Development, growth,
and remodeling of
bones
October 5 – 6 2015
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
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
D
• Osteoblasts form
C
spongy bone in the
diaphysis
B
• Cartilage in
other areas
continues
to grow
A
E
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
C
replaced
with bone
B
(compact in
diaphysis
and
spongy in
epiphyses
A
D
E
How do these x-rays demonstrate
bone development?
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.
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
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
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
Think, Pair, Share
Where in the bone does bone deposition (bone building) occur?
Where does cartilage grow?
Where is bone resorbed?
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
Compared to females, male skeletons have
Although
most
bones stop growing in late teens, some of your facial
• Heavier
bones
bones
(e.g. nose,
grow throughout life.
• Enlarged
ribjaw)
cages
• Smaller pelvic opening
• Larger brow, mandible, and occipital
protuberance
Bone Remodeling
• Throughout our lives, our bones are constantly ‘remodeled’.
Old bone is digested and new bone is formed.
• Purposes of remodeling:
•
•
•
•
Fun fact:
Spongy bone is replaced every
3-4 years; compact bone is
replaced every ~10 years.
Regulate blood calcium
Repair damaged areas
Add strength to areas that need it
Maintain proper bone shape throughout growth
Bone density
in athletes!
• Process of remodeling
• Osteoclasts break down bone tissue and release calcium into the
blood
• Osteoblasts create new bone tissue, taking calcium from the
blood
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
Calcium has many uses in our body.
Precise levels of calcium are
necessary for:
• Nerve impulse transmission
• Muscle contraction
• Blood coagulation
• Cell division
Hormonal Regulation of
Bone Remodeling
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
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.
You do:
Compare and contrast bone growth in
length, appositional growth, and bone
remodeling in terms of process, location,
timing, and regulation.
Closure
• What were our objectives and how did
we accomplish them?
• How does it relate to our overall unit
statement?
Exit Ticket
1. Differentiate between osteocyte, osteoblast, and
osteoclast.
2. Name three ways the skeletal system changes from
birth to adulthood.
3. What is bone remodeling and when does it occur?