“How are they broken down”
Bone Cells Involved?
There are 3 major bone cells involved in a process called bone remodeling. 10 11
And, at the very least, you should have a general understanding of what their role is, because you’re going to hear these 3 cells come up in future episodes and maybe even in discussions with your doctor. So, I’m going to simplify and help you understand each of these cells.
Osteoclasts
These are the cells that break down or “resorb” old, worn and weakened bone by secreting an acidic mix of enzymes, chemicals, and hydrogen ions. That’s the basic function of osteoclasts in one simplified sentence.
An easy way to remember this is “clasts” chew.
This chewing up and tunneling through bone is called resorption, and it leaves "scooped out" or excavated regions of bone matrix to later be filled in by our next cell, the osteoblasts.
Osteoblasts
Osteoblasts are the bone-building cells that form new bone. They deposit that collagen and minerals that will eventually form mature bone.
An easy way to remember this is “blasts” build.
The actions of both osteoclasts and osteoblasts are closely related and are considered a coupled process.
Our third type of bone cells, osteocytes, are what keep this process in balance.
Osteocytes
95% of bone cells in the adult skeleton are osteocytes. Their job is to maintain the correct oxygen and mineral levels in the bone.
You can think of osteocytes as the orchestrators of the bone-remodeling process. They’re housed in, or embedded in, the bone matrix.
Here’s what they do, and I actually think this is pretty fascinating, these cells, these osteocytes, they sense mechanical loading, and microcracks, and microdamage and then transmit a signal to the other bone cells to initiate the remodeling process.
Kind of like when a car accident happens, and the dispatcher has to transmit the signal or message to all different first responders to get the area cleaned up and repaired or restored. The car accident would be like a fracture, the dispatcher would be the osteocyte and those first responders and clean up crew would be those other bone cells.
So, in summary, You have the osteocytes that sense the damage and communicate to the other cells that repair is needed. You have the osteoclasts that come in and excavate the damaged bone. Then you have the osteoblasts that follow and build the new bone. 12
Stem Cells
Finally we have stem cells. These are cells found in the bone marrow that can become specialized cells in the bone remodeling process based upon certain factors or conditions in the body. Osteoclasts are derived from hematopoietic stem cells and osteoblasts are derived from mesenchymal stem cells.
So, in summary, You have the osteocytes that sense the damage and communicate to the other cells that repair is needed. You have the osteoclasts that come in and excavate the damaged bone. Then you have the osteoblasts that follow and build the new bone. 13
Bone Remodeling
This process is called bone remodeling. It occurs throughout our entire lives.
It balances bone resorption and bone formation.
We need this to happen for a variety of reasons...
We need this to happen for a variety of reasons...
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Regulate calcium balance
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Repair microscopic cracks sustained during normal activity
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Create new, mechanically stronger bone to adapt to changing biomechanical forces
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Heal fractures if, and when, we get them. 14
Bone remodeling is a necessary and important process. BUT...we need a balance between the amount of bone being broken down and the amount being formed. It’s when we have a negative balance, where resorption exceeds formation that you’ll have a net bone loss that can lead to osteoporosis.
There are different types of osteoporosis (primary and secondary), different causes and conditions, and risk factors. In the next episode, we’re going to discuss these causes and risk factors of low bone density in depth. So if you’re just starting out on your bone health journey and you’re still trying to understand how you have osteoporosis or what could be a potential cause, you’ll want to check that out.
And be sure to hit that button and subscribe to the Bone Coach podcast to make sure you never miss an episode. We have some amazing bone health experts that will be joining us on the show and I want to make sure you can learn from their expertise.
I think that’s it for this one...I’m your Bone Coach, Kevin Ellis, See you in the next episode!
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Sources
1 - https://www.ncbi.nlm.nih.gov/books/NBK441968/
2 - https://www.ncbi.nlm.nih.gov/books/NBK541132/#article-18456.s1
3 - https://www.healthline.com/health/bone-health/bone-function#function
4 - https://www.ncbi.nlm.nih.gov/books/NBK541132/
5 - https://www.ncbi.nlm.nih.gov/books/NBK45504/
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3316917/?fbclid=IwAR34L79OhPyNlAOJEzUQAO4rwFxLizBnkberAx36SAhDCaATXmbngE0nHKc
7 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3971652/
8 - McCormick, Keith. “Your Body's Regulatory Hormones-The Pulse On Skeletal Health.” The Whole Body Approach To Osteoporosis, edited by Kayla Sussell, Raincoast Books, 2008. Pp 10-12.
9 - https://www.sciencedirect.com/topics/materials-science/bone-matrix
10 - https://www.ncbi.nlm.nih.gov/books/NBK499863/
11 - https://www.ncbi.nlm.nih.gov/books/NBK441968/
12 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5994176/#R45
13 - https://courses.washington.edu/bonephys/physremod.html
14 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5994176/
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