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?