Bone Growth & Repair

Question 1

Intramembranous ossification

Answer 1

Bone forms in mesenchyme or in/under dense collagenous CT

Question 2

Endochondral ossification

Answer 2

Bone replaces cartilage (usually hyaline)

Question 3

What aspects are the same whether a bone forms via intramembranous or endochondral ossification?

Answer 3

1. First bone tissue to be deposited is primary/woven bone
2. Primary bone is later resorbed
3. Secondary/lamellar bone is deposited in same place. Takes longer to make but stronger. Secondary bone is initially spongy and can remain spongy or become compact. If compacted, additional bone deposited to fill holes

Question 4

Where can osteoblasts deposit osteoid? What does this mean?

Answer 4

Onto existing mineralized surfaces

Means all bone tissue forms via appositional growth

Question 5

What undergoes intramembranous ossification? How does it work?

Answer 5

Main ossification process for flat bones

All bones grow in circumference by depositing new primary bone tissue just below the inner layer of the periosteum

Question 6

What is the process of intramembranous ossification in mesenchyme?

Answer 6

Mesenchymal cells condense in the region where bone will form - ossification center

Mesenchymal cells divide and differentiate into osteoprogenitor cells

Osteoprogenitor cells divide and differentiate into osteoblasts which then synthesize and secrete osteoid and initiate its mineralization

Question 7

What undergoes endochondral ossification?

Answer 7

Main ossification process for long bone (femur ribs) and vertebrae

Question 8

What is the process of endochondral ossification?

Answer 8

1. Hyaline cartilage template forms within mesenchyme
2. Cartilage in middle of template (diaphysis) becomes calcified, eroded, and replaced with bone tissue - primary ossification center
3. Same process occur later in ends of the template (eventual epiphyses) - secondary ossification centers
4. Band of hyaline cartilage persists between diaphysis and epiphysis - growth plate
5. Growth plate replaced with bone tissue

Question 9

How is the hyaline cartilage template formed in endochondral ossification?

Answer 9

Chondrification center forms in mesenchyme.

Hyaline cartilage grows interstitially and appositionally to form a template

Template keeps shape as it grows

Question 10

How does primary ossification center form in diaphyses?

Answer 10

Forms in center of diaphysis

Template’s perichondrium becomes vascularized

Increase in oxygen triggers chondorgenic cells to transform into osteoprogenitor cells. Perichondrium becomes periosteum

Osteoblasts secrete osteoid under periosteum - layer new bone around middle of template’s diaphysis - subperiosteal bone collar

Osteoclasts dissolve holes through bone collage and holes allow a periosteal (osteogenic) bud to travel through collar

Cartilage calcifies and osteoprogenitor cells becomes osteoblasts

Question 11

How does the secondary ossification center in epiphyses?

Answer 11

Forms months or years after primary ossification center form in diaphysis

Similar formation like diaphysis but without bone collar

Cartilage in epiphyses replaced by bone

Question 12

Where does hyaline cartilage persist even after epiphyses replaced by bone?

Answer 12

Articular surface - articular cartilage

In forming growth plates

Question 13

Bone cannot grow ____

Answer 13

Interstitially! Only appositional growth

Question 14

How do long bones elongate?

Answer 14

Endochondral ossification of growth plates

Question 15

How does new cartilage and bone form at growth plate edges?

Answer 15

New cartilage forms on epiphysis side of growth plate

Cartilage replaced with bone on diaphysis side of growth plate

Diaphysis elongates as long as rate of chondrogenesis is greater than rate of ossification

Once rate of chondrogenesis slows down, the entire growth plate with close and bone can’t elongate anymore

Question 16

What are the 5 distinct tissue zones of growth plates?

Answer 16

Zone of Resting (reserve) cartilage

Zone of proliferation

Zone of hypertrophy (= maturation) - matrix not calcified

Zone of calcification (=calcified cartilage) - become mineralized by calcium

Zone of ossification (=resorption and ossification) - process of becoming bone tissue

Question 17

How do bones grow in circumference?

Answer 17

Depositing new bone to existing outer bone surface underneath the periosteum

Appositional growth

Intramembranous ossification

Can occur throughout life

Question 18

What is bone remodeling?

Answer 18

Process by which existing bone is resorbed and then new bone tissue forms in the same area

Either secondary bone replaces primary bone or secondary bone replaces older secondary bone

Question 19

What is the purpose of bone remodeling? What is a complicating factor?

Answer 19

To adapt to changing biomechanical forces
To repair microfractures in mineralized matrix
To release/store minerals needed elsewhere

Complicating factor is vessels cannot penetrate mineralized tissues so bone must remodel around locations of existing vessels

Question 20

How does remodeling occur in spongy bone?

Answer 20

1. Signaling microdamage - osteocytes at site of microdamage undergo apoptosis
2. Activation to recruit osteoclast precurors
3. Osteoclast precursors are recruited and form an osteoclast
4. Osteoclasts digest minerals with acid, release calcium and phosphate
5. Osteoblasts come in a lay down new osteoid, which then mineralizes and forms new bone
6. Bone lining cell layer is reestablished and trapped osteoblasts becomes osteocytes

Question 21

How does remodeling occur in compact bone?

Answer 21

Cells coordinate and work in phases to resorb old bone and form new osteons.

Osteoclasts work from a vessel outward and osteoblasts arrive in stages and deposit new concentric lamellae from outside of cylinder to vessel.

Question 22

During bone fracture repair, bone forms via

Answer 22

Intramembranous ossification

Endochondral ossification of fibrocartilage

Bone remodeling (secondary bone replaces primary bone)

Question 23

What forms from a bone fracture?

Answer 23

Fracture hematoma - forms immediately after fracture which then clots

Hematopoietic cells release growth factors, cytokines, to recruit WBCs and fibroblasts and to promote healing

Osteoprogenitor cells in periosteum and endosteum proliferate and surround fracture site and begin to penetrate the clot and space between the ends of the damaged bone

Question 24

What occurs with formation of soft callus?

Answer 24

Fibroblasts form a vascularized loose CT called granulation tissue in clot center

Ishcemia causes the osteoprogenitor cells to transform into chondrogenic cells

Chondrogenic cells and fibroblasts form a soft callus made of fibrocartilage (chondrogenesis)

Osteoprogenitor cells divide and differentiate into osteoblasts and secrete osteoid on outer surface of soft callus and form hard shell of new primary bone (intramembranous ossification)

Question 25

What occurs with formation of the hard callus?

Answer 25

Fibrocartilage in soft callus calcifies

Primary bone replaces fibrocartilage (endochondral ossification)

Fibrocartilage replaced with bone - hard callus

Question 26

What happens after hard callus in bone repair?

Answer 26

Bone remodeling - primary bone in hard callus eventually replace with compact secondary bone.

Extra bone removed by osteoclasts and bone remodeled back to its original shape

Question 27

What are the steps of bone fracture repair?

Answer 27

1. Fracture affects bone - fracture hematoma
2. Formation of soft callus
3. Formation of hard (bony) callus
4. Bone remodeling