Bone Growth & Repair Flashcards
Intramembranous ossification
Bone forms in mesenchyme or in/under dense collagenous CT
Endochondral ossification
Bone replaces cartilage (usually hyaline)
What aspects are the same whether a bone forms via intramembranous or endochondral ossification?
- First bone tissue to be deposited is primary/woven bone
- Primary bone is later resorbed
- 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
Where can osteoblasts deposit osteoid? What does this mean?
Onto existing mineralized surfaces
Means all bone tissue forms via appositional growth
What undergoes intramembranous ossification? How does it work?
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
What is the process of intramembranous ossification in mesenchyme?
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
What undergoes endochondral ossification?
Main ossification process for long bone (femur ribs) and vertebrae
What is the process of endochondral ossification?
- Hyaline cartilage template forms within mesenchyme
- Cartilage in middle of template (diaphysis) becomes calcified, eroded, and replaced with bone tissue - primary ossification center
- Same process occur later in ends of the template (eventual epiphyses) - secondary ossification centers
- Band of hyaline cartilage persists between diaphysis and epiphysis - growth plate
- Growth plate replaced with bone tissue
How is the hyaline cartilage template formed in endochondral ossification?
Chondrification center forms in mesenchyme.
Hyaline cartilage grows interstitially and appositionally to form a template
Template keeps shape as it grows
How does primary ossification center form in diaphyses?
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
How does the secondary ossification center in epiphyses?
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
Where does hyaline cartilage persist even after epiphyses replaced by bone?
Articular surface - articular cartilage
In forming growth plates
Bone cannot grow ____
Interstitially! Only appositional growth
How do long bones elongate?
Endochondral ossification of growth plates
How does new cartilage and bone form at growth plate edges?
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
What are the 5 distinct tissue zones of growth plates?
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
How do bones grow in circumference?
Depositing new bone to existing outer bone surface underneath the periosteum
Appositional growth
Intramembranous ossification
Can occur throughout life
What is bone remodeling?
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
What is the purpose of bone remodeling? What is a complicating factor?
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
How does remodeling occur in spongy bone?
- Signaling microdamage - osteocytes at site of microdamage undergo apoptosis
- Activation to recruit osteoclast precurors
- Osteoclast precursors are recruited and form an osteoclast
- Osteoclasts digest minerals with acid, release calcium and phosphate
- Osteoblasts come in a lay down new osteoid, which then mineralizes and forms new bone
- Bone lining cell layer is reestablished and trapped osteoblasts becomes osteocytes
How does remodeling occur in compact bone?
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.
During bone fracture repair, bone forms via
Intramembranous ossification
Endochondral ossification of fibrocartilage
Bone remodeling (secondary bone replaces primary bone)
What forms from a bone fracture?
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
What occurs with formation of soft callus?
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)
What occurs with formation of the hard callus?
Fibrocartilage in soft callus calcifies
Primary bone replaces fibrocartilage (endochondral ossification)
Fibrocartilage replaced with bone - hard callus
What happens after hard callus in bone repair?
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
What are the steps of bone fracture repair?
- Fracture affects bone - fracture hematoma
- Formation of soft callus
- Formation of hard (bony) callus
- Bone remodeling
