1. Development, growth, and remodeling of bones
October7 2016

2. Important vocabulary What is an osteocyte? A mature bone cell
Two new words:
Osteoblast – bone forming cell
Osteoclast – bone dissolving cell
Mnemonic:
osteoblast – bone building
osteoclast – bone consuming

3. Bone Growth & Development
Embryonic skeleton (A) is made of hyaline cartilage
As a fetus (B),
Osteoblasts form a bone “collar” around the diaphysis
Osteoclasts digest cartilage in center of diaphysis
Osteoblasts form
spongy bone in the
diaphysis
Cartilage in
other areas
continues
to grow D C E B A

4. Bone Growth & Development
As a newborn (C),
The medullary cavity forms
The epiphyses begin to ossify
Throughout childhood (D) and adolescence (E)
Bones grow in length & width (more on this)
Cartilage is
replaced
with bone
(compact in
diaphysis
and
spongy in
epiphyses D C E B A

5. How do these x-rays demonstrate bone development?

6. How do these x-rays demonstrate bone development?
The infant has a high percentage of cartilage (which doesn’t show on x-ray), whereas the 8 year old’s hand is mostly bony
Notice the epiphyseal plates.

7. Growth in bone length Epiphyseal end of growth plate
Cartilage on the side of the epiphyseal plate close to the end of the bone grows by mitosis
Diaphysis end of growth plate
Cartilage on the side of the epiphyseal plate close to the diaphysis calcify and die
Osteoblasts develop spongy
bone around the dying cartilage
Osteoclasts dissolve the dead
cartilage

8. Growth in bone length
During most of childhood, the rate of cartilage growth equals the rate of cartilage destruction / bone formation, so the epiphyseal plate width stays the same length
Near the end of adolescence, cartilage growth slows and stops, so the epiphyseal plates narrow then close (are replaced by bone)
Fun fact:
Ages of plate closure
Girls: ~18 years
Boys: ~21 years

9. Growth in bone width
Bones grow in width using a process called appositional growth
Osteoblasts under the periosteum secrete bone matrix
Osteoclasts near the medullary cavity dissolve bone, enlarging the cavity.
The osteoblasts
create new bone at
a slightly faster rate
than the
osteoblasts destroy
bone. This means
the bones get wider
and slightly thicker
as they grow.
bone growth

10. Think, Pair, Share
Where in the bone does bone deposition (bone building) occur? Where does cartilage grow? Where is bone resorbed?

11. Regulation of Bone Growth
Hormones regulate bone growth during youth.
Growth Hormone (secreted by pituitary) is the primary hormone responsible for bone growth in infancy and childhood
Testosterone and Estrogen (secreted by testes / ovaries)
Cause a growth spurt in adolescence
Cause the masculinization / feminization of the skeleton
Later cause epiphyseal plate closure
Although most bones stop growing in late teens, some of your facial bones (e.g. nose, jaw) grow throughout life.
Compared to females, male skeletons have
Heavier bones
Enlarged rib cages
Smaller pelvic opening
Larger brow, mandible, and occipital protuberance
The termination of growth is done by estradiol metabolites, which is why it occurs later in men

12. Bone Remodeling
Throughout our lives, our bones are constantly ‘remodeled’. Old bone is digested and new bone is formed.
Purposes of remodeling:
Regulate blood calcium
Repair damaged areas
Add strength to areas that need it
Maintain proper bone shape throughout growth
Process of remodeling
Osteoclasts break down bone tissue and release calcium into the blood
Osteoblasts create new bone tissue, taking calcium from the blood
Fun fact:
Spongy bone is replaced every 3-4 years; compact bone is replaced every ~10 years.
Bone density in athletes!
We obtain microscopic cracks in our bones due to daily activities. Older women are encouraged to do weight-bearing exercises to increase bone density; astronauts lose density

13. Regulation of Bone Remodeling
Bone remodeling is controlled by two factors:
Hormones that regulate blood calcium levels – determines whether more deposition or resorption occurs
Mechanical stress –
determines where more bone is needed
We obtain microscopic cracks in our bones due to daily activities. Older women are encouraged to do weight-bearing exercises to increase bone density; astronauts lose density
Calcium has many uses in our body. Precise levels of calcium are necessary for:
Nerve impulse transmission
Muscle contraction
Blood coagulation
Cell division

14. Hormonal Regulation of Bone Remodeling
We obtain microscopic cracks in our bones due to daily activities. Older women are encouraged to do weight-bearing exercises to increase bone density; astronauts lose density

15. The Role of Mechanical Stress
Although the role of mechanical stress (how much tension and compression bones experience) is not fully understood we do know that:
Areas that experience greater stress have greater bone deposition
Areas that experience little stress have greater bone resorption
We obtain microscopic cracks in our bones due to daily activities. Older women are encouraged to do weight-bearing exercises to increase bone density; astronauts lose density
Without gravity, astronauts have little stress on their bones. As a result, they lose bone density at an alarming rate.
Astronauts on Mir lost 1.5% bone mass per month. Compare this to post menopausal women, who lose about 2.5% per year.

16. You do:
Compare and contrast bone growth in length, appositional growth, and bone remodeling in terms of process, location, timing, and regulation.

17. Closure What were our objectives and how did we accomplish them?
How does it relate to our overall unit statement?

18. Exit Ticket
Differentiate between osteocyte, osteoblast, and osteoclast.
Name three ways the skeletal system changes from birth to adulthood.
What is bone remodeling and when does it occur?