Ch. 7. pt. 2 ( Bone Structure and Function )

What is interstitial growth?

Growth from inside the cartilage, making it grow in length. ( inside cells make length )

Where does interstitial growth occur?

Inside the lacunae, where existing chondrocytes divide.

Chondrocyte

• cell that lives in cartilage making

• keeps it healthy by making matrix

Matrix

• stuff OUTSIDE the cell

• helps tissue give structure and support

Steps of interstitial growth (in order)

1. Chondrocyte divides in a lacunae

2. Two cells (chondroblasts) form in one space

3. They secrete ( expands ) matrix and push apart

4. Each becomes a chondrocyte → cartilage grows internally

What is appositional growth?

Growth on the outer surface of cartilage, making it grow in width/thickness ( added layers grows surface) .

Where does appositional growth occur?

In the perichondrium (outer covering of cartilage).

Steps of appositional growth (in order)

1. Stem cells in perichondrium divide

2. New chondroblasts form at the surface

3. They secrete matrix into the outside layer

4. Cartilage becomes thicker and wider

When do both types of cartilage growth occur most?

During embryonic and childhood development.

What happens to cartilage growth as you age?

Interstitial growth slows and most growth becomes appositional; after maturity, growth stops unless there is injury.

Interstitial:

• grows inside, increases length

Appositional:

• grows outside, increases width

What is ossification (osteogenesis)?

The process of bone formation happening by developing bone connective tissue. ( how bones grow and repair )

• Begins in the embryo and continues through childhood and adolescence.

When does the skeleton begin to form?

Between the 8th and 12th weeks of embryonic development from either intramembranous or endochondral ossification.

What is intramembranous ossification? ( Dermal Ossification )

Bone development within a membrane

• It forms flat bones of the skull, facial bones, mandible, and the center of the clavicle .

• NO cartilage

• the formation directly from connective tissues without using cartilage

Step 1 of intramembranous ossification

Ossification centers form in thickened mesenchyme. Some cells become osteoprogenitor cells; others become osteoblasts that secrete osteoid

- cells in tissue become bone starting cells whilr others turn into bone builders that make bone material

Step 2 of intramembranous ossification

- osteoid is soft bone material

-hardens with calcium

- traps bone building cells (crystalizes)

- turns into bone maintaining cells

Step 3 of intramembranous ossification

-Woven bone (primary, immature bone) forms quickly

-and tissue surrounding mesenchyme ( flexible tissue) becomes periosteum (strong layer)

Step 4 of intramembranous ossification

. - immature bone is replaced by mature bone

- strong outer layer and spongy layer inside

- diploae sits between compact layers

Example of a bone formed by intramembranous ossification

Flat bones of the skull, facial bones, mandible, and clavicle.

What is endochondral ossification?

-Bone formation that replaces hyaline cartilage with bone.

-It forms most bones in the body — especially long bones like the femur and humerus

- cartilage becomes bone

When does endochondral ossification begin?

During the second month of embryonic development and continues into adolescence until growth plates close.

Intramembranous

Bone develops directly from mesenchyme → flat bones.

Endochondral

Bone replaces hyaline cartilage model → long bones.

Interstitial growth →

increases bone length

Appositional growth

increases bone width (thickness).

Where does interstitial growth occur?

at the epiphyseal ( end part of bone)

(growth) plate within long bones.

What happens during interstitial growth?

• cells inside cartilage divide

• produces new material

• cartilage grows longer from inside

• The bone lengthens until the plate fully ossifies (turns to bone).

When does interstitial growth stop?

When the epiphyseal plate closes and becomes the epiphyseal line (around late teens

Where does appositional growth occur?

Beneath the periosteum (outer bone surface).

How does appositional growth work?

• Osteoblasts in the periosteum lay new bone layers.

• Osteoclasts in the endosteum remove bone from the inside.

• This balances bone thickness and keeps medullary cavity size appropriate.

What is bone remodeling?

The continuous process of breaking down old bone and forming new bone throughout life.

Why is bone remodeling important?

• Maintains bone strength.

• Repairs microdamage.

• Adjusts bone structure to stress and activity.

What are the major hormones that affect bone growth?

• Growth hormone

• Thyroid hormone

• Sex hormones (estrogen & testosterone)

• Glucocorticoids

• Serotonin

Function of growth hormone (GH)?

Stimulates cartilage growth at the epiphyseal plate (via IGF/somatomedin).

Growth Hormone (Steps)

• Growth hormone makes liver produce ( IGF-1)

• IGF-1 tells cartilage in growth plate to grow

• helps bone get longer

Function of thyroid hormone?

Regulates metabolism and helps maintain normal bone growth rates.

Function of sex hormones?

• Stimulate bone growth during puberty.

• Cause epiphyseal plate closure, ending growth.

Effect of glucocorticoids (stress hormones)?

• In high amounts, they increase bone loss and can stunt growth in children.

Role of serotonin in bone remodeling?

• Acts as a regulator for bone cell activity.

• Too much serotonin can prevent osteoblasts from forming bone.

Why is calcium regulation important?

Calcium is needed for muscle contraction, nerve impulses, blood clotting, and enzyme function

What two main hormones regulate blood calcium?

• Parathyroid hormone (PTH)

• Calcitriol (active Vitamin D)-

What are the 3 steps in Vitamin D activation?

1. UV light converts skin cholesterol → Vitamin D₃ (cholecalciferol).

2. Liver converts Vitamin D₃ → calcidiol.

3. Kidneys convert calcidiol → calcitriol (active form) under influence of PTH.

( First Step ) UV light converts skin cholesterol →

Vitamin D₃ (cholecalciferol).

( Second Step) Liver converts Vitamin D₃

calcidiol

Kidneys convert calcidiol →

calcitriol (active form) under influence of PTH.

When is PTH released?

When blood calcium levels drop.

What does PTH do?

• raises by blood calcium by pulling calcium from bone

• saving in kidneys and helping gut absorb more

What hormone decreases blood calcium levels?

Calcitonin ( messenger)

• calcitonin tells bones to keep calcium inside

• tells kidneys to release more calcium into urine

• secreted by the thyroid gland (source)

How does calcitonin work?

• Inhibits osteoclast activity.

• Promotes calcium storage in bone.

• Secreted when blood Ca²⁺ is too high (especially after meals).

How does exercise affect bones?

• Weight-bearing exercise increases bone density and strength.

• Stimulates osteoblast activity.

What happens to bones with lack of exercise?

Bone mass decreases, making bones weaker and more likely to fracture.

What happens to bone with aging?

• Bone resorption > bone formation.

• Loss of calcium and collagen → bones become brittle.

What is osteopenia?

• Natural, mild loss of bone mass due to aging.

• less severe

What is osteoporosis?

Severe bone density loss, leading to fragile bones and high fracture risk (especially in postmenopausal women).

• more advanced

Which bones are most affected by osteoporosis?

Vertebrae, hip, and jaw —> these are high pressure areas making it likely to lose density and break

How can you help prevent osteoporosis?

• Regular exercise

• Calcium and Vitamin D

• Avoid smoking and excess alcohol

What is a fracture?

A break in bone due to stress or trauma.

What are some common fracture types?

• Simple (closed) – bone doesn’t pierce skin.

• Compound (open) – bone pierces skin.

• Stress – small crack from repetitive use.

• Pathologic – caused by disease (like cancer).

Simple (closed) —>

bone doesn’t pierce skin.

Compound (open)

bone pierces skin

Stress

small crack from repetitive use.

Pathologic

caused by disease (like cancer).

Step 1 ( of healing a broken bone) : Fracture Hematoma Formation

• when bone breaks, blood vessels break too

• causing blood clot ( hematoma) to form at the fracture in a few hours

Step 2 ( of healing a broken bone:) Fibrocartilaginous (soft) Callus Formation

- cells make collagen and cartilage

- creating soft callus that connects broken bone ends

Step 3: Bony (hard) Callus Formation

-Osteoblasts replace the soft callus with spongy bone (hard callus)

- soft callus becomes hard bone to stabilize the break

Step 4: Bone Remodeling

-compact bone replaces spongy bone;

- bone shape is restored to its original structure.

How long does complete bone healing take?

Usually 3 months or more, depending on age and health.

Bone fractures heal through four steps

hematoma → soft callus → hard callus → remodeling.

• Healing restores bone strength but may leave a slight thickening at the site.