To finish talking about the microscopic anatomy of bone, we're going to talk about the trabeculae, which are found in spongy bone. We just want to start by saying, remember, spongy bone is named for its sponge-like appearance, which features many open spaces. Although it has many open spaces like a sponge, it is not soft like a sponge. In spongy bone, the parts that aren't open spaces are the trabeculae; these are small rods or struts in the spongy bone. Before we go on, we want to look at our diagram here, which shows the edge of a bone with compact bone on the edge and the spongy bone inside filled with trabeculae that connect in various directions, giving the bone its strength.
Each trabecula is short and small, but they connect in different directions which seems chaotic and random at a close-up view, but it's not completely so, as we'll discuss. We are now going to pull out one trabecula and talk about its structure. It contains lamellae—the layers of the bone matrix. It has osteocytes, which are living in the lacunae, spaces inside the bone arranged along the lamellae. It also has canaliculi, the tiniest spaces in the bone that allow osteocytes to exchange materials. What you'll notice is missing is that there is no well-organized osteon and no central canal. The lamellae are not in rings but are lined up in a line. Sometimes in larger ones, they're in a ring, and in the largest ones, there might be a blood vessel going through, but it's not like compact bone where you have regular osteons with a central canal.
These are all different, and usually, there's no blood vessel in the middle. They can get away with this because they are so small, and all of these osteocytes are relatively close to the edge of the trabecula. As long as there's blood supply going through the spongy bone, with all these blood vessels wrapping through, the bone cells in the lacunae can get the nutrients they need and get rid of waste.
When you look at this, it may look 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 that weight is passing through the bone; the trabeculae are usually aligned in a way that they take that weight head-on right through the strut in the strongest way possible. There are also cross pieces that hold everything together, much like scaffolding, which also has cross pieces but definitely has those bars running straight up and down because that's where the weight is applied.
Before we advance, I just want to note that we've been calling this spongy bone, and you're probably fine calling it spongy bone in your course, but you might see it called trabecular bone, named after the trabeculae, or sometimes you'll see it called cancellous bone. Trabecula comes from the Greek meaning "a beam" or "a timber," referencing the rods or struts, while cancellous comes from the Latin meaning "lattice" or "latticework," symbolizing a crosswise structure with spaces in between.
With that, we have finished the microscopic anatomy of bones. Great work, everyone, and I'll see you in the next video.