Microscopic Anatomy of Bones - Bone Matrix - Online Tutor, Practice Problems & Exam Prep

Bone is a unique connective tissue characterized by its hardness and strength, primarily due to its extracellular matrix. This matrix consists of an inorganic component, hydroxyapatite, made of calcium (Ca²⁺) and phosphate, which contributes to bone hardness. The organic matrix, or osteoid, mainly composed of collagen fibers, provides flexibility and strength. Together, these components create a tough structure, similar to a cast made of plaster and gauze, ensuring bones are both resilient and durable against fractures.

Bone Matrix

Bone is a connective tissue, but it's unique among connective tissues in that it's not only very strong but also hard. What gives bone those properties, hardness and strength, is the bone matrix. So we're going to talk about the bone matrix in more detail here. We're going to start off just by saying the extracellular matrix of bone has 2 basic components, and those 2 basic components are going to contribute separately to that strength and the hardness. We're going to start out by talking about the inorganic matrix.

The inorganic matrix is going to be made of mineral crystals, and the mineral we're really talking about here is hydroxyapatite. Hydroxyapatite is a crystal that's made of calcium, which I'm just going to write as Ca2+, and phosphate. And you probably know to have strong bones, you're supposed to eat a lot of calcium. Well, that's where that calcium is going. It's going into these hydroxyapatite crystals, and you need a lot of it because that, that hydroxyapatite, those inorganic crystals, are going to make up 2 thirds of your bone mass.

Now the crystal is pretty heavy, but that's still a lot of bone that that calcium and phosphate is making up. What that hydroxyapatite crystal is doing, it's making your bones hard. Now when you think about hardness, though, I want you to think about something like plaster, something that's hard but fragile, or maybe think of something like a dinner plate. Right? Your dinner plate's really hard, but if you were to drop it on the floor, it's just going to smash.

You don't want your bones to do that. So to keep your bones from smashing like that, you want organic matrix in there or what we call the osteoid. The osteoid is going to be mostly collagen fibers and also some ground substance. So that ground substance, it's just a few proteins that we don't really need to worry about those nearly as much as we need to worry about the collagen. Remember, collagen is this protein fiber that's really strong but it's flexible like a rope.

Now there's a lot of collagen fiber in that organic matrix in there. It's the remaining 1 third of your bone mass, and this is what's going to make bones strong. So when I say strong here, I want you to think of a rope or something here. I say think of gauze. Gauze, you know, you want to wrap a wound with gauze.

It's really flexible. You can wrap it around your arm. You can ball it up. Whatever. But go ahead and try and rip gauze.

It's really difficult to rip because it's really tough. It has all those fibers woven together, and each one is really difficult to break. Now if you take gauze and you put it inside plaster, you end up with something like a cast, and a cast is both rigid and tough. And to show this, we have a picture of someone with a broken arm with a cast on it. Now this cast here is actually probably not plaster and gauze.

It's probably fiberglass, but that uses the same technology. Right? Fiber. There's fiber in fiberglass. The same technology in carbon fiber.

In the olden days, they used to make walls by putting horsehair in the plaster. So all these technologies take a fiber that's strong but flexible and put it in something that's hard but fragile, and you get something really tough out. I don't know if you've ever broken a bone, but in college I broke my arm and I had a cast on. I used to landscape in college. I used that cast to hammer in fence posts.

It was really tough. It's probably not advisable, but it worked. Alright. So that idea that we have these 2 different things that contribute separately to make bones both hard and strong, we're going to talk about a lot more going forward, and I'll see you there.

Microscopic Anatomy of Bones - Bone Matrix Example 1

This example tells me that there are 2 bones. One was treated with an acid while one was treated with bleach. Based on the properties of each bone after the treatment, predict what component of bone bleach breaks down and what component of bone is dissolved by acid. Alright. Instead of just looking at the picture, I went ahead and did this. So let's take a look. Alright. I have 3 bones on the counter here. The first one's a normal bone. The second one is treated with an acid. The third one with bleach. Okay. This first bone, I'm going to try and break. It's hard to break. Bones are hard to break. Now it's a chicken bone. I can do it, but if I try to break it more than this, it'd be really hard without something like pliers or a hammer. Alright. Let's check this one treated with an acid. Alright. Pretty hard to break. That bone became really flexible. It's just a floppy, bendy bone. So think, what did the acid dissolve to make the bone like that? Alright. Now the bleach. The bleach breaks pretty easily. It doesn't just crumble, but I can easily just keep breaking it up with my fingers if I push it down into the counter. It just makes all these little bone shards. So, therefore, if it's able to break up easily, it doesn't bend. What did the bleach break down in the protein? Okay. Now that we're thinking about that, let's go back to our paper here. So we have our normal bone, and when we treat it with bleach, the bone is brittle and it shattered easily. So what do you think it is that the bleach breaks down? Well, I'm going to say that the bleach breaks down collagen. Remember, collagen is that protein, that fiber that gives bone its strength, its toughness. Collagen is really flexible, but it's also really hard to break. So when you take it out, you're just left with that hydroxyapatite crystal, those calcium crystals. Those are hard, but they're really fragile, and the bones just shatter pretty easily. You don't want your bones to do that. Alright. So now let's think with the acid. With the acid, when you treat it with acid, I just use vinegar to do that, so it's pretty easy to do. After soaking it in vinegar, the bone became very flexible, and it bent easily. So therefore, what did the acid dissolve, do you think? Well, I'm going to say that the acid dissolved that hydroxyapatite crystal. I'm going to just write that in shorthand as the calcium²⁺, the calcium crystals. Remember, calcium is hard, so when you take it out, you're just left with the collagen. Collagen does not break easily, but it's really flexible, and so you're left with this floppy bone. Again, you don't want your bones to do that. You want both these properties, so you want both those things in your bones. Alright. Like always, we have practice problems to follow. Give them a try.

At her annual checkup, the doctor tells Pilar that she should add leafy greens like spinach and collard greens to her diet to help keep her bones strong. What vitamin or mineral in the vegetables would contribute to bone strength?

• A. Iron
• B. Calcium
• C. Collagen
• D. Vitamin D

If the hydroxyapatites were replaced with more typical gel-like extracellular matrix, how would you expect the bones to be affected?

• A. The inorganic matrix would lack the hardness necessary for bones to bear weight. 
• B. The organic matrix would lack the hardness necessary for bones to bear weight.
• C. The inorganic matrix would be unable to provide tensile strength.
• D. The organic matrix would be unable to provide tensile strength.

Here’s what students ask on this topic:

What is the bone matrix and what are its components?

The bone matrix is the extracellular material in bone tissue that provides its unique properties of hardness and strength. It consists of two main components: the inorganic matrix and the organic matrix. The inorganic matrix is primarily made up of hydroxyapatite crystals, which are composed of calcium (Ca²⁺) and phosphate. These crystals contribute to the hardness of the bone. The organic matrix, also known as the osteoid, is mainly composed of collagen fibers and some ground substance. Collagen fibers provide flexibility and strength, making the bone tough and resilient against fractures.

How does hydroxyapatite contribute to the properties of bone?

Hydroxyapatite is a mineral crystal composed of calcium (Ca²⁺) and phosphate, and it makes up about two-thirds of the bone mass. This inorganic component of the bone matrix is responsible for the hardness of bones. The presence of hydroxyapatite crystals ensures that bones are rigid and can withstand compressive forces. However, while hydroxyapatite makes bones hard, it also makes them brittle, similar to how plaster is hard but fragile. This is why the bone also needs the organic matrix to provide flexibility and strength.

What role does collagen play in the bone matrix?

Collagen is a protein fiber that is a major component of the organic matrix, or osteoid, in bones. It provides flexibility and tensile strength to the bone, making it tough and resilient. Collagen fibers are strong yet flexible, similar to a rope or gauze, which makes it difficult to break. This flexibility helps prevent bones from shattering under stress. Collagen works in conjunction with the inorganic hydroxyapatite crystals to create a bone structure that is both hard and strong, ensuring durability and resistance to fractures.

Why is it important to have both inorganic and organic components in the bone matrix?

Having both inorganic and organic components in the bone matrix is crucial for achieving the optimal balance of hardness and strength in bones. The inorganic component, primarily hydroxyapatite crystals, provides the necessary hardness to withstand compressive forces. However, hardness alone would make bones brittle and prone to shattering. The organic component, mainly collagen fibers, adds flexibility and tensile strength, making bones tough and resilient. This combination ensures that bones are not only rigid but also capable of absorbing impact and resisting fractures, similar to how a cast made of plaster and gauze is both hard and tough.

How does the bone matrix compare to materials like plaster and gauze?

The bone matrix can be compared to materials like plaster and gauze to illustrate its unique properties. Plaster is hard but fragile, similar to the inorganic hydroxyapatite crystals in the bone matrix. Gauze, on the other hand, is flexible and strong, akin to the collagen fibers in the organic matrix. When combined, plaster and gauze create a cast that is both rigid and tough, much like how the bone matrix combines hydroxyapatite and collagen to create a structure that is both hard and strong. This analogy helps explain why bones are resilient and durable against fractures.