Ossification

Ossification is the gradual transition from a fibrous or cartilaginous template to bone. This process takes place at different rates and is completed at different ages depending on the site of ossification. There are two distinct types of ossification, intermembraneous and endochondral.

Intermembranous Ossification: This is the less common form of bone formation, being limited primarily to the flat bones of the skull such as the parietal, parts of the temporal, and parts of the maxilla. In this form, bone is deposited between two fibrous membranes. The bone formed in this way is normally quite porous and does not display the Haversian System of other bones.

Endochondral Ossification: In this type of bone formation a cartilage template, surrounded by the perichondrium, is entered by blood vessels to begin the process. As the template grows, ossification begins in the central portion of the template, which will become the diaphysis. This area is termed the primary ossification center. Later, the extremities of the template each develop ossification centers called secondary ossification centers, which are located in the epiphyses (proximal and distal ends of the bone). Once the template has begun to grow the perichondrium is referred to as the periosteum.

During endochondral ossification, the area between the primary and secondary ossification centers that remains cartilage is called the epiphyseal plate and is an example of a synchondroses. The final ossification of these plates takes place gradually and is completed at different chronological ages.

Haversian Systems develop in compact bone. These consist of the following parts which should be studied with the aid of a textbook diagram:

• Haversian canals
• lamellae
• lacunae
• canniculi
• osteocytes (within the lacunae)
• Volkmann canals
Within the spongy bone, the Haversian systems do not form. This more porous bone is also known as cancellous bone.

Often the epiphyses of bones that form part of an articulation are capped by an area of cartilage. This is the articular surface or articular cartilage.