Structure of Bone | Lamellar Bone | Compact and Cancellous Bone | Bone Histology

0:00Hello. Welcome to Byte Size Med. This video is on the structure of a long bone.

0:08There are different types of bones in the body and based on their shape,

0:11they're classified as long, short, flat or irregular bones. But to

0:16study the structure of bone, we usually use a long bone like the humerus or the femur.

0:23Most long bones have three different parts. The shaft of the bone is called the Diaphysis,

0:28the ends are called the Epiphyses. The expanded ends of the diaphysis that join the epiphysis

0:35are called the metaphyses. In a growing bone, the diaphysis and the epiphysis are joined

0:41by a plate of cartilage called the epiphyseal growth plate. Over time, ossification takes place

0:47and eventually once the bone reaches its full length, the two fuse and there's no more cartilage.

0:54If we were to take a longitudinal section of this bone, it would look something like this.

0:59Here we have the diaphysis and this is the epiphysis. In the centre, there's a space and

1:05that's called the medullary cavity. And around it there are two different kinds of bone. If

1:14we take a cross section of this bone, its structure varies depending on the level of that section.

1:20But macroscopically, there are two types of bone. Compact bone and cancellous bone.

1:26Compact bone is also called dense bone, because it is dense and its thickest in the diaphysis. It

1:33tapers towards the ends. It's also called cortical bone, because it's on the outside. Like the cortex.

1:42Compact bone is solid bone versus cancellous bone, which is also called spongy bone. Cancellous bone

1:49consists of rods and plates forming what's called trabeculae, with spaces or cavities in between.

1:56So cancellous bone is also called trabecular bone.

2:01These spaces are filled with bone marrow. In the diaphysis, the cancellous bone is thin underneath

2:08the compact bone. So the compact bone is the thickest at the diaphysis with lesser cancellous

2:13bone and at the centre of the diaphysis is the medullary cavity filled with bone marrow.

2:19The epiphysis on the other hand doesn't have the medullary cavity.

2:23But it's filled with more cancellous bone surrounded by a thin layer of compact bone.

2:32Bone marrow could be of two types, yellow or red. Red bone marrow is hematopoietic, meaning it can

2:39synthesize blood cells. But with age, a lot of the red bone marrow gets replaced by yellow marrow

2:45which is fat. So it's got adipocytes. The diaphysis of most adult long bones usually has a yellow bone

2:52marrow. But cancellous bone of the epiphysis and some long bones can have red bone marrow.

2:59There are some areas where red marrow persists for life, like the sternum and

3:03the iliac crest. But for now our focus is more on the structures of the compact and cancellous bone.

3:10Whichever it may be, compact or cancellous, they are made up of bone tissue and bone tissue is

3:16a form of connective tissue. A specialized connective tissue.

3:20Now we'll do a quick recap of general connective tissue. Connective tissue consists of cells in an

3:27extracellular matrix, which includes protein fibres like collagen and elastin and the well-hydrated

3:33ground substance with glycosaminoglycans, proteoglycans and adhesive glycoproteins.

3:39Bone is a special form of connective tissue, because that extracellular matrix is calcified.

3:47In addition to these organic components, bone has an inorganic matrix with lots of minerals,

3:53the predominant one being calcium hydroxyapatite. The organic components contribute to the tensile

3:59strength of bone while the inorganic ones give it compressional strength. So

4:04both of them are important for bone strength. That's the matrix. But what about the cells?

4:12Bone cells have special names too. There are osteoprogenitor cells,

4:17which are the mesenchymal stem cells capable of differentiating into osteoblasts when required.

4:23The osteoblasts are the active cells that synthesize all these matrix components.

4:28The organic uncalcified matrix is called osteoid. That's what first gets synthesized.

4:34It then gets mineralized to form calcified bone matrix. Again regulated by the osteoblasts.

4:43When they form the matrix, some of the cells get trapped in the matrix and

4:47differentiate into mature bone cells called osteocytes. Those that don't form osteocytes

4:53either undergo apoptosis or they can remain as bone lining cells, lining the surface of bone.

5:01So we have osteoprogenitor cells or osteogenic cells, osteoblasts, osteocytes, bone lining cells and

5:09the final type of bone cell is the osteoclast. Now these are multi-nucleated giant cells which are

5:15derived from monocytes. They are bone digesting cells. They're responsible for bone resorption.

5:24Osteoblasts synthesize the bone matrix, osteocytes maintain it and osteoclasts destroy it.

5:33Osteoblasts synthesize the bone matrix. The abundant protein fibre in bone tissue is type

5:401 collagen. So that is synthesized by osteoblasts. The ground substance proteins are also produced by

5:47the osteoblasts. That's glycosaminoglycans like chondroitin sulfate, proteoglycans, adhesive

5:54glycoproteins like osteocalcin, osteonectin. They're all synthesized by osteoblasts.

6:00Things like osteocalcin and osteonectin are important because they can avidly bind

6:05to calcium and that's needed for mineralization of bone. That's calcification of the bone matrix.

6:12Those are the important components of bone tissue and that tissue is arranged in a certain way.

6:20If we pick up the diaphysis we already saw on a cross section how it's got compact bone, then thin

6:26cancellous bone and a central medullary cavity. Now we're going to take a closer look at those layers.

6:33For that I'm going to schematically arrange the layers first.

6:37Compact bone - cancellous bone - medullary cavity. Bone is specialized connective tissue because it's

6:44calcified, but it's covered by dense connective tissue which is not calcified. That's the sheath

6:51covering the bone that you can see here. Now let's peel this layer off the surface. This

6:59layer is called the periosteum. So the periosteum is a layer of dense connective tissue on the outer

7:05surface of bone and it has two layers. It's got an outer fibrous layer and an inner cellular layer.

7:12The outer fibrous layer has Type 1 Collagen fibres synthesized by fibroblasts. It's also got nerves

7:19and blood vessels, which can provide nourishment to the outer surface of bone. The inner cellular

7:26layer has cells, osteoprogenitor cells which can differentiate into osteoblasts. It's got those bone

7:33lining cells.It can have osteoclasts. Thus making this layer important for bone growth and repair.

7:40Fibres from this periosteum can extend inside and anchor it to the surface of bone. These are

7:46called the Perforating Fibres. They're more dense at sites where tendons attach the periosteum.

7:54So the periosteum is needed for bone growth and repair. It also forms the outer limiting layer

8:00of bone. It provides nourishment to the bone and forms a site of attachment for tendons of muscles.

8:08Periosteum is on the outside. The endosteum is on the inside. Peri- outside and Endo- inside.

8:16It lines the marrow cavities and we'll get to that. This endosteum also has a similar

8:22cellular layer with osteoprogenitor cells, osteoblasts, bone lining cells and even osteoclasts.

8:30The innermost part of the diaphysis is the central medullary cavity.

8:35Between the periosteum and the medullary cavity is the compact bone and cancellous bone.

8:42Bone forms by ossification and both types of bone, when they first form are woven bone which is

8:49irregularly arranged collagen and not much mineral. It's immature. It's quickly formed it's not strong

8:55enough. Bone realizes this and so remodels itself to form a stronger version where bone tissue is

9:02arranged in regular layers. This is called mature lamellar bone. Most adult bones are mature lamellar

9:11bones and here the Type I Collagen fibres are arranged in layers. The fibres run parallel to each

9:19other in a single layer. But between the layers, the angle of the fibres changes. That contributes to

9:25the strength of bone. These layers are arranged in concentric circles around a central canal.

9:32So these circles would be how it would look from above in a cross section.

9:38Central canals have vessels, arteries and veins and nerves and they run through these cylindrical

9:45structures. These are called Haversian canals. If we go back to that schematic diagram, this

9:52would be a longitudinal section of compact bone. It's incomplete now. But we'll fill it up soon

9:59and these central canals are the longitudinal canals with vessels and nerves. The central

10:06canal is thus surrounded by concentric layers of calcified bone matrix and these are the lamellae.

10:16The cells that synthesize these are the osteoblasts and as they do so, some of them

10:21get trapped between the layers and differentiate into osteocytes. So those cells are what you see

10:28trapped between the layers. Osteocytes. There are thus spaces called lacunae in between the layers

10:35and each lacuna has a single osteocyte. These osteocytes have cytoplasmic extensions which

10:43connect one cell to another via Gap Junctions. These processes are located in canaliculi.

10:52The calcified matrix obviously doesn't allow diffusion to happen. So nutrition exchange

10:57happens between these cells and the extracellular fluid located in the lacunae and the canaliculi.

11:06Now this whole structure, the central Haversian canal and

11:10those layers is called an Osteon or a Haversian system.

11:15The outer boundary of each osteon has a layer with

11:19lesser collagen and more non-collagenous proteins. This is called the cement line.

11:27Like I said before, this is a longitudinal section. So there are layers and layers of

11:32calcified bone with trapped osteocytes. But to see the osteons, we need a cross-section.

11:39There are many osteons that make up the structure of lamellar bone. Between the central

11:44canals of these osteons and going towards the periosteum and the endosteum are transverse canals.

11:52These are called the Perforating Volkmann's canals, which also have vessels and nerves. So

11:58the longitudinal canals are the Haversian canals and the transverse canals are the Volkmann's canals.

12:06These are not the first osteons to form. Remember that bone first forms as woven

12:11bone and then remodels into lamellar bone. So there are primary osteons, which then get

12:17replaced with secondary osteons and such. The remnants from previous osteons that

12:23have then gotten remodeled remain between these osteons as lamellae called interstitial lamellae.

12:32Towards the periosteum, there are longitudinal lamellae called the external circumferential

12:38lamellae. External - outside. Similarly towards the cancellous bone surface, which is going to be our

12:45next layer are the internal circumferential lamellae. So there's external circumferential

12:51lamellae on the outer side, internal circumferential lamellae on the inner

12:56side and the interstitial lamellae in between the osteons. And now we've reached cancellous bone.

13:07Cancellous bone, remember is made up of rods and plates forming trabeculae.

13:12It also consists of calcified bone matrix with those trapped osteocytes. This arrangement in

13:20the form of trabeculae reduces the weight of bone, while still providing support and remember in the

13:26spaces between the trabeculae is bone marrow and the marrow cavities are lined by endosteum, which

13:33has those osteogenic cells, osteoblasts, bone lining cells and some osteoclasts.

13:40This endosteum is thus important for bone growth, repair and remodelling because it's

13:46got all of those cells. And this cancellous bone is then followed by that central medullary cavity.

13:55So we have the nourishing periosteum surrounding dense compact bone, which

14:00is then followed by the trabeculae of cancellous bone, the spaces of which

14:05are filled with marrow and lined by endosteum and in the centre, we have the medullary cavity.

14:13And that is the structure of a long bone. I hope this video was helpful. If it was, you can give

14:19it a like and subscribe to my channel. Thanks for watching and I'll see you in the next one! :)