Post on 24-Dec-2015
Development, growth, and remodeling of bones
October 17-20, 2014
Important vocabulary
What is an osteocyte? A mature bone cell
Two new words:• Osteoblast – bone forming cell• Osteoclast – bone dissolving cell
Mnemonic: osteoblast – bone buildingosteoclast – 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• Osteoblasts form spongy bone in the diaphysis• Cartilage in other areas continues to grow
A
B
CD
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 replaced with bone (compact in diaphysis and spongy in epiphyses A
B
CD
E
Bone growth
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 mitosisDiaphysis 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, ShareWhere 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
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
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
Bone density in athletes!
Fun fact: Spongy bone is replaced every 3-4 years; compact bone is replaced every ~10 years.
Regulation of Bone RemodelingBone 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 StressAlthough 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
question? Our mini question?
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?