1. Bone Remodeling
Bone remodeling = combination of bone deposition and bone resorption.
Deposition = taking minerals (Ca2+, Mg2+, Mn2+, phosphate) from the bloodstream and using it to build hard bone tissue.
Resorption (re + absorption) = breaking down hard bone tissue into the above materials and adding them back to the blood.
In order for the a bone to maintain its size and strength: rate of resorption = rate of deposition

2. Developmental Aspects of Bones
By age 25, nearly all bones are completely ossified
In old age, bone resorption predominates
A single gene that codes for vitamin D binding to cells determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life

3. Long Bone Growth and Remodeling
Growth in length – cartilage continually grows and is replaced by bone as shown
Remodeling – bone is resorbed and added by appositional growth as shown
Figure 6.10

4. Hormonal Regulation of Bone Growth During Youth
During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
During puberty, by testosterone and estrogens
Initially promote adolescent growth spurts
Later induce epiphyseal plate closure, ending longitudinal bone growth

5. Bone Remodeling
Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces

6. Bone Deposition Accomplished by osteoblasts (“bone builders”)
Occurs where bone is injured or added strength is needed
Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese

7. Bone Resorption Accomplished by osteoclasts (“bone crackers”)
Resorption involves osteoclast secretion of:
Lysosomal enzymes that digest organic matrix
Acids that convert calcium salts into soluble forms
Dissolved matrix is secreted into the interstitial fluid (fluid between cells) and then into the blood

8. Importance of Ionic Calcium in the Body
Calcium is necessary for:
Transmission of nerve impulses
Muscle contraction
Blood coagulation

9. Control of Remodeling Two control loops regulate bone remodeling
Hormonal mechanism that maintains calcium homeostasis in the blood (PTH and calcitonin)
Mechanical and gravitational forces acting to the skeleton (“stresses”)

10. Hormonal Mechanism
Rising blood Ca2+ levels trigger the thyroid to release calcitonin
Calcitonin stimulates calcium salt deposit in bone
Figure 6.11

11. Hormonal Mechanism
Falling blood Ca2+ levels signal the parathyroid glands to release PTH
PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood
Figure 6.11

12. Response to Mechanical Stress
Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it
Figure 6.12

13. Response to Mechanical Stress
Long bones are thickest midway along the shaft (where bending stress is greatest)
Curved bones are thickest where they are most likely to buckle
Figure 6.12

14. Response to Mechanical Stress
Trabeculae (bone fibers) form along lines of stress
Large, bony projections occur where heavy, active muscles attach
Figure 6.12