Osteogenesis (Bone Formation): Endochondral Ossification (Nursing)

00:00 In the second kind of ossification, we have endorchondral ossification.

00:05 This is different from intramembranous ossification.

00:09 In that we are going to go through a cartilage intermediate, instead of going straight from our mesenchymal cells to bone cells.

00:19 In the first step of endochondral ossification, we're going to develop a cartilage model.

00:26 This happens when the mesenchymal cells develop into chondroblasts instead of osteoblasts like they did in intramembranous ossification.

00:36 These chondroblasts will form a cartilage model, which will look like the bones of the body.

00:45 In the second step, what happens are these chondroblasts are going to secrete extracellular matrix.

00:53 This extracellular matrix is going to surround the chondroblasts.

00:58 After this happens and they become completely surrounded or in caged by the extracellular matrix, they are going to mature into chondrocytes.

01:08 As well, the extracellular matrix begins to calcify or harden a little bit.

01:17 After this, a nutrient artery is going to penetrate the middle or the center of what will soon be the diaphysis.

01:26 This is called the development of a primary ossification center.

01:32 At this point, we are now going to start modeling or creating the shape of the actual bone.

01:38 And we're going to replace the cartilage with bone tissue.

01:43 We do this by replacing the chondrocytes with osteoblasts and eventually osteocytes.

01:52 This occurs from the outside toward the inside of the bone.

01:58 The type of bone that we are creating in this step is spongy bone.

02:06 In the next step, we start to develop our medullary cavity.

02:12 So as we are building bones from the outside in osteoclasts or those immune cells that are able to break down bone begin to break down the bone from the inside.

02:24 So now we are starting to hollow out the diaphysis of the bone.

02:32 And the next step, we are also going to be doing the same thing that we did in the diaphysis in the epiphysis.

02:40 In this step we are going to create a secondary ossification center at both ends of the bone.

02:47 Just like with the primary ossification center, this is triggered by the introduction of an artery into the epiphysis of the bone.

02:58 This artery is referred to as the epiphyseal artery.

03:04 Finally, we are going to create the bone in the epiphysis.

03:10 The bone is going to replace the chondrocytes that were formed during the secondary ossification.

03:17 This kind of bone is going to be spongy bone.

03:21 Unlike in primary ossification, where we created a medullary cavity, we do not create a medullary cavity in the epiphysis.

03:30 So the spongy bone will be throughout the epiphysis and will not be hollow.

03:36 At the ends of the bone, we also have the articular cartilage.

03:42 These are the remnants of the cartilage model.

03:46 The type of cartilage found here is hyaline cartilage just like the cartilage that we began with.

03:53 As well on the other side on the side closest to the diaphysis.

03:57 We also create an epiphyseal plate.

04:01 It is at this point that we allow for interstitial growth or the lengthening of the bone.