Full transcript
0:03hello anatomy colleagues this is Dr. Alsup and in this video we have quite a bit to discuss a lot of
0:09really interesting things and and you can see there's quite a few learning objectives here
0:14i want to talk big picture regarding bone formation as well as bone growth
0:19and a little bit about bone remodeling as always please make sure that you are
0:25taking the time to answer these learning objectives in your own words after viewing
0:30this is such an excellent way to make sure you're on track keep in mind these learning objectives
0:34can't really be answered in one word so take that time it is really the best way to study
0:42okay so all bone formation is going to begin with what's referred to as the formation of an
0:49embryonic skeleton and this is going to be composed of mesenchyme so it's not a true skeleton
0:56it's going to be composed of mesenchyme which is that embryonic tissue that almost all connective
1:02tissues are derived from and since bone is a type of connective tissue that makes sense that
1:07we're starting with mesenchyme generally this formation of this mesenchymal skeleton begins
1:15around the sixth week in utero and from there bone formation follows one or one of two patterns
1:25and these types of ossification patterns are referred to as intramembranous and
1:32endochondral but before we get into the description of these i want to distinguish
1:38between the terms ossification and calcification because they're often used interchangeably
1:45albeit that's incorrect so ossification is the process by which bone forms so that's what we're
1:53talking about here this is how bone will form and this happens in four specific situations
2:00ossification will occur with that initial formation of bone which is what we're focusing
2:05on right here ossification also is growth of a bone during the juvenile and adolescent period
2:12it's also remodeling a bone and lastly it's repair of bone which sometimes remodeling and repair
2:18are often discussed together now as part of bone formation and ossification osteoblasts will lay
2:26down the organic materials of that extracellular matrix which will initiate calcification
2:33and calcification is really just tissue hardening so calcification is a process within ossification
2:44but it's not the same thing as ossification so calcification is hardening ossification
2:49is bone formation so don't use those terms interchangeably and i'll try not to do it myself
2:56okay so let's return to these two main types of ossification the first one we're going to discuss
3:02is intramembranous and we're starting with this one because it's the simpler of the two
3:07and in this process bone is going to directly replace that mesenchymal skeleton
3:15all right so bone directly replaces that mesenchymal skeleton whereas in endochondral
3:22ossification that mesenchymal skeleton is going to be actually replaced by a
3:29cartilage model first so this hyaline cartilage model grows first and then eventually it will be
3:36replaced by bone so there's that extra step in endochondral in comparison to intramembranous
3:44now i don't want you to memorize these specific steps so this is intramembranous here and this
3:50is endochondral here and it gives you kind of these big picture steps by which this occurs
3:56but i do want you to at least understand these basics in terms of intramembranous is through
4:02the mesenchymal skeleton and endochondral has that extra step of the cartilage model
4:10a minority of bones form through intramembranous ossification initially and these include many flat
4:17bones of the skull so you can see we're looking at a juvenile skull here you can see the frontal bone
4:22parietal bones here occipital back here see these are all forming through intramembranous
4:28ossification and between these bones of the skull you will have what's referred to as
4:33fontanels you can see a few here fontanels are often colloquially referred to as soft spots
4:40these are super important in allowing that large skull to exit the relatively small birth canal
4:46so allowing some flexibility here and after birth these areas will undergo intramembranous
4:51ossification these fontanelles and harden and be replaced entirely by bone so same kind
4:57of process as what you have the actual bones here other bones are going to form through intra-
5:02membranous ossification they include the face many of the facial bones the mandible so that lower jaw
5:09and the medial part of the clavicle often you hear the clavicle referred to as the collarbone so a
5:15relatively few bones form initially through intramembranous ossification but i want you
5:20to focus in on this box right here likely intramembranous ossification's more important
5:27role is lifelong thickening, strengthening, and remodeling of bones so it is the method
5:34for depositing new tissue on the bony surface even past the age where bones can no longer
5:40grow in length so this is a really important function here of intramembranous ossification
5:48now as we mentioned earlier endochondral ossification is more complex than
5:52intramembranous and this is because of that these middle steps here where you have
5:57the cartilage model and not only do you have a cartilage model but that cartilage
6:02model actually grows so it's going to grow both in length as well as width
6:08and most bones of the body initially form through endochondral ossification
6:13but this type of ossification process stops after skeletal maturity so any other type
6:19of bone formation say that occurs during adulthood on the in terms of bone width
6:25that's going to occur through intramembranous rather than endochondral ossification
6:32a few notes regarding the cartilage model in this region where the bone so in the region
6:40say where the bone is going to form chemical messages will actually cause those mesenchymal
6:45cells to crowd together and form this general shape of the future bone and this is where you
6:51have that cartilage model development these cells will differentiate into chondroblasts
6:57and so if you have -blast in a term you know it's building something so specifically it's secreting
7:04cartilage extracellular matrix matrix components so producing this cartilage model chondro- you
7:11need to be thinking cartilage specifically here we're talking about hyaline cartilage
7:19and this cartilage model will grow both through interstitial
7:24or growth from within which will result in the cartilage model actually growing in length
7:30as well as appositional growth which is going to be growth on the
7:35outer surface so the cartilage model will actually grow in width as well
7:41you can see through these steps bone will eventually begin to replace the cartilage model
7:47first this will occur in the diaphyseal region and then it will begin to occur in the epiphyseal
7:53region and you can see that you will have this plate of cartilage that will persist and allow
8:02for bone length growth to occur so this is that epiphyseal plate
8:08right here so in that metaphysis region eventually the hyaline cartilage model will be completely
8:15replaced with bone except on the very ends of bone where you will have articular cartilage
8:23persist into adulthood articular cartilage is extremely important in terms of joints and
8:28we'll talk about that in much greater detail when we get to those lectures
8:34I do want to return to the terms interstitial and appositional growth and make sure we clearly
8:39define those interstitial growth is growth from within and here i want us to be thinking of those
8:47epiphyseal plates you can see generally where that would be located here on this
8:52radiograph that region is that transition from cartilage to bone it is here that you can have
9:00increases in bone length because of the elongation really occurring as a result of cartilage growth
9:08so the cartilage will grow and be replaced by bone the cartilage will grow be replaced by bone
9:14so it's really cartilage that's playing or it's going to to grow in terms of interstitial growth
9:20eventually we'll have the complete replacement of cartilage with bone and bone will not
9:24be capable of bone length growth however with appositional growth
9:30or growth or deposition of new tissue to the outer surface this can occur with both cartilage
9:39and bone so cartilage can enlarge by both interstitial and appositional growth but in
9:44bone osteocytes are embedded within that calcified extracellular matrix so it really
9:49has little room to deposit matrix internally so bone growth can really only be appositional
9:57bony appositional growth occurs by intramembranous ossification at the bone
10:02surface so those osteoblasts and the internal layer of the periosteum they're going to deposit
10:07tissue on the bone surface that will calcify or harden and become trapped as an osteocyte and
10:13eventually this will form those circumferential lamellae that surround the entirety of the bone
10:22lastly bone remodeling is a natural process that is vital in repairing micro fractures, reshaping
10:28bone in response to use or disuse, and also in releasing minerals into the blood when necessary
10:35healthy typical bone remodeling is built upon the collaborative actions of the osteoblasts as
10:41well as the osteoclast so remember osteoblasts are the bone builders osteoclasts are the bone resorp
10:48resorbers or bone resorption if one of these cells outpaces the other this can have implications in
10:56bone health and mass and we will talk about an example of one of these disorders or osteoporosis
11:01in an upcoming video but i do want to note that typically remodeling is healthy it's a
11:06necessary life process it's just when you have an imbalance here that things can start to go wrong
11:14all right excellent thanks for sticking with me through a denser topic here let's do a quick
11:20question review before moving on so the question here the parietal bone of the skull so it's one
11:26of those flat bones if you put your hands on the top of your head you're touching or the parietal
11:31bones are underneath the skin there so those probable bones initially form through which
11:36type of ossification process is it endochondral or intramembranous pause the video if needed and when
11:44ready what did you think what was your answer well the parietal bone is one of the few bones that is
11:52going to form through intramembranous ossification recall the flat bones of the skull most of the
11:58facial bones the mandible or the lower jaw and a portion of the clavicle or the collarbone is going
12:04to form via intramembranous ossification all other bones of the body
12:10initially form via endochondral ossification with that cartilage model first all right excellent
12:17thank you so much for your time and attention here and i will see you in the next video