1. Hyaline cartilage model is completely covered with bone matrix (a bone "collar") by bone-building cells called osteoblasts
2. Enclosed cartilage model is digested away, opening up a medullary cavity
Bone remodeling
Remodeled in response to changes in calcium levels in the blood and the pull of gravity and muscles on the skeleton
Bone remodeling in response to calcium levels
1. ↓blood calcium, parathyroid gland releases parathyroid hormone (PTH) which activates osteoclasts to break down bone matrix and release calcium
2. ↑blood calcium, calcium is deposited in bone matrix as hard calcium salts
Bone remodeling in response to gravity and muscle pull
1. Bones become thicker and form large projections to increase their strength in areas where bulky muscles are attached
2. Osteoblasts lay down new matrix and become trapped within it, becoming osteocytes
Bones of the bedridden or physically inactive people tend to lose mass and to atrophy, because they are no longer subjected to stress
Rickets
A disease of children in which the bone fail to calcify, causing the weight-bearing bones of the legs to become bowed
Rickets is usually due to lack of calcium in the diet or lack of vitamin D, which is needed to absorb calcium into the bloodstream
Fracture
A break in the bone, resulting from exceptional trauma that twists or smashes the bones
Fracture repair
1. Hematoma forms
2. Fibrocartilage callus forms
3. Bony callus forms
4. Bone remodeling occurs
The repair of bone fractures involves four major events: hematoma formation, fibrocartilage callus formation, bony callus formation, and bone remodeling
