Difference Between Endochondral Ossification and Intramembranous Ossification

The main difference between endochondral ossification and intramembranous ossification is that the endochondral ossification is the method of forming a bone through a cartilage intermediate while the intramembranous ossification directly forms the bone on the mesenchyme. Furthermore, endochondral ossification is involved in the formation of long bones while intramembranous ossification is involved in the formation of flat bones.

Endochondral ossification and intramembranous ossification are the two methods of bone formation. The process of the formation of bones is known as ossification or osteogenesis. Osteoblasts are the cells involved in the bone formation.

Key Areas Covered

1. What is Endochondral Ossification
     – Definition, Types of Bones, Steps
2. What is Intramembranous Ossification
     – Definition, Types of Bones, Steps
3. What are the Similarities Between Endochondral Ossification and Intramembranous Ossification
     – Outline of Common Features
4. What is the Difference Between Endochondral Ossification and Intramembranous Ossification
     – Comparison of Key Differences

Key Terms

Endochondral Ossification, Flat Bones, Intramembranous Ossification, Long Bones

What is Endochondral Ossification

Endochondral ossification is a type of ossification that proceeds through the formation of intermediate cartilage. Generally, this intermediate cartilage is hyaline cartilage. Here, the cartilage only serves as a template. Endochondral ossification is involved in the formation of long bones as well as the bones at the base of the skull.

Figure 1: Endochondral Ossification

Endochondral Ossification of Long Bones – Steps

1. Around 6-8 weeks after conception, mesenchymal cells differentiate into chondrocytes, which form the cartilaginous bone precursor. Perichondrium, which is the envelope of the cartilage appears soon after the formation of the cartilage.
2. The matrix of the cartilage calcifies. This results in the death of chondrocytes and blood vessels invade through the forming spaces called lacuna.
3. The osteogenic cells also migrate into the spaces and become osteoblasts.
4. Penetration of the growing cartilage by blood capillaries initiates the transformation of perichondrium into the bone-producing periosteum.
5. In the compact bones, osteoblasts form a periosteal collar/bone collar around the shaft of the log bone called the diaphysis.
6. Within the second or third month of the fetal life, ossification ramps up, creating the primary ossification center deep in the periosteal collar where ossification begins.
7. In the meanwhile, chondrocytes grow the cartilage at the two ends of the bone, increasing the length.
8. When the skeleton fully forms, the cartilage can be found between the diaphysis and epiphysis as the epiphyseal plate and at the joint surface as articular cartilage.
9. After birth, a secondary ossification center forms at the epiphyseal plate, which helps the longitudinal growth of bone.

What is Intramembranous Ossification

Intramembranous ossification is the type of ossification in which the compact and spongy bones directly develop on a sheet of mesenchyme. The formation of flat bones in the face, skull and the clavicle occur through intramembranous ossification.  

Figure 2: Intramembranous Ossification

Intramembranous Ossification of Flat Bones – Steps

1. The mesenchyme in the embryonic skeleton differentiates into capillaries and osteoblasts. Osteoblasts appear in a cluster called an ossification center.
2. Osteoblasts secrete osteoid, which is an unclacified matrix, which calcifies later. The osteoblasts trapped in the calcified matrix become osteocytes.
3. The osteoid secreted around blood vessels become trabecular matrix called woven bone. Periosteum is the condensed mesenchyme around the woven bone. It forms a protective layer around the compact bone.
4. Trabecular matrix thickens and is replaced later with mature lamellar bone, forming the compact bone plates.
5. Spongy bone consists of distinct trabecular matrices, and its vascular tissue becomes red marrow.

Similarities Between Endochondral Ossification and Intramembranous Ossification

• Endochondral ossification and intramembranous ossification are the two methods of ossification/osteogenesis.
• Osteoblasts help in the synthesis of bones in both processes.
• Both processes are essential in the healing of bone fractures.

Difference Between Endochondral Ossification and Intramembranous Ossification

Definition

Endochondral ossification refers to a type of ossification that takes place from the centers arising in the cartilage and involving the deposition of lime salts in the cartilage matrix followed by secondary absorption and replacement by the true bony tissue. The intramembranous ossification refers to the development of osseous tissue within the mesenchymal tissue without prior cartilage formation.

Method

In endochondral ossification,  a cartilage forms first and the bone is laid down on it while in intramembranous ossification, the bone directly forms on mesenchyme.

Intermediate Cartilage

Endochondral ossification proceeds through intermediate cartilage while the intramembranous ossification does not form intermediate cartilage.

Importance

Endochondral ossification is important in the formation of long bones while intramembranous ossification is important in the formation of flat bones.

Time

Endochondral ossification takes a longer time to form a bone while intramembranous ossification takes less time to form a bone.

Stopping Point

Endochondral ossification stops at year two while intramembranous ossification stops at year 18 in females and 21 in males.

Conclusion

Endochondral ossification is the method of forming a bone through a cartilage intermediate while intramembranous ossification directly forms the bone on the mesenchyme. Endochondral ossification involves in the formation of long bones while intramembranous ossification involves in the formation of flat bones. The main difference between endochondral ossification and intramembranous ossification is the method of bone formation and types of bones formed.

Reference:

1. “38 6.4 Bone Formation and Development.” Anatomy and Physiology, BC Open Textbooks, 6 Mar. 2013, Available Here

Image Courtesy:

1. “608 Endochrondal Ossification” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia  
2. “611 Intramembraneous Ossification” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia