Full transcript
0:00This is the third video over chapter seven.
0:03We will be going over bone development. Ossification or osteogenesis is the
0:10formation of bone. In the human fetus and infant, bone
0:15develops by two methods, intramembranous ossification and
0:21endochondrial ossification. Intramembranous ossification,
0:27you can remember this because intra means
0:31inside. So this is where you form bone inside a membrane,
0:37and this happens to be a fibrous connective tissue membrane.
0:41It has mesenchymal cells in it. Those are embryonic stem cells.
0:48These stem cells make connective tissue. In this case, the mesenchymal cells are
0:54going to do mitosis, and they produce osteoblasts. The
0:58osteoblasts make the matrix, and then they turn into
1:02osteocytes. You make spongy bone first, then later
1:07you can convert the spongy bone to compact. But compact is very organized;
1:13that doesn't come first. The type of bones that use
1:18intramembranous ossification are the flat bones of the skull,
1:22the clavicles, and the fontanelles. The fontanelles are the soft spots you have
1:28on the skull of an infant. Most of those bones are going to be remodeled.
1:34That means they're destroyed and reformed as we grow to an adult
1:38size. We look at this diagram here, on this first figure you see mesenchymal
1:45cells. They're going to divide; they're going to
1:47produce the osteoblasts. Once the osteoblasts are trapped in the
1:53matrix, then you call them osteocytes. You notice
1:58that it has these wide areas, so we have spongy bone.
2:03Then afterwards, you can see compact bone being made
2:07on the top and on the bottom of this.
2:12The second type of ossification is endochondrial
2:16ossification. In this case, bone forms by replacing hyaline cartilage.
2:24This method forms most of the bones of the body below the skull,
2:28except for the clavicle. You start out with the mesenchyme.
2:33That's again, the embryonic stem cells that form connective tissue.
2:38In this case, it forms chondroblasts. Now the chondroblasts are going to make
2:44hyaline cartilage. Then that hyaline cartilage is going to
2:49be destroyed, and you replace the chondrocytes with
2:53the osteoblasts. Osteoblasts
2:57first make spongy bone, then it can convert it to compact bone.
3:03When we look at endochondrial ossification,
3:06you start out with the cartilage, and so you have a perfect model of
3:13that bone, but it's all made out of hyaline cartilage.
3:17Primary ossification, that means the first
3:20part of that, that becomes bone, is going to be the diaphysis.
3:26Secondary ossification is going to be the epiphysis,
3:30so those are the ends of the long bones. If we look at this diagram, you start out
3:36with a cartilage model, and then the diaphysis starts getting
3:41this bony collar. It hollows out in the center of it,
3:46and then the epiphysis will start turning into spongy bone.
3:54We look at the skeleton in a fetus. Here's a long bone. This is going to be
4:00made out of hyaline cartilage. When we do primary
4:04ossification, the diaphysis then, is converted to bone.
4:10And then this is going to hollow out, you're going to get that
4:14medullary cavity in there. Then secondary ossification is going to work
4:19on the ends of the bones, the epiphyses. It starts out in the center of it.
4:27So this is where you're destroying that cartilage, and it's being replaced with
4:32osteoblasts. That center area is going to enlarge. You will leave
4:39two strips of cartilage. On the outside is going to be your articular cartilage,
4:46and hopefully you keep this for the rest of your life.
4:50And then you will have a strip of cartilage between
4:54the epiphyses and the diaphysis. That's your epiphyseal plate; that is the
5:00growth plate. During infancy and childhood, the
5:05epiphyses fill with spongy bone. The cartilage that's left is
5:10the articular cartilage, and the epiphyseal plate. That is that
5:16growth zone for making the bones longer.
5:20If we look at this fetal skeleton here, on the cranial bones, remember, that's
5:26intramembranous ossification. So you start out
5:30in the center, and it gets wider and wider.
5:34In between those cranial bones is still the membrane,
5:38and so that will form the soft spots that you have in an
5:42infant. If we look at the legs and the arms,
5:46you can see that only the diaphysis is ossified. The ends are still
5:53hyaline cartilage, and that's why it doesn't show up on this
5:57image. Ossification continues throughout life,
6:03and so you have growth, and then you have remodeling
6:07once you finish with growth. When we look at
6:11growth, bones grow in two directions, lengthwise and by width. If we look on
6:18lengthwise, that occurs at the epiphyseal plates.
6:22Those cartilage cells do mitosis. It is not the bone cells, it's the cartilage
6:28cells. So as you accumulate more cartilage cells, it starts pushing
6:35the epiphysis away from the diaphysis. Then
6:40the cartilage cells start to die off, and are replaced by bone.
6:46When growth is finished, there will be no more
6:50cartilage at that plate, and so we call it the epiphyseal
6:55line, and so that bone is no longer able to grow in length. We say
7:02that the plate has closed. When we look at a child's hand, it still
7:08has the growth plates on it. On the
7:12metacarpals, you can see growth plates here
7:16at the very ends. That's where cartilage is;
7:20that's why it looks like it's a gap. Because the cartilage does not show up
7:26on the x-ray. You can see another epiphyseal plate
7:29here at the end of the radius, here on the phalanges.
7:35That allows the hand and the fingers to get longer.
7:39When growth is done, these plates will close,
7:43and this will be solid bone. We have a type of dwarfism in the United
7:49States, it's called achondroplastic dwarfism.
7:53In this case the long bones stopped growing in childhood. So this person has
7:59a normal torso, but the arms and legs
8:03are abnormally short, and that's because you don't have cartilage growth in
8:09that epiphyseal plate. This occurs because of a spontaneous mutation.
8:16There's another type of dwarfism that's called pituitary dwarfism.
8:21That is a lack of growth hormone. That person would have
8:25normal proportions, but a very short stature.
8:29We almost never see pituitary dwarfism in the United States anymore.
8:36How do the bones widen and thicken? This is called appositional growth, and
8:42that occurs at the bone surface. So this is how
8:47bones get wider and wider. This is a type of intramembranous
8:53ossification where we're looking at the osteoblasts
8:58on the inside of the periosteum. Remember, the periosteum is your membrane
9:05that is on the outside of every bone, and so that lays down matrix in layers
9:11and you get lamellae. Now while you are thickening up the bone,
9:18the inside of the bone is going to be hollowed out. So the osteoclasts
9:25and they're going to be on the inside membrane, the
9:28endosteum, will enlarge that marrow cavity,
9:33otherwise, the bone gets too heavy. So let's look at bone growth and width.
9:41We are looking on the outside of the bone. There is a blood vessel that
9:47has been produced, so running lengthwise on your bone. And what will happen,
9:54is that around that blood vessel, your bone cells and the membrane starts
10:01to come up. You're going to enclose that blood
10:05vessel in a tunnel, and so now it seals up.
10:11The periosteum continues on the outside and then you have now in this tunnel,
10:19the endosteum. Then you'll have your osteoblast
10:24start making the matrix, and you get these lamella,
10:29and it'll stop as soon as it hits the blood vessel.
10:34And now I've added an osteon on the side of my
10:37bone, and this is how my bone gets wider and wider.
10:42Ten percent of your skeleton is remodeled
10:46every year, that means they're broken up and rebuilt.
10:50So about every 10 years, you have a new skeleton.
10:54How it's remodeled is the osteoblasts. They're the ones that make the bone, and
11:00the osteoclasts. They're the ones that break down bone. We call that
11:06bone resorption. Bone grows or remodels in response to the
11:12demands placed on it. This is Wolff's law. If you put
11:17pressure against the bone, that bone responds. So if I
11:23am using a muscle that is pulling on a part of the bone, I will
11:28add bone density right where the muscle is attached.
11:33I strengthen that bone at that place, and it's because I put demands on it.
11:42So exercise is more than promoting muscle health, it is also promoting bone
11:48growth. Lack of exercise, if you have a patient
11:52that's bedridden, for instance, the bones atrophy, and then those bones
11:58become very weak. This is the end of the third
12:02video. The last video will cover physiology
12:06and some disorders.