Dr. Cohn BIOL 319 Lecture Exam 3 - Outline 8

This set covers lecture notes from outline 8 for lecture exam 3.

What is the extracellular matrix (ECM) of bone?

Non-living material around cells that gives bone strength and structure

What are the two main components of bone ECM?

Organic matrix + inorganic matrix

What is the organic matrix (osteoid)?

Protein-rich portion of bone made by osteoblasts

What are proteoglycans?

Large molecules made of proteins + carbohydrates that hold water

What is the function of proteoglycans in bone?

Provide support and help resist compression slightly

What is hyaluronic acid?

Slippery carbohydrate molecule in ground substance

What is the function of hyaluronic acid?

Helps with lubrication and diffusion of nutrients

What is ground substance?

Gel-like material made of proteoglycans + hyaluronic acid

What is the function of ground substance?

Supports cells and allows nutrient movement

What are collagen fibers?

Strong protein fibers (Type I collagen)

What is the role of collagen fibers in bone?

Provide flexibility and tensile strength (resist pulling)

Which cells produce the organic matrix?

Osteoblasts

What is the inorganic matrix of bone?

Mineral portion that gives hardness

What is hydroxyapatite?

Mineral crystal made of calcium phosphate

Is hydroxyapatite a salt?

Yes

What are the components of hydroxyapatite?

Calcium (Ca²⁺) + phosphate (PO₄³⁻)

What is the function of hydroxyapatite?

Provides rigidity and compressive strength

Which cells are responsible for hydroxyapatite formation?

Osteoblasts

What is compressive (compression) strength?

Ability to resist being crushed

What part of the matrix provides compressive strength?

Inorganic matrix (hydroxyapatite)

Example of compressive strength in bone?

Supporting body weight (standing/walking)

What is tensile strength?

Ability to resist being stretched or pulled apart

What part of the matrix provides tensile strength?

Organic matrix (collagen fibers)

Example of tensile strength in bone?

Muscles pulling on bones during movement

What happens when compressive + tensile strength are combined?

Bone is strong, flexible, and resistant to breaking

What is a synergistic property of combining both strengths?

Bone can handle multiple forces without failing

What contributes to compressive strength?

Hydroxyapatite (inorganic matrix)

What contributes to tensile strength?

Collagen fibers (organic matrix)

How is bone like steel-reinforced concrete?

Concrete = hydroxyapatite (compression) AND Steel = collagen (tension)

Why is the steel/concrete analogy in the construction of bone matrix important?

Shows how two materials combine for strength

Can you evaluate compressive strength without tensile strength?

Yes, remove collagen → bone becomes brittle

What happens when collagen is removed?

Bone can resist compression but breaks easily

Can you evaluate tensile strength without compressive strength?

Yes, remove minerals → bone becomes flexible

What happens when hydroxyapatite is removed?

Bone bends easily (rubbery)

What happens if collagen production is deficient?

Bone becomes brittle and fractures easily

What happens if hydroxyapatite is deficient?

Bone becomes soft and weak

What happens if BOTH are deficient?

Bone loses strength completely (very fragile)

How would an old, unpreserved femur differ from a living femur?

It has lost organic components (collagen)

What is the result of losing collagen in old bone?

Bone becomes brittle and breaks easily

How does a living femur compare to an old preserved one?

Has both collagen + hydroxyapatite → strong and flexible

What is endochondral ossification?

Bone formation by replacing a hyaline cartilage model

Why is it called endochondral ossification?

“Endo” = within, “chondral” = cartilage → bone forms within cartilage

What is intramembranous ossification?

Bone forms directly from mesenchyme (no cartilage model)

Why is it called intramembranous ossification?

“Membranous” refers to mesenchymal tissue layer where bone forms

What is the skeleton made of up to ~5 weeks post conception?

Mostly hyaline cartilage (and mesenchyme)

At ~5 weeks, what ossification begins and where?

Intramembranous ossification begins in flat bones (skull, clavicle)

Which bones undergo intramembranous ossification?

Flat bones → skull + clavicle

At ~8–12 weeks, what ossification increases and where?

Endochondral ossification in long bones (femur, etc.)

What is the first step of endochondral ossification?

A hyaline cartilage model surrounded by perichondrium

What is the perichondrium?

Dense connective tissue around cartilage

How does perichondrium change during ossification?

Becomes periosteum

What is the periosteum in the endochondral ossification process?

Outer bone layer that produces osteoblasts

What is the bone collar?

A thin layer of bone forming around the diaphysis

What is the periosteal bud?

Invasion of blood vessels, osteoblasts, osteoclasts into cartilage

What happens during chondrocyte hypertrophy?

Cartilage cells enlarge

What happens during chondrocyte calcification?

Matrix hardens, cutting off nutrients

What happens during chondrocyte apoptosis?

Chondrocytes die, leaving cavities

What do osteoblasts do in this chondrocyte apoptosis?

Lay down bone matrix (woven bone)

What is the primary ossification center?

First site of bone formation in the diaphysis

What is woven bone?

Immature, disorganized bone formed early

What do osteoclasts do during ossification?

Break down bone to form the medullary cavity

What is the medullary cavity?

Hollow center of bone that holds marrow

What is secondary ossification?

Bone formation in the epiphyses

What are the key events of endochondral ossification from beginning to end (in order)?

1. Hyaline cartilage model forms surrounded by perichondrium

2. Perichondrium becomes periosteum

3. Bone collar forms around diaphysis (via osteoblasts)

4. Chondrocyte hypertrophy (cells enlarge)

5. Chondrocyte calcification (matrix hardens)

6. Chondrocyte apoptosis (cells die → cavities form)

7. Periosteal bud(s) invade (blood vessels + osteoblasts + osteoclasts)

8. Primary ossification center forms in diaphysis

9. Osteoblasts lay down woven bone (spongy bone)

10. Osteoclasts break down bone → production of the medullary cavity

11. Secondary ossification occurs in epiphyses (after birth)

Describe the temporal changes in endochondral ossification throughout life.

1. Early (embryo): Entire skeleton = cartilage model (perichondrium present)

2. Mid-development: Ossification begins in diaphysis (primary ossification) → periosteum + bone collar form

3. Late fetal/after birth: Secondary ossification in epiphyses

4. Childhood: Bone lengthening continues at epiphyseal plates

5. Adulthood: Plates close → become epiphyseal lines (no more length growth)

Describe the spatial changes in endochondral ossification throughout life.

1. Diaphysis = primary ossification + medullary cavity

2. Epiphyses = secondary ossification (spongy bone remains)

What is the starting tissue for intramembranous ossification?

Mesenchyme

What are ossification centers?

Clusters where osteoblasts begin bone formation

What do osteoblasts do at the ossification centers?

Secrete matrix → form woven bone

How do ossification centers grow?

Extend into fingerlike projections

What structure do these projections form?

Trabeculae (spongy bone)

Where does spongy bone form first?

Interior of the developing bone

What happens to woven bone over time?

Remodeled into spongy bone

What forms from mesenchyme on the outside?

Periosteum

How is compact bone formed from mesenchyme on the outside?

Forms on the outer surface under periosteum

Where does endochondral ossification occur?

Inside cartilage models (long bones)

Where does primary ossification occur?

Diaphysis (shaft)

Where does secondary ossification occur?

Epiphyses (ends)

Where does intramembranous ossification occur?

Within mesenchyme (flat bones)

Where does spongy bone form first in intramembranous ossification?

Inside (trabeculae network)

Where does compact bone form in intramembranous ossification?

Outer surface (under periosteum)

What is the key difference between the two ossification processes?

Endochondral = cartilage → bone AND Intramembranous = direct bone formation

Which process is used for long bones?

Endochondral ossification

Which process is used for flat bones?

Intramembranous ossification

What are the key events of intramembranous ossification from beginning to end (in order)?

1. Mesenchyme condenses in the region where bone will form

2. Ossification centers develop within the mesenchyme

3. Osteoblasts differentiate and begin secreting bone matrix

4. Ossification centers extend into fingerlike projections, forming woven bone (occurs in the interior)

5. Osteoclasts help remodel early bone structure

6. Woven bone is remodeled into spongy bone (trabeculae)

7. Surrounding mesenchyme differentiates into periosteum

8. Compact bone forms on the outer surface beneath the periosteum