Ossification & Growth of Bones: Types, Process & Development
Ossification, also called osteogenesis, is the process of bone formation. Bone formation begins during embryonic development and continues during skeletal growth.
There are two main types of ossification: intramembranous ossification and endochondral ossification. Both ultimately produce bone, but they differ in the tissue from which bone develops.
Interactive Bone Formation Guide
Click the buttons to view the major stages and structures involved in ossification and bone growth.
What Is Ossification?
Ossification is the process through which new bone tissue is formed. It is an essential part of skeletal development, growth and repair.
The cells primarily responsible for new bone formation are osteoblasts. Osteoblasts produce the organic bone matrix, called osteoid, which subsequently becomes mineralized.
Some osteoblasts become surrounded by the matrix they produce and develop into mature bone cells called osteocytes.
Types of Ossification
| Feature | Intramembranous Ossification | Endochondral Ossification |
|---|---|---|
| Basic process | Bone develops directly from mesenchymal tissue. | Bone replaces a pre-existing hyaline cartilage model. |
| Cartilage model | Absent | Present |
| Typical examples | Many flat bones of the skull and much of the clavicle. | Long bones and many other bones of the skeleton. |
| Main early tissue | Mesenchymal connective tissue | Hyaline cartilage |
Intramembranous Ossification
In intramembranous ossification, bone develops directly within mesenchymal connective tissue. No cartilage model is formed first.
Main Steps
- Mesenchymal cells condense at the site where bone will develop.
- Some mesenchymal cells differentiate into osteoblasts.
- Osteoblasts begin producing osteoid.
- The osteoid becomes mineralized.
- Some osteoblasts become trapped within the matrix and become osteocytes.
- Bone spicules and trabeculae develop around blood vessels.
- The developing bone becomes organized and remodeled.
- The surrounding connective tissue develops into the periosteum.
Examples
- Many flat bones of the skull
- Parts of the facial skeleton
- Much of the clavicle
Endochondral Ossification
In endochondral ossification, bone develops by replacing a pre-existing hyaline cartilage model.
This is the principal process involved in the formation of long bones.
The cartilage does not directly transform into bone. Instead, the cartilage model is progressively modified and replaced by newly formed bone.
Main Steps
- A hyaline cartilage model develops from mesenchymal tissue.
- The cartilage model grows in length and width.
- A bone collar develops around the shaft.
- A primary ossification center develops in the diaphysis.
- Chondrocytes enlarge and the surrounding cartilage matrix becomes calcified.
- The calcified cartilage provides a temporary framework for bone deposition.
- Blood vessels and osteogenic cells enter the developing region.
- Osteoblasts deposit bone matrix.
- The medullary cavity develops and expands as bone is remodeled.
- Secondary ossification centers develop in the epiphyses.
Primary Ossification Center
The primary ossification center is the first major center of bone formation in a typical long bone.
It develops in the diaphysis, or shaft. Bone formation progressively replaces the cartilage in the shaft while the bone continues to grow.
Secondary Ossification Centers
Secondary ossification centers develop in the epiphyses of long bones.
Their appearance varies among different bones and occurs at characteristic stages of development.
After ossification of the epiphyses, cartilage remains at the articular surfaces and, during skeletal growth, at the epiphyseal plate.
Growth of Bones
Bone growth occurs in two important dimensions: growth in length and growth in thickness.
Growth in Length
Long bones increase in length through activity at the epiphyseal plate, also called the growth plate.
The growth plate is a region of hyaline cartilage located between the epiphysis and metaphysis of a growing long bone.
Zones of the Epiphyseal Plate
| Zone | Main Feature |
|---|---|
| Resting zone | Contains relatively small chondrocytes and provides an attachment between the growth plate and epiphysis. |
| Proliferative zone | Chondrocytes divide and form columns. |
| Hypertrophic zone | Chondrocytes enlarge. |
| Calcification zone | Cartilage matrix becomes mineralized and chondrocytes die. |
| Ossification zone | Osteoblasts deposit bone on the remaining cartilage framework. |
How Does Longitudinal Growth Occur?
New cartilage is produced on the epiphyseal side of the growth plate. The newly formed cartilage then undergoes maturation, calcification and replacement by bone toward the metaphyseal side.
As this process continues, the bone increases in length.
Therefore, longitudinal growth depends on the coordinated activity of chondrocytes in the growth plate and osteoblasts in the adjacent bone-forming region.
Closure of the Epiphyseal Plate
When longitudinal growth is complete, the epiphyseal plate is replaced by bone.
The remaining bony structure is called the epiphyseal line.
Epiphyseal plate = active growth plate.
Epiphyseal line = remnant after growth plate closure.
Growth in Thickness
Bones also increase in diameter through appositional growth.
Osteoblasts in the periosteum add new bone to the outer surface. At the same time, osteoclast activity on internal surfaces helps enlarge the medullary cavity.
This coordinated process allows the bone to become wider while maintaining an appropriate internal structure.
Bone Modeling and Remodeling
Bone Modeling
Modeling changes the size and shape of bone during growth. Bone formation and resorption can occur at different surfaces, allowing the developing skeleton to adapt its shape.
Bone Remodeling
Remodeling is the continuous replacement of old bone with new bone. It continues throughout life.
- Osteoblasts form new bone.
- Osteoclasts resorb bone.
- Osteocytes maintain and regulate the surrounding bone matrix.
Clinical Importance
- Growth plate injury: Damage to the epiphyseal plate may interfere with normal growth of a long bone.
- Rickets: Defective mineralization of growing bone can affect the growth plate and produce skeletal deformities.
- Osteomalacia: Defective mineralization of newly formed bone matrix occurs in adults.
- Fracture repair: Bone healing involves both intramembranous and endochondral bone formation during different stages of repair.
- Osteoporosis: An imbalance between bone resorption and formation can contribute to reduced bone mass and increased fracture risk.
High-Yield MBBS Points
- Ossification is also called osteogenesis.
- There are two major types: intramembranous and endochondral.
- Intramembranous ossification occurs directly within mesenchymal tissue.
- Endochondral ossification occurs by replacement of a hyaline cartilage model.
- The primary ossification center develops in the diaphysis of a typical long bone.
- Secondary ossification centers develop in the epiphyses.
- Longitudinal growth occurs at the epiphyseal plate.
- Growth in diameter occurs by appositional growth.
- Osteoblasts form bone, while osteoclasts resorb bone.
- After growth plate closure, the epiphyseal plate becomes the epiphyseal line.
- Bone remodeling continues throughout adult life.
Summary
Bone formation occurs through two major mechanisms. In intramembranous ossification, bone develops directly from mesenchymal tissue. In endochondral ossification, bone develops by replacing a pre-existing hyaline cartilage model.
In a typical long bone, the primary ossification center develops in the diaphysis and secondary ossification centers develop in the epiphyses. Longitudinal growth occurs at the epiphyseal plate, while increase in diameter occurs through appositional growth.
After skeletal growth is complete, bone continues to undergo remodeling. This ongoing process helps maintain the structure and mechanical integrity of the skeleton.
Image Credits & Licenses
1. Intramembranous Ossification:
OpenStax College,
611 Intramembraneous Ossification.jpg.
Licensed under CC BY 3.0.
Wikimedia Commons
2. Endochondral Ossification:
OpenStax College,
608 Endochrondal Ossification.jpg.
Licensed under CC BY 3.0.
Wikimedia Commons
3. Longitudinal Bone Growth:
OpenStax College,
622 Longitudinal Bone Growth.jpg.
Licensed under CC BY 3.0.
Wikimedia Commons
4. Epiphyseal Growth Plate:
Veronika Šromová, Dinara Sobola and Pavel Kaspar,
Epiphyseal growth plate.jpg.
Licensed under CC BY 4.0.
Wikimedia Commons