Bone develops from fibrous membranes and cartilage. Which of the following is the term referring to the development of bone from cartilage?
A. Endochondral ossification
B. Intramembranous ossification
C. Periosteal ossification
D. Remodeling
The correct answer and explanation is :
The correct answer is A. Endochondral ossification.
Explanation:
Endochondral ossification refers to the process of bone development that occurs from a cartilage model. It is one of the two primary processes by which bones form, the other being intramembranous ossification. Endochondral ossification is crucial during the formation of long bones, such as the femur, tibia, and humerus, and is responsible for the growth of bones in length.
Here’s a step-by-step breakdown of endochondral ossification:
- Formation of cartilage model: Initially, during fetal development, a cartilage model of the bone is formed. This cartilage is hyaline cartilage, and it serves as a template for bone formation. The model resembles the shape of the future bone, with a central region called the primary ossification center.
- Bone Collar Formation: As the cartilage continues to grow, a bone collar made of compact bone forms around the diaphysis (shaft) of the cartilage model. This bone collar helps to stabilize the structure while the cartilage begins to break down inside.
- Cartilage Calcification and Breakdown: The chondrocytes (cartilage cells) in the center of the model hypertrophy (increase in size), and the surrounding cartilage matrix starts to calcify. This calcification cuts off nutrients to the chondrocytes, causing them to die. This results in the formation of cavities in the cartilage.
- Invasion by Blood Vessels: Blood vessels invade the calcified cartilage, bringing in osteoblasts (bone-forming cells). These osteoblasts begin to replace the cartilage with bone, starting at the primary ossification center.
- Formation of Medullary Cavity: As the ossification spreads, the cartilage in the central region is replaced by spongy bone, and a medullary (bone marrow) cavity begins to form.
- Secondary Ossification: In long bones, secondary ossification centers appear in the epiphyses (ends of the bone), leading to further bone formation. However, some cartilage remains at the epiphyseal plates (growth plates), allowing the bones to continue growing in length until early adulthood when the growth plates eventually close.
Why the Other Choices Are Incorrect:
- B. Intramembranous ossification: This is a process where bone develops directly from mesenchymal (embryonic connective tissue) membranes. It is typical in the formation of flat bones like the skull and clavicle.
- C. Periosteal ossification: This term refers to the process of bone formation along the periosteum (outer membrane of bone) but is not a specific type of ossification.
- D. Remodeling: Bone remodeling is the ongoing process where old bone tissue is replaced by new bone tissue throughout life, which helps maintain bone strength and mineral balance. It’s not directly related to the initial formation of bone.
Thus, endochondral ossification is the correct term for the development of bone from cartilage.