Learn about intramembranous ossification vs. endochondral ossification. Study the process of bone formation and growth, and examine how bones are repaired.
Updated: 03/11/2022
Intramembranous and Endochondral Ossification
What Does Ossification Mean?
Ossification is the process of bone formation. This can begin as early as a few weeks after conception. The process can be broken down into two types of bone growth: intramembranous ossification and endochondral ossification. These types of ossification will be discussed in more detail throughout the lesson. During the bone growth process, there are important types of cells involved: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are bone cells that build bone and are essential when bones are growing. When osteoblasts mature they are known as osteocytes. Osteoclasts are bone cells that break down and remodel bone. These bone cells as well as others will be discussed in more detail throughout the lesson.
Two Processes of Bone Formation
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We obviously have a lot of growing to do after we're born. Not only do our bones need to get longer and thicker as we grow, but an adult skeleton is very different from a fetal or infant skeleton. A fetal skeleton has about 275 bones and a fairly high percentage of cartilage (this helps the skeleton to be somewhat flexible). Compare that to an adult skeleton, which has 206 bones and a much smaller percentage of cartilage. We fuse several bones together as we age, which accounts for the decrease in bone number.
There are two main processes that occur during fetal development that contribute to our bone formation. These are intramembranous ossification and endochondral ossification. The term 'ossification' refers to the process of forming bone. There are a couple of things that distinguish these two processes from one another.
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Intramembranous Ossification vs. Endochondral Ossification
There are two types of bone growth: intramembranous and endochondral ossification. Intramembranous ossification is the formation of bones specifically in the skull as well as the clavicles and mandible, whereas endochondral ossification is the formation of all other bones including the long, short and the ends of the irregular bones. The process of ossification has many differences which will be discussed in the next sections of the lesson.
Intramembranous Bone Growth
The first type of bone growth is intramembranous ossification, which occurs in fetal development specifically in the skull bones. When a baby is born this area between bones is not ossified. This is to help with the birth of a baby through the birth canal. The growth and development of these bones will continue into adulthood. An important component of intramembranous bone development is mesenchymal cells. Mesenchymal cells are groups of stem cells that do not have a specific function yet. This means they can become any cell depending on the location. In intramembranous bone growth the mesenchymal cells become osteoblasts. Below is the process of how these cells work in order to ossify tissue.
Intramembranous ossification occurs in these steps:
- Mesenchymal cells bunch together in the embryonic stage into mesenchymal connective tissue.
- The tissue differentiates into osteoblasts which are specialized for bone growth.
- Osteoblasts gather in an area called the primary ossification center or the area where the bone begins to ossify. In embryos, this is in the center of the bone or the diaphysis.
- Osteoblasts release what is called osteoid - a mix of collagen, proteins, and uncalcified matrix.
- Mineral salts are deposited, which hardens or calcifies the area.
- The osteoblasts become trapped in the calcified area, becoming osteocytes.
- Surrounding cells continue steps 1-7, adding calcified tissue allowing for bone growth.
- Osteoclasts are constantly remodeling the bone during the process.
Endochondral Bone Formation
The second type of bone growth is endochondral bone formation, which occurs during adolescence in the long bones. Endochondral bone formation is different from that of intramembranous because it replaces areas of cartilage with bone tissue. This is not to confuse the cartilage turning into bone, but to serve as a placeholder for the bone. Endochondral bone growth also takes a longer amount of time to complete, which can be seen in the formation process below.
Endochondral ossification occurs in these steps:
- In the embryo some mesenchymal cells differentiate into chondroblasts or cartilage cells.
- The area of cartilage is known as a matrix which is composed of hyaluronic acid, collagen fibers, chondroitin sulfate and water.
- The matrix surrounds the chondroblasts producing chondrocytes.
- More matrix is produced which allows the cartilage to grow.
- Osteoblasts deposit bone rings around the diaphysis.
- Nutrients cannot be taken up by the chondrocytes due to the bone growth so they die and the center remains hollow, called the medullary cavity.
- Osteoblasts will continue to grow in the primary ossification center, the center of the bone.
- Chondrocytes continue to grow at the ends of the bone, epiphysis, this is known as the secondary ossification center.
- A small line of cartilage remains here to allow for growth until adulthood, this is called the epiphyseal plate or growth plate.
- The ends of the long bones remain cartilage for cushion between joints.
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Process of Bone Formation and Growth
Bone formation and growth occurs until a person has reached adulthood. The process begins by the release of growth hormones in the pituitary gland. This will stimulate the production of osteoblasts. As stated in endochondral ossification, after birth most cartilage will be replaced by bone. This occurs first in the primary ossification center. The primary center focuses on appositional bone growth or the addition to the thickness of the bone. The center of the bone will continue to grow and develop throughout life. Growth can be triggered by stress or weight gain. The ossification will occur in the epiphysis of the bones or the secondary ossification center. The epiphyseal plate will create more chondrocytes which will eventually be replaced by osteoblasts to form bone. The addition of the bone material to the epiphyses will elongate the bone in a process called elongation. This will continue until growth hormones have ceased. Once growth is complete, the epiphyseal plate will also change the bone. When observing an x-ray of an adolescent, it can be determined if they are still growing based on the epiphyseal plate. If the plate can be seen as in the diagram displaying the child's foot, bone growth is still occurring. If the plate cannot be seen, bone growth is complete.
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Bone Formation Issues and Repair
When a bone is broken, the bone is immobilized with a cast or splint to allow the bone to heal properly on its own. Bones have a series of steps in order to heal properly. Immediately after a break, blood vessels begin to clot forming a hematoma. The hematoma helps to keep the bone in place and seals the broken ends. Capillaries will grow into the hematoma allowing for the process of new bone formation. White blood cells called phagocytes clear away the dead cells while fibroblasts and osteoblasts begin bone formation. Fibroblasts are cells that create collagen fibers similar to chondrocytes in endochondral bone formation. These fibers will hold the broken bones together. Osteoblasts will create spongy bone. After a few months the spongy bone and cartilage will be ossified with the assistance of osteoblasts and bone matrix. Osteoclasts will continue to modify and remodel the bone. Although osteoclasts are helpful in the bone remodeling process they can be related to bone issues.
There are many bone disorders that can be affected by flaws in bone cells. An example of this is osteoporosis or brittle bone disease. A major cause of this is due to low levels of calcium in the body. The body compensates by taking calcium from the bones for the body's utilization. Osteoclasts will break down the bone creating a bone that is porous and brittle. This can lead to bone fractures and other complications. Brittle bones can also be caused by deficiencies of vitamins such as vitamin C and D. This can lead to disorders such as scurvy or rickets where there is improper bone development. Vitamins are vital to bone health and missing vitamin C can lead to Scurvy. Symptoms of scurvy that are related to bone health include brittle bones and teeth loss. Rickets is another disorder which occurs in children who do not have proper nutrition. This can lead to improper bone growth where the bones are malleable or can bend. Children have bow legs or curved bones due to the inability to build bone correctly. Proper bone formation is essential to avoid complications and disorders of the bones.
Lesson Summary
Ossification is the creation of bone that is either classified as intramembranous or endochondral ossification. Intramembranous ossification occurs in the early embryo to create flat bones such as the skull and clavicle. Endochondral ossification forms ossified tissue to replace areas of cartilage. This occurs in the long bones as well as other irregular bones. Mesenchymal cells or connective tissue are stem cells that are an important factor in the bone growth process. These cells differentiate into other cells such as the osteoblasts. Bone growth begins in the primary ossification center which is in the center of the bone or the diaphysis. The process of adding bone to the width is known as appositional bone growth. The secondary ossification center is at the ends of the bone or epiphysis adding to the length of the bones. The process of adding bone to epiphysis is called elongation. Elongation occurs at the epiphyseal plate or the growth plate. Cartilage is added to this area and eventually will be replaced by ossified bone in a similar process to endochondral bone growth. The epiphyseal plate is only visual during adolescence when the bones are elongating. If an xray shows the epiphyseal plate to be sealed, bone growth is complete.
Intramembranous Ossification
Intramembranous ossification occurs primarily during the initial formation of the flat bones of our skull. This process is also responsible for forming our jaw and clavicles, or collar bones. Intramembranous ossification also helps with healing bone fractures. The bone is formed from a specific type of connective tissue called mesenchyme connective tissue.
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Mesenchyme connective tissue is made up of mesenchymal, or stem, cells. These cells are interesting because they haven't differentiated yet. This means that it hasn't been determined what this cell is specifically going to be. It could be a bone cell, cartilage cell, muscle cell, or even a fat cell! When you were little, how many times did you change your answer when asked what you wanted to be when you grew up? You could have said an astronaut, a doctor, or a fireman; anything was possible. This is how a mesenchymal cell works. It's going to be something, but that specific something hasn't been determined yet.
Endochondral Ossification
Endochondral ossification is essential for the formation of long bones (bones that are longer than they are wide, such as the femur, or thigh, bone and the humerus - the bone in your upper arm), as well as short bones (bones that are shorter than they are wide, such as the carpals and tarsals that make up your wrist and ankle). This process also forms the ends of flat and irregular bones (flat bones are flat, such as your ribs, and irregular bones are irregularly shaped, such as your vertebrae). Endochondral ossification is also part of the process that lengthens long bones, as well as the natural healing of small bone fractures.
Like intramembranous ossification, endochondral ossification starts with mesenchymal cells. However, the primary way endochondral ossification is distinguished from intramembranous ossification is the fact that cartilage is present during endochondral ossification.
How Ossification Works
Intramembranous ossification starts by developing the ossification center, or the point where bone formation starts to occur. The next step is calcification, or the accumulation of calcium to help form bone tissue. This step continues until the bones of our skull are formed. This is similar to snow accumulation. You start with a single snowflake and keep adding more and more until you end up with a few feet of snow! Finally, the periosteum is formed, which is the outside lining of all bones.
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Endochondral ossification is a bit more complicated; not surprising, considering this is how the majority of our bones are formed! Remember, this process requires the presence of cartilage, specifically hyaline cartilage, which is the same type of cartilage found in your nose. So, the first step in this process is to develop a cartilage model. This cartilage model grows in length by cellular division of chondrocytes, the living cells of hyaline cartilage. This growth continues until the primary ossification center develops. The primary ossification center is responsible for developing the diaphysis, or middle section, of our long and short bones. After primary ossification, the periosteum forms.
Bone Growth After Birth
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Right about the time we're born, a secondary ossification center appears at each end (or epiphysis) of our bones. The cartilage in between the primary and secondary ossification centers is called the epiphyseal plate, or growth plate. These plates (one on each end), continue to make new cartilage, which is replaced by new bone. As this new bone forms, the bone is lengthened, which means we grow! This process is referred to as bone elongation. Once we're 'fully grown' (which doesn't happen until our mid-twenties), this plate is replaced by bone, and our bones no longer lengthen. However, despite the fact that we stop elongating our bones at a certain point, we are constantly strengthening and repairing our bones throughout our lifetime. Endochondral and intramembranous ossification are both essential to this process.
Lesson Summary
There are two processes that form our bones before we are born: intramembranous ossification and endochondral ossification. Intramembranous ossification is primarily responsible for forming the bones in our skull, and bones are formed from a specific type of connective tissue, called mesenchymal connective tissue. Endochondral ossification not only forms our long bones, short bones, and the ends of irregular and flat bones, but is also responsible for bone elongation after we're born. This is accomplished by a cartilage growth plate, known as the epiphyseal plate.
Learning Outcomes
After this lesson, you'll be able to:
- Understand why adults have fewer bones than infants
- Compare and contrast intramembranous ossification and endochondral ossification, and name some bones that each is responsible for forming
- Summarize what mesenchymal cells are
- Explain the steps involved in each type of ossification
- Define ossification center, calcification, periosteum, and diaphysis
- Describe the process of bone elongation, noting which type of ossification this process uses, and explain how secondary ossification centers and epiphyseal plates are essential to this process
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Video Transcript
Two Processes of Bone Formation
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We obviously have a lot of growing to do after we're born. Not only do our bones need to get longer and thicker as we grow, but an adult skeleton is very different from a fetal or infant skeleton. A fetal skeleton has about 275 bones and a fairly high percentage of cartilage (this helps the skeleton to be somewhat flexible). Compare that to an adult skeleton, which has 206 bones and a much smaller percentage of cartilage. We fuse several bones together as we age, which accounts for the decrease in bone number.
There are two main processes that occur during fetal development that contribute to our bone formation. These are intramembranous ossification and endochondral ossification. The term 'ossification' refers to the process of forming bone. There are a couple of things that distinguish these two processes from one another.
Intramembranous Ossification
Intramembranous ossification occurs primarily during the initial formation of the flat bones of our skull. This process is also responsible for forming our jaw and clavicles, or collar bones. Intramembranous ossification also helps with healing bone fractures. The bone is formed from a specific type of connective tissue called mesenchyme connective tissue.
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Mesenchyme connective tissue is made up of mesenchymal, or stem, cells. These cells are interesting because they haven't differentiated yet. This means that it hasn't been determined what this cell is specifically going to be. It could be a bone cell, cartilage cell, muscle cell, or even a fat cell! When you were little, how many times did you change your answer when asked what you wanted to be when you grew up? You could have said an astronaut, a doctor, or a fireman; anything was possible. This is how a mesenchymal cell works. It's going to be something, but that specific something hasn't been determined yet.
Endochondral Ossification
Endochondral ossification is essential for the formation of long bones (bones that are longer than they are wide, such as the femur, or thigh, bone and the humerus - the bone in your upper arm), as well as short bones (bones that are shorter than they are wide, such as the carpals and tarsals that make up your wrist and ankle). This process also forms the ends of flat and irregular bones (flat bones are flat, such as your ribs, and irregular bones are irregularly shaped, such as your vertebrae). Endochondral ossification is also part of the process that lengthens long bones, as well as the natural healing of small bone fractures.
Like intramembranous ossification, endochondral ossification starts with mesenchymal cells. However, the primary way endochondral ossification is distinguished from intramembranous ossification is the fact that cartilage is present during endochondral ossification.
How Ossification Works
Intramembranous ossification starts by developing the ossification center, or the point where bone formation starts to occur. The next step is calcification, or the accumulation of calcium to help form bone tissue. This step continues until the bones of our skull are formed. This is similar to snow accumulation. You start with a single snowflake and keep adding more and more until you end up with a few feet of snow! Finally, the periosteum is formed, which is the outside lining of all bones.
|
Endochondral ossification is a bit more complicated; not surprising, considering this is how the majority of our bones are formed! Remember, this process requires the presence of cartilage, specifically hyaline cartilage, which is the same type of cartilage found in your nose. So, the first step in this process is to develop a cartilage model. This cartilage model grows in length by cellular division of chondrocytes, the living cells of hyaline cartilage. This growth continues until the primary ossification center develops. The primary ossification center is responsible for developing the diaphysis, or middle section, of our long and short bones. After primary ossification, the periosteum forms.
Bone Growth After Birth
|
Right about the time we're born, a secondary ossification center appears at each end (or epiphysis) of our bones. The cartilage in between the primary and secondary ossification centers is called the epiphyseal plate, or growth plate. These plates (one on each end), continue to make new cartilage, which is replaced by new bone. As this new bone forms, the bone is lengthened, which means we grow! This process is referred to as bone elongation. Once we're 'fully grown' (which doesn't happen until our mid-twenties), this plate is replaced by bone, and our bones no longer lengthen. However, despite the fact that we stop elongating our bones at a certain point, we are constantly strengthening and repairing our bones throughout our lifetime. Endochondral and intramembranous ossification are both essential to this process.
Lesson Summary
There are two processes that form our bones before we are born: intramembranous ossification and endochondral ossification. Intramembranous ossification is primarily responsible for forming the bones in our skull, and bones are formed from a specific type of connective tissue, called mesenchymal connective tissue. Endochondral ossification not only forms our long bones, short bones, and the ends of irregular and flat bones, but is also responsible for bone elongation after we're born. This is accomplished by a cartilage growth plate, known as the epiphyseal plate.
Learning Outcomes
After this lesson, you'll be able to:
- Understand why adults have fewer bones than infants
- Compare and contrast intramembranous ossification and endochondral ossification, and name some bones that each is responsible for forming
- Summarize what mesenchymal cells are
- Explain the steps involved in each type of ossification
- Define ossification center, calcification, periosteum, and diaphysis
- Describe the process of bone elongation, noting which type of ossification this process uses, and explain how secondary ossification centers and epiphyseal plates are essential to this process
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Create your account
Frequently Asked Questions
What are the steps in endochondral bone formation?
The steps in endochondral bone formation are as follows:
1. Mesenchymal cells differentiate into chondroblasts or cartilage cells
2. Bone matrix surrounds chondroblasts creating chondrocytes
3. Cartilage grows while osteoblasts deposit bone at the diaphysis
4. Chondrocytes die leaving the center of long bones hallow
5. Bone forms in the primary ossification center
6. Chondrocytes add to the epiphysis elongating the bone at the secondary ossification center
7. The epiphyseal plate continues to elongate the bone until adulthood
What is the difference between intramembranous ossification and endochondral ossification?
Intramembranous ossification occurs in the growing embryo to form bones from osteoblasts. Endochondral ossification is areas of cartilage, chondrocytes, that are replaced by bone in adolescence, which occurs specifically in long bones.
What bones form by intramembranous ossification?
Intramembranous ossification form flat bones in the growing embryo. Flat bones include the bones of the skull, clavicle and mandible.
What bones are formed by endochondral bone formation?
Endochondral bone formation creates all the long bones in the body. The epiphyseal plate adds cartilage which later becomes bone tissue elongating the bones.
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BackBone Formation Process & Growth Types | Intramembranous & Endochondral Ossification
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