Endochondral ossification

Endochondral ossification is a critical process in the development of the skeletal system of vertebrates, whereby cartilage is transformed into bone. It is one of the two methods by which bone is formed, the other being intramembranous ossification. Endochondral ossification is primarily responsible for the formation of the long bones of the body, as well as the base of the skull and other complex structures.

Process

The process of endochondral ossification begins with the development of a cartilage model that approximates the shape of the future bone. This model grows in size by cell division of chondrocytes (cartilage cells) and the secretion of extracellular matrix. The next steps involve the calcification of the cartilage matrix and the invasion of the calcified cartilage by blood vessels, bringing in osteoblasts (bone-forming cells) that begin to lay down bone matrix.

1. 1. 1. Phases of Endochondral Ossification###

1. Mesenchymal cells aggregate into the shape of the future bone and differentiate into chondrocytes. 2. The chondrocytes enlarge and the surrounding matrix begins to calcify. Eventually, the chondrocytes die and disintegrate, leaving cavities within the cartilage. 3. Blood vessels invade the cavities, bringing osteoblasts that start forming spongy bone at a primary ossification center. Ossification proceeds towards the ends of the cartilage model. 4. As the bone grows, osteoclasts break down newly formed bone near the center of the model, creating a medullary cavity. 5. Secondary ossification centers develop in the epiphyses (ends of the bone) after birth, leading to the formation of spongy bone in these areas. The cartilage that remains between the primary and secondary ossification centers is known as the epiphyseal plate, which allows for growth in length of the bone. 6. Eventually, when bone growth in length is complete, the epiphyseal plate ossifies, becoming the epiphyseal line and marking the end of bone growth in length.

Regulation

The process of endochondral ossification is tightly regulated by various hormones, including growth hormone, thyroid hormone, and sex hormones, which play significant roles in the regulation of bone growth and development. Additionally, several growth factors such as Insulin-like Growth Factors (IGFs) and Fibroblast Growth Factors (FGFs) are crucial for the proliferation and differentiation of chondrocytes.

Clinical Significance

Disruptions in the process of endochondral ossification can lead to various bone disorders and diseases. For example, achondroplasia, the most common form of dwarfism, is caused by mutations affecting the growth factor receptors involved in chondrocyte proliferation. Similarly, osteogenesis imperfecta, or brittle bone disease, can result from defects in collagen production, affecting both intramembranous and endochondral ossification.

See Also

• Bone marrow
• Cartilage
• Osteoblast
• Osteoclast
• Skeletal system