Microscopic structure of bone - the Haversian system

- [Instructor] All right so we're going to be talking about the microscopic structure of bone and in particular, we're going to be talking about what is called the haversian system. Now let's take a piece of bone and cut it in half and see what it looks like on the inside. Here, we basically have a cross section of a piece of bone. So let's take a look inside. Now the inner most portion of this bone is made up of, what is called, spongy bone, which is otherwise known as cancellous or trabecular bone. And here you can see this arrow pointing to the trabecula of spongy bone. So it's no surprise that if you take a look at spongy bone, that it looks pretty much a lot like a sponge and in fact, because of all these various trabeculae or cavities, the surface area of spongy bone is 10 times that of the outer layer of compact bone. So basically spongy bone is just this porous network of spikes surrounding the innermost portion of bone marrow. And the overall effect of this spongy network in the center of the bone is that of making the bone lighter. Now if you look at the periphery of the bone, you have what is the harder, denser layer that surrounds the spongy bone. That is called compact bone. Compact bone, it's no surprise, that it's, well, more compact than spongy bone. It has fewer gaps and spaces, but what really makes compact bone different from spongy bone, is that it has a specific type of organization made up of these osteons, these repeating functional units. Here's a blown up view of an osteon. Another word for these osteons is the haversian system. So let's talk more about this haversian system. So each of these osteons looks like of like a cylinder and it has multiple concentric layers of bone, or sheets really, that wrap around each other to form this osteon. Each of these layers is called a lamellae. In the center of these layers is a canal called the haversian canal, or central canal. In this canal travels blood vessels, lymph vessels and nerves as well. Now in between these sheets of lamellae are these tiny channels that are called canaliculi, which you can kind of see here. They branch out from the central haversian canal to these empty spaces that are called lacunae. Whenever you see the word lacuna or lacunae, you should think empty space. So each of these lacunae is really just an empty space or osteocytes or bone cells. These osteocytes have these long cellular processes that branch through the canaliculi to contact other osteocytes via gap junctions which allow these cells to communicate with each other and exchange nutrients and signals with each other. Finally, you have these volkmann canals which are canals that run perpendicular to the haversian canals. And these connect osteons to one another and also, as you can see, carry their own set of small blood vessels. Let's not forget that the very outer most superficial layer of bone is called the periosteum. Peri meaning around or surrounding and so that's the layer of bone that is on the outermost that you can actually see with the naked eye.