Hello. Welcome to Byte Size Med. This video is on the structure of a long bone. There are different types of bones in the body and based on their shape, they're classified as long, short, flat or irregular bones. But to study the structure of bone, we usually use a long bone like the humerus or the femur. Most long bones have three different parts. The shaft of the bone is called the Diaphysis, the ends are called the Epiphyses. The expanded ends of the diaphysis that join the epiphysis are called the metaphyses. In a growing bone, the diaphysis and the epiphysis are joined by a plate of cartilage called the epiphyseal growth plate. Over time, ossification takes place and eventually once the bone reaches its full length, the two fuse and there's no more cartilage. If we were to take a longitudinal section of this bone, it would look something like this. Here we have the diaphysis and this is the epiphysis. In the centre, there's a space and that's called the medullary cavity. And around it there are two different kinds of bone. If we take a cross section of this bone, its structure varies depending on the level of that section. But macroscopically, there are two types of bone. Compact bone and cancellous bone. Compact bone is also called dense bone, because it is dense and its thickest in the diaphysis. It tapers towards the ends. It's also called cortical bone, because it's on the outside. Like the cortex. Compact bone is solid bone versus cancellous bone, which is also called spongy bone. Cancellous bone consists of rods and plates forming what's called trabeculae, with spaces or cavities in between. So cancellous bone is also called trabecular bone. These spaces are filled with bone marrow. In the diaphysis, the cancellous bone is thin underneath the compact bone. So the compact bone is the thickest at the diaphysis with lesser cancellous bone and at the centre of the diaphysis is the medullary cavity filled with bone marrow. The epiphysis on the other hand doesn't have the medullary cavity. But it's filled with more cancellous bone surrounded by a thin layer of compact bone. Bone marrow could be of two types, yellow or red. Red bone marrow is hematopoietic, meaning it can synthesize blood cells. But with age, a lot of the red bone marrow gets replaced by yellow marrow which is fat. So it's got adipocytes. The diaphysis of most adult long bones usually has a yellow bone marrow. But cancellous bone of the epiphysis and some long bones can have red bone marrow. There are some areas where red marrow persists for life, like the sternum and the iliac crest. But for now our focus is more on the structures of the compact and cancellous bone. Whichever it may be, compact or cancellous, they are made up of bone tissue and bone tissue is a form of connective tissue. A specialized connective tissue. Now we'll do a quick recap of general connective tissue. Connective tissue consists of cells in an extracellular matrix, which includes protein fibres like collagen and elastin and the well-hydrated ground substance with glycosaminoglycans, proteoglycans and adhesive glycoproteins. Bone is a special form of connective tissue, because that extracellular matrix is calcified. In addition to these organic components, bone has an inorganic matrix with lots of minerals, the predominant one being calcium hydroxyapatite. The organic components contribute to the tensile strength of bone while the inorganic ones give it compressional strength. So both of them are important for bone strength. That's the matrix. But what about the cells? Bone cells have special names too. There are osteoprogenitor cells, which are the mesenchymal stem cells capable of differentiating into osteoblasts when required. The osteoblasts are the active cells that synthesize all these matrix components. The organic uncalcified matrix is called osteoid. That's what first gets synthesized. It then gets mineralized to form calcified bone matrix. Again regulated by the osteoblasts. When they form the matrix, some of the cells get trapped in the matrix and differentiate into mature bone cells called osteocytes. Those that don't form osteocytes either undergo apoptosis or they can remain as bone lining cells, lining the surface of bone. So we have osteoprogenitor cells or osteogenic cells, osteoblasts, osteocytes, bone lining cells and the final type of bone cell is the osteoclast. Now these are multi-nucleated giant cells which are derived from monocytes. They are bone digesting cells. They're responsible for bone resorption. Osteoblasts synthesize the bone matrix, osteocytes maintain it and osteoclasts destroy it. Osteoblasts synthesize the bone matrix. The abundant protein fibre in bone tissue is type 1 collagen. So that is synthesized by osteoblasts. The ground substance proteins are also produced by the osteoblasts. That's glycosaminoglycans like chondroitin sulfate, proteoglycans, adhesive glycoproteins like osteocalcin, osteonectin. They're all synthesized by osteoblasts. Things like osteocalcin and osteonectin are important because they can avidly bind to calcium and that's needed for mineralization of bone. That's calcification of the bone matrix. Those are the important components of bone tissue and that tissue is arranged in a certain way. If we pick up the diaphysis we already saw on a cross section how it's got compact bone, then thin cancellous bone and a central medullary cavity. Now we're going to take a closer look at those layers. For that I'm going to schematically arrange the layers first. Compact bone - cancellous bone - medullary cavity. Bone is specialized connective tissue because it's calcified, but it's covered by dense connective tissue which is not calcified. That's the sheath covering the bone that you can see here. Now let's peel this layer off the surface. This layer is called the periosteum. So the periosteum is a layer of dense connective tissue on the outer surface of bone and it has two layers. It's got an outer fibrous layer and an inner cellular layer. The outer fibrous layer has Type 1 Collagen fibres synthesized by fibroblasts. It's also got nerves and blood vessels, which can provide nourishment to the outer surface of bone. The inner cellular layer has cells, osteoprogenitor cells which can differentiate into osteoblasts. It's got those bone lining cells.It can have osteoclasts. Thus making this layer important for bone growth and repair. Fibres from this periosteum can extend inside and anchor it to the surface of bone. These are called the Perforating Fibres. They're more dense at sites where tendons attach the periosteum. So the periosteum is needed for bone growth and repair. It also forms the outer limiting layer of bone. It provides nourishment to the bone and forms a site of attachment for tendons of muscles. Periosteum is on the outside. The endosteum is on the inside. Peri- outside and Endo- inside. It lines the marrow cavities and we'll get to that. This endosteum also has a similar cellular layer with osteoprogenitor cells, osteoblasts, bone lining cells and even osteoclasts. The innermost part of the diaphysis is the central medullary cavity. Between the periosteum and the medullary cavity is the compact bone and cancellous bone. Bone forms by ossification and both types of bone, when they first form are woven bone which is irregularly arranged collagen and not much mineral. It's immature. It's quickly formed it's not strong enough. Bone realizes this and so remodels itself to form a stronger version where bone tissue is arranged in regular layers. This is called mature lamellar bone. Most adult bones are mature lamellar bones and here the Type I Collagen fibres are arranged in layers. The fibres run parallel to each other in a single layer. But between the layers, the angle of the fibres changes. That contributes to the strength of bone. These layers are arranged in concentric circles around a central canal. So these circles would be how it would look from above in a cross section. Central canals have vessels, arteries and veins and nerves and they run through these cylindrical structures. These are called Haversian canals. If we go back to that schematic diagram, this would be a longitudinal section of compact bone. It's incomplete now. But we'll fill it up soon and these central canals are the longitudinal canals with vessels and nerves. The central canal is thus surrounded by concentric layers of calcified bone matrix and these are the lamellae. The cells that synthesize these are the osteoblasts and as they do so, some of them get trapped between the layers and differentiate into osteocytes. So those cells are what you see trapped between the layers. Osteocytes. There are thus spaces called lacunae in between the layers and each lacuna has a single osteocyte. These osteocytes have cytoplasmic extensions which connect one cell to another via Gap Junctions. These processes are located in canaliculi. The calcified matrix obviously doesn't allow diffusion to happen. So nutrition exchange happens between these cells and the extracellular fluid located in the lacunae and the canaliculi. Now this whole structure, the central Haversian canal and those layers is called an Osteon or a Haversian system. The outer boundary of each osteon has a layer with lesser collagen and more non-collagenous proteins. This is called the cement line. Like I said before, this is a longitudinal section. So there are layers and layers of calcified bone with trapped osteocytes. But to see the osteons, we need a cross-section. There are many osteons that make up the structure of lamellar bone. Between the central canals of these osteons and going towards the periosteum and the endosteum are transverse canals. These are called the Perforating Volkmann's canals, which also have vessels and nerves. So the longitudinal canals are the Haversian canals and the transverse canals are the Volkmann's canals. These are not the first osteons to form. Remember that bone first forms as woven bone and then remodels into lamellar bone. So there are primary osteons, which then get replaced with secondary osteons and such. The remnants from previous osteons that have then gotten remodeled remain between these osteons as lamellae called interstitial lamellae. Towards the periosteum, there are longitudinal lamellae called the external circumferential lamellae. External - outside. Similarly towards the cancellous bone surface, which is going to be our next layer are the internal circumferential lamellae. So there's external circumferential lamellae on the outer side, internal circumferential lamellae on the inner side and the interstitial lamellae in between the osteons. And now we've reached cancellous bone. Cancellous bone, remember is made up of rods and plates forming trabeculae. It also consists of calcified bone matrix with those trapped osteocytes. This arrangement in the form of trabeculae reduces the weight of bone, while still providing support and remember in the spaces between the trabeculae is bone marrow and the marrow cavities are lined by endosteum, which has those osteogenic cells, osteoblasts, bone lining cells and some osteoclasts. This endosteum is thus important for bone growth, repair and remodelling because it's got all of those cells. And this cancellous bone is then followed by that central medullary cavity. So we have the nourishing periosteum surrounding dense compact bone, which is then followed by the trabeculae of cancellous bone, the spaces of which are filled with marrow and lined by endosteum and in the centre, we have the medullary cavity. And that is the structure of a long bone. I hope this video was helpful. If it was, you can give it a like and subscribe to my channel. Thanks for watching and I'll see you in the next one! :)