The first process, called intramembranous ossification, occurs when fibrous membranes are replaced by bone tissue. The process, occurring only in certain flat bones, such as the flat bones of the skull, sternum, and clavicle, is summarized in two basic steps:
1. Spongy bone tissue begins to develop at sites within the membranes called centers of ossification.
2. Red bone marrow forms within the spongy bone tissue, followed by the formation of compact bone on the outside.
The second ossification process, called endochondral ossification, occurs when hyaline cartilage is replaced by bone tissue. The process, occurring in most bones of the body, follows these steps:
1. At a primary ossification center, in the center of a cartilage model, hyaline cartilage breaks down, forming a cavity.
2. A periosteal bud, consisting of osteoblasts, osteoclasts, red marrow, nerves, and blood and lymph vessels, invades the cavity. The osteoblasts produce spongy bone tissue.
3. A medullary cavity forms as osteoclasts break down the newly produced spongy bone tissue. The medullary cavity expands as it follows the spread of the primary ossification center to the ends of the bone.
4. Compact bone tissue replaces cartilage on the outside of the bone.
5. In long bones, secondary ossification centers form in the epiphyses. As in the shaft, a periosteal bud develops. However, the spongy bone tissue that subsequently develops is not replaced by a medullary cavity.
6. Articular cartilage is formed from cartilage remaining on the outside of the epiphyses.
7. The epiphyseal plate is formed from cartilage remaining between the expanding primary and secondary ossification centers.