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Intramembranous and Endochondral Ossification

Heather Eighme, Laura Enzor
  • Author
    Heather Eighme

    Heather has taught high school science and college credit courses for over 6 years in biological sciences. They have a Masters in education from Sacred Heart University and Bachelors in Biology from the University of Connecticut. They also have teacher certifications in Biology and General Science.

  • Instructor
    Laura Enzor

    Laura has a Master's degree in Biology and is working on her PhD in Biology. She specializes in teaching Human Physiology at USC.

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

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

A fetal skeleton has 275 bones, while a adult skeleton has only 206 due to bone fusion.
Fetal and Adult Skeleton

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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  • 0:05 Two Processes of Bone…
  • 0:56 Intramembranous Ossification
  • 1:56 Endochondral Ossification
  • 2:51 How Ossification Works
  • 4:07 Bone Growth After Birth
  • 5:02 Lesson Summary
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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:

  1. Mesenchymal cells bunch together in the embryonic stage into mesenchymal connective tissue.
  2. The tissue differentiates into osteoblasts which are specialized for bone growth.
  3. 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.
  4. Osteoblasts release what is called osteoid - a mix of collagen, proteins, and uncalcified matrix.
  5. Mineral salts are deposited, which hardens or calcifies the area.
  6. The osteoblasts become trapped in the calcified area, becoming osteocytes.
  7. Surrounding cells continue steps 1-7, adding calcified tissue allowing for bone growth.
  8. 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:

  1. In the embryo some mesenchymal cells differentiate into chondroblasts or cartilage cells.
  2. The area of cartilage is known as a matrix which is composed of hyaluronic acid, collagen fibers, chondroitin sulfate and water.
  3. The matrix surrounds the chondroblasts producing chondrocytes.
  4. More matrix is produced which allows the cartilage to grow.
  5. Osteoblasts deposit bone rings around the diaphysis.
  6. 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.
  7. Osteoblasts will continue to grow in the primary ossification center, the center of the bone.
  8. Chondrocytes continue to grow at the ends of the bone, epiphysis, this is known as the secondary ossification center.
  9. A small line of cartilage remains here to allow for growth until adulthood, this is called the epiphyseal plate or growth plate.
  10. The ends of the long bones remain cartilage for cushion between joints.


Parts of the long bone including the primary and secondary ossification centers

Parts of the long bone grown during endochondral bone formation, including primary and secondary ossification centers


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.

Mesenchymal cells are cells that have not been differentiated yet.
Mesenchyme Cells

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.

The periosteum is formed after the calcification of cartilage.
Periosteum

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Video Transcript

Two Processes of Bone Formation

A fetal skeleton has 275 bones, while a adult skeleton has only 206 due to bone fusion.
Fetal and Adult Skeleton

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.

Mesenchymal cells are cells that have not been differentiated yet.
Mesenchyme Cells

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.

The periosteum is formed after the calcification of cartilage.
Periosteum

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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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