Dr. Cohn 319 Lecture Exam 3 - Outline 9

This set covers material over outline 9 for lecture exam 3.

What is the epiphyseal plate and what does it do?

Hyaline cartilage region that allows bone lengthening

Where is the epiphyseal plate located?

Between the epiphysis and diaphysis (at the metaphysis)

What is the correct order of zones from epiphysis → diaphysis?

Resting → Proliferation → Hypertrophy → Calcification → Ossification

What is another name for the zone of proliferation?

Growth zone / hyperplasia

Which zone is closest to the epiphysis?

Zone of resting cartilage

Which zone is closest to the diaphysis?

Zone of ossification

In what direction do cells move in the epiphyseal plate?

From epiphysis → diaphysis

What happens in the zone of resting cartilage?

Resting chondrocytes; anchors plate to epiphysis

What happens in the zone of proliferation?

Chondrocytes rapidly divide (mitosis) → push toward diaphysis

How does the zone of proliferation look histologically?

Cells in stacked columns

What happens in the zone of hypertrophy?

Chondrocytes enlarge (increase in size)

How does the zone of hypertrophy look histologically?

Cells appear large and swollen

What happens in the zone of calcification?

Matrix calcifies and chondrocytes die

How does the zone of calcification look histologically?

Cells appear dead with empty spaces

What happens in the zone of ossification?

Osteoblasts build bone and osteoclasts remodel

What is the overall “big picture” of chondrocytes moving through the plate?

Cartilage is replaced by bone → bone lengthens

How does the epiphyseal plate appear on an X-ray?

As a dark line

Why does the epiphyseal plate appear dark on an X-ray?

It is cartilage (less dense than bone) so fewer X-rays are absorbed

What is bone widening?

Increase in bone diameter (thickness of diaphysis)

Where does bone widening occur?

At the diaphysis of long bones

How can the diaphyseal wall thicken while the medullary cavity also widens?

Bone is added to the outside while being removed from the inside

Which two cell types are responsible for bone widening?

Osteoblasts and osteoclasts

Where are osteoblasts located during bone widening?

On the outer surface (periosteum)

Where are osteoclasts located during bone widening?

On the inner surface (endosteum/medullary cavity)

What do osteoblasts do in bone widening?

Build new bone → thickens diaphyseal wall

What do osteoclasts do in bone widening?

Break down bone → widens medullary cavity

How must osteoblast activity compare to osteoclast activity during widening?

Osteoblast activity must always be greater than osteoclast activity

Why must osteoblast activity be greater than osteoclast activity?

So the bone thickens overall while still allowing the cavity to expand

What happens if osteoclast activity is greater than osteoblast activity?

Bone becomes thin and weak

Is bone widening an example of bone remodeling?

Yes

Why is bone widening considered bone remodeling?

Because it involves simultaneous bone formation and resorption

Does bone widening stop in adulthood?

No

How can bone still widen in adulthood?

Through remodeling in response to stress (Wolff’s Law)

Under what conditions does bone widening occur in adults?

Exercise, mechanical stress, increased load

What is the overall goal of bone widening?

To increase strength without making bone too heavy

What happens if bone is not used?

Bone atrophies (loses density and strength)

Who was Julius Wolff?

A scientist who observed that bone changes based on stress placed on it

What is Wolff’s Law?

Bone remodels in response to mechanical stress, becoming stronger where stress is greater

How would Wolff’s Law appear in a tennis player, pitcher, or bowler?

The dominant arm bones are thicker and denser than the non-dominant arm

Why does the dominant arm become thicker?

More mechanical stress → increased osteoblast activity → more bone formation

Is changing trabeculae organization an example of bone remodeling?

Yes

How do trabeculae demonstrate Wolff’s Law?

They realign along lines of stress to better support forces

To make bone thicker and stronger, what must happen between osteoblasts and osteoclasts?

Osteoblast activity must exceed osteoclast activity

Does the diaphysis thicken uniformly throughout the bone?

No

Where does the diaphysis thicken the most?

In areas of greatest mechanical stress (weight-bearing regions)

Which side of the diaphysis thickens the most?

The side experiencing the most compression/load

Is there a corollary to Wolff’s Law?

Yes → bone is a “use it or lose it” tissue

What are examples of bone atrophy?

Lack of exercise, immobilization (cast), and space travel (no gravity)

Should you take advantage of Wolff’s Law throughout life?

Yes

Why is lifelong exercise important for bone health?

It maintains bone density and strength through constant remodeling

What is the main takeaway of Wolff’s Law?

Bone adapts to stress → stronger with use, weaker without it