This again is not the mature bone, but is known a woven bone, which will be eventually replaced by compact bone. 5 Chondrocytes within the cartilage bone will undergo hypertrophy, or an increase in cell size. Minerals will then be deposited in the area, which lessens nutrient delivery to the cartilage cells, which then die. A periosteal bud invades the newly formed cavity in the cartilage. The periosteal bud contains blood and lymph vessels, nerves, and osteoblasts and osteoclasts.
Ossification spreads in both directions from the primary ossification center.
Osteoclasts will then remove the newly formed bone, and open up the medullary cavity. After birth, secondary ossification centers form in the epiphyses, and the same steps will occur, except that the spongy bone is not reabsorbed by the osteoclasts and no medullary cavity is formed. A child's long bones grow in length via activity in the epiphyseal plates, but bones also need to become thicker as a child grows. This thickening process is known as appositional growth. This process is very similar to endochondral ossification.