So in this video we're going to look at bones as organs. They contain many different tissue types. Of course synonymous with bones is the connective tissue known as osseous tissue.
In addition to the osseous tissue we have cartilage, fibrous connective tissue. So again all falling under the umbrella of connective tissue but you have different types that serve different functions. You include nervous tissue, you have epithelial tissue, you have muscle tissue. So organs, again, are defined by multiple different tissue types. So we're going to examine three hierarchical levels of bone structure.
So first we're going to look at the big picture. We're going to examine the gross anatomy. And when we examine the gross anatomy, there are two different types of bone textures. In the first, compact bone, this tends to be sort of along the margins of the bone. It's very dense, smooth, and solid.
Then you have sort of the interior parts. These are spongy, and they're made up of very small, flat pieces of bone called trabiculae, and they kind of go mishmash at sort of odd angles. And this allows for the resistance from multiple planes.
We will then continue our discussion in later videos examining both the microscopic as well as the chemical anatomy. So let's look at the two major groups of bones. The short, irregular, and flat bones versus the long bones. So a couple of things that I want to point out here is that when we look at the flat bones that are found in the skull, you have very thin plates of compact bone, and nestled in the middle, you have spongy bone. So specifically the spongy bone.
spongy bone, excuse me, in the skull is known as diplo. Outside, you have what's called another connective tissue called a periosteum. So this is what confers blood vessels and therefore nutrients to the underlying bone as well as where the nerves are. And then you have a layer of connective tissue at the juncture between the compact and the spongy bone.
And this is known as the endosteum. So within the flat bones, there are no shafts. There isn't a structure called an epiphysis.
We'll talk about this more for the long bones. And the bone marrow that you have in the spongy bone. Usually, it can be sort of red bone marrow, but there isn't a cavity like you have in long bones. In addition, any kind of articulation you have, so that's when two bones come together, you're going to have hyaline cartilage. So again, that's not going to be any different from long bones.
Now, if we zoom into the spongy bone, You can see these fragments, and this is the trabeculae. So it's very, very porous, so it's not very heavy, it's not very dense, but the benefit is that multiple angles of compression can occur without the bone sort of buckling. When we examine the long bone, we're going to look at the various membranes at the end, But there are certain unique components of the long bone. So you have the sort of shaft of the bone.
This is known as the diaphysis. So the shaft of this diaphysis, you have this sort of hollow interior called a medullary cavity. So spongy bone does not have a medullary cavity. This is going to be usually found.
associated with the compact bone, which is part of the diaphysis. You have compact bone here and here, so you form basically this hollow cylinder. But towards the end of the diophysis, you have a region where bone can actually grow.
This region is known as the metaphysis. Some textbooks just kind of bundle this in with the epiphysis, but the metaphysis is the area that bone can grow. grow and it consists of spongy bone.
So this spongy bone, similar to what we just talked about in the cranium, is not overly solid. You have trabiculae. There is no cavity there. There is a marrow.
Additionally, you have what's called an epiphyseal line or when you're growing it's actually called an epiphyseal plate. This consists of cartilage and the way the bone is going to grow and we'll kind of detail this is that the cartilage grows provides a matrix upon which bone can then sort of seed and extend the long bones lengthwise. So within the epiphysis, you are going to have compact bone on the perimeter, but on the inside, you're going to have spongy bone. Now when we compare the types of marrow, usually within this spongy bone, you're going to have red bone marrow.
where you can have hematopoiesis. That's the formation of the blood cells. And in the medullary cavity, you're actually going to have yellow bone marrow where you can store fat. Now surrounding the entire bone, so on the perimeter, right over here, you're going to have the hematopoiesis.
periosteum and so I can kind of show you it over here the periosteum would be right over here and then lining the interior of the medullary cavity just gonna put a little star here that's where you would have the endosteum So let's specifically talk about these two very significant membranes. First off, we'll discuss the periosteum. So it is a white double-layered membrane.
It's going to cover all of the external surfaces of the bone, with the exception of where there are joints. It consists of dense, irregular connective tissue, and in this dense, irregular connective tissue, You actually have collagen that sort of percolates into the underlying compact bone. So these collagen fibers go by the name of penetrating fibers, perforating fibers, or some textbooks will refer to them as Sharpies fibers. So it allows you to secure this membrane to the actual bone matrix.
Now within the periosteum, you have what's called an osteogenic layer. So osteo for bone, genic to generate. And within it, you have these primitive cells.
Now primitive, the term is a little bit misleading. It's not primitive. I would say it's more sort of in the context of it's undifferentiated. So it's not...
specialized yet. So we refer to these as osteogenic cells, bone forming cells, because it is these cells then that will ultimately give rise to the other cells needed to build bone up and to break bone down. So within the periosteum, as shown in the image, you have arteries, which we call nutrient arteries, as well as nerve fibers that then penetrate deeper into the shaft of the bone.
So it does go into the compact bone itself. So the periosteum is also very useful in that it serves as an anchoring point for both tendons as well as ligaments. Now if we actually zoom into the periosteum, you see there's the rugged fibrous layer and then underneath it you have the cellular layer where those osteogenic cells are going to be.
So that then in turn allows the formation of osteoblasts and osteoclasts that can then penetrate deeper into the bone. So the endosteum is a little bit more delicate. It's protected by the compact bone, but it does juxtapose the trabeculae of the spongy bone. So it helps to sort of... line the interior of the compact bone, so where the medullary cavity is located.
So within the endosteum, you also have osteogenic cells that can differentiate to other bone cells. Very important if there's any kind of bone repair that's needed or bone remodeling. So within the context of these bones you do have marrow. Specifically the red bone marrow is going to be very important in the formation of blood cells.
So the red bone marrow is usually found in the small little cavities in between the trabeculae in flat bones like the sternum. and also in sort of the ends of the long bones. You find red bone marrow also in the medullary cavities of newborns, but this later transitions into sort of yellow bone marrow. So we very ha- we have very little red bone marrow per se, however during periods where you might need lots of red blood cell production or blood cell production, the yellow marrow can convert into red temporarily under anemic conditions.
So here we have a particular image where you see the outer surface of the bone, you see the sort of open cavity regions of the spongy bone marrow, excuse me, of the spongy bone, and nestled. Sort of deep in here, you have the yellow bone marrow, which contains fat.