MSK1: Bone Formation, Growth, & Remodeling

Summary

Dr. Alsup's video delves into the fascinating processes of bone formation, growth, and remodeling. It distinguishes between intramembranous and endochondral ossification, explaining how bones form from mesenchyme or a cartilage model. The video emphasizes the importance of understanding these processes for studying anatomy, highlighting the roles of osteoblasts and osteoclasts in bone health and the critical differences between ossification and calcification.

Takeaways

• 🦴 Bone formation begins with the formation of an embryonic skeleton composed of mesenchyme, starting around the sixth week in utero.
• 🔄 Ossification is the process of bone formation and occurs in four situations: initial bone formation, juvenile growth, bone remodeling, and bone repair.
• ❌ Calcification and ossification are not the same; calcification refers to tissue hardening, while ossification is bone formation.
• 🧠 Intramembranous ossification is the simpler of the two processes, where bone directly replaces mesenchymal tissue.
• 🦷 Endochondral ossification involves a cartilage model that grows in length and width before being replaced by bone.
• 🧠 A minority of bones, including flat bones of the skull, some facial bones, and the clavicle, form initially via intramembranous ossification.
• 💪 Intramembranous ossification plays a lifelong role in thickening, strengthening, and remodeling bones.
• 🧬 Cartilage grows both interstitially (from within) and appositionally (on the outer surface), while bone growth primarily occurs appositionally.
• 🏋️ Bone remodeling is a natural, essential process involving osteoblasts (builders) and osteoclasts (resorbers), and imbalances can lead to bone disorders like osteoporosis.
• 💀 The parietal bone of the skull forms via intramembranous ossification, unlike most bones that use endochondral ossification.

Q & A

What is the primary focus of Dr. Alsup's discussion in the video?

Dr. Alsup's discussion primarily focuses on bone formation, including the processes of bone growth and remodeling.

What is the embryonic tissue from which bone formation begins?

Bone formation begins with mesenchyme, an embryonic tissue from which almost all connective tissues are derived.

At what stage of development does the formation of the mesenchymal skeleton typically start?

The formation of the mesenchymal skeleton typically begins around the sixth week in utero.

What is the difference between ossification and calcification as discussed in the video?

Ossification is the process by which bone forms, occurring in the initial formation of bone, growth during juvenile and adolescent periods, remodeling, and repair. Calcification, on the other hand, is a process within ossification that involves tissue hardening.

What are the two main types of ossification patterns discussed in the video?

The two main types of ossification patterns discussed are intramembranous and endochondral ossification.

How does intramembranous ossification differ from endochondral ossification?

In intramembranous ossification, bone directly replaces the mesenchymal skeleton. In contrast, endochondral ossification involves an intermediate step where the mesenchymal skeleton is first replaced by a cartilage model, which then gets replaced by bone.

Which bones primarily form through intramembranous ossification, according to the video?

Many flat bones of the skull, some facial bones, the mandible, and a portion of the clavicle primarily form through intramembranous ossification.

What is the significance of fontanels in the context of bone formation?

Fontanels, often referred to as soft spots, are important for allowing the large skull to exit the relatively small birth canal. They undergo intramembranous ossification after birth, hardening and being replaced by bone.

What is the role of intramembranous ossification in bone formation after the age where bones can no longer grow in length?

Intramembranous ossification plays a crucial role in the lifelong thickening, strengthening, and remodeling of bones, even after the age where bones can no longer grow in length.

How does the cartilage model grow in endochondral ossification?

In endochondral ossification, the cartilage model grows both in length through interstitial growth and in width through appositional growth.

What is the role of the epiphyseal plate in bone growth during endochondral ossification?

The epiphyseal plate allows for bone length growth by persisting as a plate of cartilage that gets replaced by bone, thus facilitating the elongation of bones during growth.

What is the importance of bone remodeling as discussed in the video?

Bone remodeling is vital for repairing micro-fractures, reshaping bone in response to use or disuse, and releasing minerals into the blood when necessary. It involves a balance between the actions of osteoblasts and osteoclasts.

Outlines

00:00

🦴 Overview of Bone Formation Processes

Dr. Alsup introduces the video by discussing the main topics: bone formation, growth, and remodeling. She emphasizes the importance of understanding learning objectives in detail. Bone formation starts with an embryonic skeleton made of mesenchyme, which forms around the sixth week in utero. Bone develops through two processes: intramembranous and endochondral ossification. She clarifies the difference between ossification (bone formation) and calcification (tissue hardening).

05:03

💀 Intramembranous vs. Endochondral Ossification

Intramembranous ossification directly replaces the mesenchymal skeleton and is simpler compared to endochondral ossification, which involves a cartilage model before bone formation. Most flat bones of the skull, facial bones, mandible, and part of the clavicle are formed through intramembranous ossification. This process also plays a key role in thickening and strengthening bones throughout life, even after bones stop growing in length.

10:07

🛠️ Complexities of Endochondral Ossification

Endochondral ossification, which is more complex than intramembranous, involves the growth of a cartilage model in both length and width before being replaced by bone. Most bones in the body initially form this way, but after skeletal maturity, any further bone formation occurs through intramembranous ossification. Chemical signals initiate the formation of the cartilage model, which grows via interstitial (length) and appositional (width) growth before being replaced by bone.

🔄 Bone Remodeling and Maintenance

Bone remodeling is an essential process that repairs microfractures, reshapes bones, and releases minerals into the blood. It involves the coordinated activity of osteoblasts (bone builders) and osteoclasts (bone resorbers). An imbalance between these cells can lead to bone disorders like osteoporosis. Dr. Alsup concludes by reinforcing the importance of healthy remodeling and previews a future video on bone disorders.

Keywords

💡Bone formation

Bone formation refers to the process by which skeletal structures develop. In the video, it is discussed in the context of embryonic development, starting with mesenchyme, which is a type of embryonic tissue. This process is crucial for understanding how bones are initially formed in the body and is a central theme of the video.

💡Bone growth

Bone growth is the increase in size and length of bones during juvenile and adolescent periods. The video explains that bone growth is a part of ossification and is essential for the development and maintenance of the skeletal system. It is mentioned in the context of the growth of a cartilage model, which eventually gets replaced by bone.

💡Bone remodeling

Bone remodeling is the natural process of bone maintenance and repair, which involves the removal of old bone tissue and the deposition of new bone. The video emphasizes its importance in repairing micro fractures and reshaping bone in response to physical activity. It is highlighted as a vital life process that maintains bone health.

💡Mesenchyme

Mesenchyme is a type of embryonic tissue from which most connective tissues, including bones, are derived. In the video, it is described as the starting point for bone formation, highlighting its foundational role in the development of the skeletal system.

💡Ossification

Ossification is the process by which bone tissue is formed. The video distinguishes it from calcification, emphasizing that ossification involves the laying down of organic materials by osteoblasts, leading to bone formation. It is a key concept in understanding how bones develop and grow.

💡Calcification

Calcification is the process of tissue hardening, which is a part of ossification. The video clarifies that while calcification is a process within ossification, it is not synonymous with bone formation. It is an important step in the hardening of the extracellular matrix to form bone.

💡Intramembranous ossification

Intramembranous ossification is a simpler type of bone formation where bone directly replaces the mesenchymal tissue. The video uses this term to describe the formation of flat bones in the skull, emphasizing its role in the initial formation and the lifelong thickening and strengthening of bones.

💡Endochondral ossification

Endochondral ossification is a more complex process where bone formation occurs after an initial cartilage model is replaced by bone. The video explains that this process involves both interstitial and appositional growth, and it is the primary method for bone formation in most of the body's bones.

💡Fontanels

Fontanels, often referred to as 'soft spots,' are areas of the skull that have not yet ossified, allowing for flexibility during birth. The video mentions their importance in the birth process and their eventual ossification after birth, illustrating the process of intramembranous ossification.

💡Epiphyseal plate

The epiphyseal plate is a region of hyaline cartilage at the ends of long bones that allows for bone length growth. The video explains that this plate persists until skeletal maturity, after which bone length growth ceases, highlighting its role in the growth of long bones.

💡Osteoblasts and Osteoclasts

Osteoblasts are cells responsible for bone formation, while osteoclasts are involved in bone resorption. The video discusses the collaborative actions of these cells in the process of bone remodeling, emphasizing the balance needed for healthy bone maintenance.

Highlights

Introduction to bone formation, growth, and remodeling.

Emphasizing the importance of understanding learning objectives in depth.

Bone formation begins with the embryonic skeleton made of mesenchyme.

Differentiation between ossification and calcification processes.

Ossification occurs in four specific situations: initial bone formation, growth, remodeling, and repair.

Definition of intramembranous ossification where bone directly replaces mesenchyme.

Definition of endochondral ossification involving a cartilage model that is replaced by bone.

Intramembranous ossification is simpler and involves flat bones of the skull.

Fontanels, or soft spots, in the skull allow for easier birth and later ossify.

Intramembranous ossification's role in lifelong bone thickening and remodeling.

Endochondral ossification is more complex due to the cartilage model growth.

Most bones form through endochondral ossification, but this process stops after skeletal maturity.

Cartilage model development is initiated by chemical messages causing mesenchymal cells to form the future bone shape.

Growth of the cartilage model occurs both in length (interstitial) and width (appositional).

Bone eventually replaces the cartilage model, with the epiphyseal plate allowing for bone length growth.

Articular cartilage persists into adulthood and is crucial for joint function.

Bone remodeling is essential for repairing micro fractures and releasing minerals.

Collaboration between osteoblasts and osteoclasts is vital for healthy bone remodeling.

Imbalance between osteoblasts and osteoclasts can lead to bone health issues like osteoporosis.

Review question on the ossification process of the parietal bone of the skull.

Transcripts

hello anatomy colleagues this is Dr. Alsup and in  this video we have quite a bit to discuss a lot of  

really interesting things and and you can see  there's quite a few learning objectives here  

i want to talk big picture regarding  bone formation as well as bone growth  

and a little bit about bone remodeling  as always please make sure that you are  

taking the time to answer these learning  objectives in your own words after viewing  

this is such an excellent way to make sure you're  on track keep in mind these learning objectives  

can't really be answered in one word so take  that time it is really the best way to study

okay so all bone formation is going to begin  with what's referred to as the formation of an  

embryonic skeleton and this is going to be  composed of mesenchyme so it's not a true skeleton  

it's going to be composed of mesenchyme which is  that embryonic tissue that almost all connective  

tissues are derived from and since bone is a  type of connective tissue that makes sense that  

we're starting with mesenchyme generally this  formation of this mesenchymal skeleton begins  

around the sixth week in utero and from there  bone formation follows one or one of two patterns  

and these types of ossification patterns  are referred to as intramembranous and  

endochondral but before we get into the  description of these i want to distinguish  

between the terms ossification and calcification  because they're often used interchangeably  

albeit that's incorrect so ossification is the  process by which bone forms so that's what we're  

talking about here this is how bone will form  and this happens in four specific situations  

ossification will occur with that initial  formation of bone which is what we're focusing  

on right here ossification also is growth of a  bone during the juvenile and adolescent period  

it's also remodeling a bone and lastly it's  repair of bone which sometimes remodeling and repair  

are often discussed together now as part of bone  formation and ossification osteoblasts will lay  

down the organic materials of that extracellular  matrix which will initiate calcification  

and calcification is really just tissue hardening  so calcification is a process within ossification  

but it's not the same thing as ossification  so calcification is hardening ossification  

is bone formation so don't use those terms  interchangeably and i'll try not to do it myself  

okay so let's return to these two main types of  ossification the first one we're going to discuss  

is intramembranous and we're starting with  this one because it's the simpler of the two  

and in this process bone is going to  directly replace that mesenchymal skeleton  

all right so bone directly replaces that  mesenchymal skeleton whereas in endochondral  

ossification that mesenchymal skeleton  is going to be actually replaced by a  

cartilage model first so this hyaline cartilage  model grows first and then eventually it will be  

replaced by bone so there's that extra step in  endochondral in comparison to intramembranous  

now i don't want you to memorize these specific  steps so this is intramembranous here and this  

is endochondral here and it gives you kind of  these big picture steps by which this occurs  

but i do want you to at least understand these  basics in terms of intramembranous is through  

the mesenchymal skeleton and endochondral  has that extra step of the cartilage model

a minority of bones form through intramembranous  ossification initially and these include many flat  

bones of the skull so you can see we're looking at  a juvenile skull here you can see the frontal bone  

parietal bones here occipital back here see  these are all forming through intramembranous  

ossification and between these bones of the  skull you will have what's referred to as  

fontanels you can see a few here fontanels are  often colloquially referred to as soft spots  

these are super important in allowing that large  skull to exit the relatively small birth canal  

so allowing some flexibility here and after  birth these areas will undergo intramembranous 

ossification these fontanelles and harden  and be replaced entirely by bone so same kind  

of process as what you have the actual bones  here other bones are going to form through intra-  

membranous ossification they include the face many  of the facial bones the mandible so that lower jaw  

and the medial part of the clavicle often you hear  the clavicle referred to as the collarbone so a  

relatively few bones form initially through  intramembranous ossification but i want you  

to focus in on this box right here likely  intramembranous ossification's more important  

role is lifelong thickening, strengthening,  and remodeling of bones so it is the method  

for depositing new tissue on the bony surface  even past the age where bones can no longer  

grow in length so this is a really important  function here of intramembranous ossification

now as we mentioned earlier endochondral  ossification is more complex than  

intramembranous and this is because of  that these middle steps here where you have  

the cartilage model and not only do you  have a cartilage model but that cartilage  

model actually grows so it's going to  grow both in length as well as width  

and most bones of the body initially  form through endochondral ossification  

but this type of ossification process stops  after skeletal maturity so any other type  

of bone formation say that occurs during  adulthood on the in terms of bone width  

that's going to occur through intramembranous  rather than endochondral ossification

a few notes regarding the cartilage model in  this region where the bone so in the region  

say where the bone is going to form chemical  messages will actually cause those mesenchymal  

cells to crowd together and form this general  shape of the future bone and this is where you  

have that cartilage model development these  cells will differentiate into chondroblasts  

and so if you have -blast in a term you know it's  building something so specifically it's secreting  

cartilage extracellular matrix matrix components  so producing this cartilage model chondro- you  

need to be thinking cartilage specifically  here we're talking about hyaline cartilage  

and this cartilage model will  grow both through interstitial  

or growth from within which will result in  the cartilage model actually growing in length  

as well as appositional growth  which is going to be growth on the  

outer surface so the cartilage model  will actually grow in width as well

you can see through these steps bone will  eventually begin to replace the cartilage model  

first this will occur in the diaphyseal region  and then it will begin to occur in the epiphyseal  

region and you can see that you will have this  plate of cartilage that will persist and allow  

for bone length growth to occur  so this is that epiphyseal plate  

right here so in that metaphysis region eventually  the hyaline cartilage model will be completely  

replaced with bone except on the very ends of  bone where you will have articular cartilage  

persist into adulthood articular cartilage  is extremely important in terms of joints and  

we'll talk about that in much greater  detail when we get to those lectures

I do want to return to the terms interstitial  and appositional growth and make sure we clearly  

define those interstitial growth is growth from  within and here i want us to be thinking of those  

epiphyseal plates you can see generally  where that would be located here on this  

radiograph that region is that transition from  cartilage to bone it is here that you can have  

increases in bone length because of the elongation  really occurring as a result of cartilage growth  

so the cartilage will grow and be replaced by  bone the cartilage will grow be replaced by bone  

so it's really cartilage that's playing or it's  going to to grow in terms of interstitial growth  

eventually we'll have the complete replacement  of cartilage with bone and bone will not  

be capable of bone length growth  however with appositional growth  

or growth or deposition of new tissue to the  outer surface this can occur with both cartilage  

and bone so cartilage can enlarge by both  interstitial and appositional growth but in  

bone osteocytes are embedded within that  calcified extracellular matrix so it really  

has little room to deposit matrix internally  so bone growth can really only be appositional  

bony appositional growth occurs by  intramembranous ossification at the bone  

surface so those osteoblasts and the internal  layer of the periosteum they're going to deposit  

tissue on the bone surface that will calcify or  harden and become trapped as an osteocyte and  

eventually this will form those circumferential  lamellae that surround the entirety of the bone

lastly bone remodeling is a natural process that  is vital in repairing micro fractures, reshaping  

bone in response to use or disuse, and also in  releasing minerals into the blood when necessary  

healthy typical bone remodeling is built upon  the collaborative actions of the osteoblasts as  

well as the osteoclast so remember osteoblasts are  the bone builders osteoclasts are the bone resorp  

resorbers or bone resorption if one of these cells  outpaces the other this can have implications in  

bone health and mass and we will talk about an  example of one of these disorders or osteoporosis  

in an upcoming video but i do want to note  that typically remodeling is healthy it's a  

necessary life process it's just when you have an  imbalance here that things can start to go wrong  

all right excellent thanks for sticking with  me through a denser topic here let's do a quick  

question review before moving on so the question  here the parietal bone of the skull so it's one  

of those flat bones if you put your hands on the  top of your head you're touching or the parietal  

bones are underneath the skin there so those  probable bones initially form through which  

type of ossification process is it endochondral or  intramembranous pause the video if needed and when  

ready what did you think what was your answer well  the parietal bone is one of the few bones that is  

going to form through intramembranous ossification  recall the flat bones of the skull most of the  

facial bones the mandible or the lower jaw and a  portion of the clavicle or the collarbone is going  

to form via intramembranous  ossification all other bones of the body  

initially form via endochondral ossification with  that cartilage model first all right excellent  

thank you so much for your time and attention  here and i will see you in the next video