2. Bone Formation, Growth, and Remodeling 1

3. 1. Formation and Growth  Bone forms via a process called ossification or osteogenesis  As an embryo we all have a loose hyaline cartilage skeleton that eventually hardens and becomes our skeleton  Over time (sixth or seventh week of embryonic development) osetoblasts form a bone matrix over the cartilage, and over time the internal hyaline cartilage center is digested and converted into the marrow cavity. 2

4. 2. Bone Growth  Occurs at the epiphysial plate in a lengthwise direction  New cartilage is added at the diaphysis side of the epiphysial plate, and as it grows it hardens  New bone cells replace the cartilage, ossify, and die becoming bone along the way  Bones thicken at their periosteum as they grow lengthwise 3

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6. Growth Continued  New cells secrete collagen and Ca ions,  These new cells will stack up on top of each other and form a matrix  New blood vessels are enclosed with a layer of tissue, and new bone is created over the top. 5

7. 3. What does bone do when it is finished growing?  Bone responds to two things: Changes in calcium levels, or the pull of gravity on muscle tissue. 6

8. Calcium Changes  If blood Calcium levels drop, PTH (parathyroid hormone) is released.  This will activate more osteoclast activity  This increased activity will lead to the breakdown of bone and the ultimate release of Calcium back into the bloodstream for use. 7

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10. Calcium changes cont.  If blood Calcium levels increase, (Hypercalcimia) PTH levels will decrease  This will cause a decrease in the level of osteoclast activity  This in turn leads to the deposition of Calcium into bone matrix. 9

11. Stresses from Exercise and Activity  Stress forces bone to constantly remodel itself  It will lay down new bone and destroy old bone in areas where stress is applied most often.  This will lead to an increase in bone thickness which strengthens the bone 10

12. Cont.  If you become inactive for an extended period of time the opposite takes place.  Less activity leads to a decrease in bone remodeling  This will thin the bones out  And over time these bones can become brittle and easy to damage or break. 11

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