To finish talking about the microscopic anatomy of bone, we're going to discuss the trabeculae, which are found in spongy bone. So, let's start by remembering that spongy bone is named for its sponge-like appearance with many open spaces.
It is not soft like a sponge. In spongy bone, the parts that aren't open space are the trabeculae, which we previously described as small rods or struts in the spongy bone. Before we go on, let's look at our diagram here, where we have a diagram of the edge of a bone, showing the compact bone on the edge.
On the inside, we see the spongy bone with all these trabeculae connecting. Each one is pretty short and small, but they're connecting in all different directions, giving the bone its strength because they're oriented in various ways. When you look at it closely, it appears chaotic and random, but we'll explain that shortly.
We're also now going to examine one trabecula and discuss its structure. You'll see here it contains lamellae—layers of the bone matrix. It has osteocytes, and those osteocytes reside in the lacunae.
The spaces inside the bone are arranged along the lamellae. It has canaliculi, the tiniest spaces through the bone through which those osteocytes reach out so that they can exchange materials. However, what you'll notice is that there is no well-organized osteon, and there's no central canal. Often, these lamellae are not in rings but just lined up in a line.
Sometimes, in larger ones, they're in a ring. In the largest ones, there might be a blood vessel going through them, but it's not like compact bone, where you have these regular osteons with a central canal and each built the same. These are all different, and usually, there's no blood vessel in the middle. They can get away with that because they're so small, and all of these osteocytes are relatively close to the edge of the trabecula. As long as there's a blood supply going through the spongy bone, the bone cells in the lacunae can get the nutrients they need and rid of waste, etcetera.
I mentioned earlier that when you look at this, it may appear chaotic and random, but when you zoom out and look at a cross-section of bone, you'll notice that the trabeculae actually have a pattern to them. They tend to align with the lines of stress in a bone. So, imagine the way that weight would be passing through the bone; the trabeculae are usually aligned in a way that they're going to take that weight head-on right through the strut in the strongest way that the trabecula is built. There are also crosspieces holding everything together, much like scaffolding, which we likened to spongy bone earlier on. Scaffolding has cross pieces but definitely has bars running straight up and down because that's the way the weight is going to be applied.
Before we go on, I just want to note that we've been referring to this as spongy bone, and you're probably fine calling it spongy bone in your course, but you might also see it referred to as trabecular bone, named after the trabeculae, or sometimes as cancellous bone. Trabeculae comes from the Greek meaning a beam or timbers, referring to the rods or struts, while cancellous comes from the Latin meaning lattice, a crosswise structure with spaces in between.
With that, we have finished the microscopic anatomy of bones. Good work, everybody, and I'll see you in the next video.
