Bone Growth (Nursing)

00:00 Speaking of interstitial growth, this is going to occur at the epiphyseal plate.

00:06 Recall that the epiphyseal plate is a line of cartilage that separates the epiphysis from the diaphysis.

00:14 Starting from the area farthest away from the diaphysis and closest to the articular cartilage, you have a zone of resting cartilage.

00:25 This is followed by a zone of proliferating cartilage, then hypertrophic cartilage and then calcified cartilage.

00:33 So taking a closer look at the cells in each of the individual zones, starting with the zone of resting cartilage, we have chondrocytes which are not dividing.

00:44 Just below this zone, we have the proliferating cartilage.

00:49 In this zone the chondrocytes are actively going through cell division and allowing for more chondrocytes to be made.

00:58 Just below this, we have the zone of hypertrophic cartilage.

01:02 In this area, the chondrocytes are enlarging or hypertrophying and as well they are secreting extracellular matrix.

01:11 They then become engulfed or in caged in that extracellular matrix, which causes nutrients to no longer be able to get to those chondrocytes.

01:22 Because of this, it starts to calcify, and after calcification, those cells are going to die.

01:29 These dead chondrocytes are then replaced with bone cells and bone tissue.

01:37 So we've talked about interstitial growth or the lengthening of bones.

01:42 But we also need to note that in bone remodeling, there's also a thickening of the bone also referred to as appositional growth.

01:50 This occurs in four major steps.

01:53 The first step is going to be the formation of a periosteal ridge.

01:58 In the second step, we're going to form a haversian canal that is going to house the blood vessels of the bone.

02:06 And the third step, this haversian canal is going to get smaller as the bones grow inward.

02:14 And in the fourth step, the bone as a whole begins to grow outward as the periosteum creates more bone matrix from the outside.

02:23 Let's talk about each one of these steps individually.

02:26 In step one, we create the periosteal ridge.

02:30 The blood vessels in the periosteum start to be engulfed or envelope by the periosteum of the bone.

02:40 In step two, this ridge now fuses and forms a canal around the blood vessel.

02:48 This now makes what used to be the periosteum on the outside of the bone.

02:53 Now an endosteum that is spacing the inside of the bone toward the blood vessel.

03:01 And the third step, we are now going to start creating bone toward the blood vessel.

03:07 So the bone is starting to grow inward toward the blood vessel in the haversian canal.

03:16 And in the final step, we are also going to be creating bone that is going outward from the periosteum.

03:24 Please note that when we are doing this, we are going to continue to go through this process over and over around the different blood vessels in the periosteum.

03:35 The layers that are made on the outside are referred to a circumferential lamellae or that translates to basically the circumferential layer or the outer layers of the bone.

03:48 During appositional growth, we're going to be thickening the bone but we want to make sure that the bone does not become too heavy.

03:56 So as osteoblasts are depositing bone on the outer surface of the bone.

04:03 Osteoclasts are going to be working on the inside of the bone, widening the medullary cavity from within.

04:13 So now that we've discussed both types of growth, both interstitial growth where we're growing in length, and appositional growth where we're growing in thickness, let's talk about some factors that will influence this bone modeling.

04:27 One of those factors is the response to stress.

04:30 The more stress you put on your bones, the more that the bone will grow in thickness.

04:37 This means that someone who exercises a lot or puts a lot of stress on their bones will have thicker bones than those who live a sedentary lifestyle.

04:48 Another factor is growth.

04:50 Specifically, when an individual goes through puberty, there is a large amount of interstitial growth or growth in length of the long bones especially the bones in the legs.

05:03 This is controlled by different hormones.

05:06 The two main hormones that control this growth are going to be androgens and estrogens.

05:12 An example of an androgen is testosterone.

05:15 Now, while men mostly have a lot more testosterone than women and women have more estrogen than men, both men and women have some of both.

05:29 Androgens are more responsible for the lengthening of the bones.

05:33 and estrogens are actually going to reverse this process and start the process of stopping the bone modeling.

05:41 This is the reason why women stop growing before men because they have more estrogen.