Endochondral Ossification

Question 1

terms

Answer 1

long bone - paired appendages
diaphysis - shaft of long bone
epiphysis - ends of long bone
metaphysis - between diaphysis and epiphysis
epiphyseal plate - growth plate
medullary cavity - hollow cavity containing yellow marrow
perichondrium - membrane that wraps around cartilage
periosteum - dense irregular connective tissue

Question 2

Major Events of Ossification

Answer 2

Osteoblasts in perichondrium produce membrane bone

Calcification

Vascular invasion of diaphysis region

Vascular invasion of epiphysis

Appositional growth

Ephiphyseal plate closure

Question 3

Osteoblasts in perichondrium produce membrane bone

Answer 3

outer layer of perichondrium = fibrous layer with fibroblasts

inner layer of perichondrium = cambium layer with osteoprogenitor cells

osteoprogenitor cells in cambium layer of diaphysis region differentiate; osteoblasts produce a “bone collar” (membrane bone)

Question 4

Calcification

Answer 4

chondrocytes in diaphysis hypertrophy; produce Calcium salts

Chondrocyte entombment - results in chondrocyte death

Question 5

Vascular invasion of diaphysis region

Answer 5

blood vessels invade; initial medullary cavity forms - osteoclast - modified WBC

cambium layer cells (inner periosteum cells) differentiate and enter the diaphysis region with blood vessels

Additional osteoblast-lineage cells arrive in blood via invading blood vessels

PRIMARY CENTER OF OSSIFICATION = diaphysis

chondrogenesis (creation of new cartilage) continues in metaphysis

Question 6

Vascular invasion of epiphysis (parallel process), then ossification

Answer 6

SECONDARY CENTER OF OSSIFICATION

Question 7

Appositional growth

Answer 7

=continued ossification on bone surface (via osteoprogenitor cells in cambrium of periosteum)

=membrane bone

Question 8

diaphyseal ossification vs. epiphyseal ossification

Answer 8

diaphyseal ossification more rapid than epiphyseal ossification

chondrogenesis continues in metaphysis –> otherwise bone growth would stop

Question 9

Epiphyseal plate closure

Answer 9

the plate of cartilage that was mitotic is going to convert to bone – ossify

metaphyseal line - remnant of epiphyseal plate that has ossified

fate of epiphyseal plate varies across taxa:
determinant (don’t grow forever) vs. indeterminant (grow throughout life)
determinant = mammals, birds
indeterminant = lizards, turtles, alligators

Question 10

osteoclast activity

Answer 10

cause medullary cavity to enlarge as bone grows

bone isn’t static, osteoclast activity prompted when microfractures occur

microfractures common, if coalesce… new bone continually replaces older bone, new osteons formed

osteoclast activity wave followed by osteoblast activity

Question 11

acellular bone

Answer 11

• no osteocytes within mineralized matrix
• no canaliculi, no lacunae
• “dead bone”
• periosteal osteocyte activity and osteoclast invasion possible
• remodeling possible in acalcemic condition (diet OR water)
• poor fracture repair if low calcium in diet AND water

Question 12

Comparative Histology Comments

Answer 12

Gnathostomes: Haversian bone

Teleosts: usually acellular bone

Herps (reptiles/amphibians): lamellar, cellular bone

Mammals: osteonic bone

Ostracoderm: acellular

Question 13

Lines of Arrested Growth (LAG)

Answer 13

common in non-Haversian bone

tree ring analogy

can be from

• environmental stress (cold)
• nutritional state (food abundance/quality)

temperate ectotherms