Hey guys, Dr. Brown here. Another little video in histology, and this time we're gonna be going over something that a lot of folks have a hard time wrapping their head around, and that is the epiphyseal growth plate. So we look at this picture of a long bone, you can see that we're kind of zoomed in on that thin band of hyaline cartilage there in the epiphysis of the bone. That's why it's called an epiphyseal growth plate. And the growth plate is actually divided into five distinct layers. There's a zone of reserve cartilage. Immediately beneath that, a zone of proliferation, meaning that's the area where we're gonna make new cartilage tissue. The zone of hypertrophy, where the cartilage cells themselves, the chondrocytes, are going to enlarge and then die off. The zone of calcification, where the holes left by the dead cartilage cells are going to be

infiltrated by osteoblasts, which are going to start to calcify the cartilage tissue. And then finally, deepest down the zone of ossification, where the calcified cartilage is remodeled into lamellar bone in the zone of ossification. So how these zones relate to the direction of bone growth is something a lot of people tend to struggle with, and so I've put together just a little animation that will hopefully make this make just a little bit more sense. Let's look at just the growth plate all by itself. The helices is called the zone of proliferation for a reason; it's going to be making new tissue. So as chondrogenesis begins in the zone of proliferation, new cartilage tissue is going to be made. Hey, zone of proliferation got to proliferate, right? Now what's going to happen next is deep down in the zone of proliferation, lower, those are the

cells that are the oldest. Those proliferated the longest time ago, and they're eventually going to begin to undergo hypertrophy. So these cells are going to undergo a morphological change, they're gonna get bigger, they're gonna get bigger. Right? They're going to continue to grow. Meanwhile, underneath these cells down here that were hypertrophied, they're going to begin to undergo calcification. So these recently hypertrophied cells begin to calcify. Notice that the zone of proliferation is still pushing up. So these cells that were in the zone of proliferation have now differentiated, and this has become the zone of hypertrophy. So the bottom part of the zone of proliferation is now not part of the zone of proliferation, it's part of the zone of hypertrophy. Likewise, the zone of hypertrophy has become the zone of calcification, and then finally down here the zone of

calcification is going to be remodeled into new bones. So the calcified cartilage is remodeled into bone, and now what was the zone of calcification is the zone of ossification. So essentially, as the zone of proliferation grows, the cells that were left behind from the previous iteration of growth in the zone of proliferation become the zone of hypertrophy. The cells that were in the zone of hypertrophy become the zone of calcification, and the cells that were in the zone of calcification become ossification. Let's kind of wrap this up with a side-by-side. So here's our original cartoon of the epiphysis growth plate, and here's the cartoon we ended up with at the end. So you can see that it has gotten longer, but it has gotten longer by extending upward...right...toward the zone of proliferation. So we can kind of think of the epiphyseal growth plate as like a

snail, and as the snail crawls along, it's gonna leave a trail of bone behind it. So that the bones are gonna...gonna...gonna...gonna...gonna...pardon me...the zones are gonna... ..once more...the bone is gonna grow toward the Epiphanes. Meaning the bones are gonna grow from the edges outward. A lot of people want to think that the bone is growing inward but that is not the case. As cartilage is being produced at the top of the zone of proliferation, everything beneath it is being turned into more bone. More cartilage is made at the top; everything beneath it becomes bone. So I hope that helped make some sense out of how those layers in the epistle growth plate contribute to bone growth, and why bone is growing in the direction it's growing. Alright, I hope that helps. Study hard. Have fun. See you later.

HSCI 4590 Histology--Epiphyseal growth plate

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