L15: Bone Formation

Question 1

Two mechanisms by which bones develop

Answer 1

1. ) intramembranous ossification

2. ) endochondral ossification

Question 2

Describe intramembranous ossification

Answer 2

• Intramembranous ossification is where bone forms directly within a membrane of highly vascular mesenchyme (embryonic CT)
• This is the mechanism by which flat bones of skull, face, mandible, clavicle etc. form
  Process:
  1.) Initial site of osteogenesis = mesenchymal membrane. Mesenchymal cells differentiate into osteoprogenitor cells, which then become osteoblasts
  2.) Osteoblasts synthesize and deposit osteoid
  3.) First bone laid down is primary bone
  4.) Soft osteoid undergoes mineralization process, osteoblasts become trapped in lacunae and are referred to as osteocytes
  5.) Mesenchymal cells continuously undergo mitosis giving rise to more progenitor and ultimately osteoblast cells. These osteoblasts deposit bone matrix on surface of existing spicules and trabeculae via appositional bone growth
  6.) Primary bone later resorbed by osteoclasts and gradually replaced with secondary bone. Highly vascular CT surrounding bone spicules and trabeculae differentiates into bone marrow
  7.) Mesenchymal tissue also gives rise to periosteum and endosteum

Question 3

Describe endochondral ossification

Answer 3

• Cartilage model of the bone-to-be is formed first, then calcified, eroded and replaced
• This is mechanism by which long bones of limbs and vertebrae form
  Process:
  1.) Primary ossification center (in diaphysis)
• Hyaline cartilage model of bone-to-be, derived from mesenchyme, forms
• It grows appositionally (width) and interstitially (length), maintaining shape/outline of bone
• Perichondrium becomes vascularized, signaling chondrogenic cells in inner layer to differentiate into osteoprogenitor cells
• Osteoprogenitor cells differentiate into osteoblasts and periochondrium differentiates into periosteum
• Osteoblasts in inner layer of periosteum synthesize and secrete osteoid to form bony collar
• As bony collar forms, cartilage cells hypertrophy (accumulate glycogen and form vacuoles) and produce alkaline phosphatase. Matrix where they live becomes calcified
• Nutrients can no longer diffuse across this medium and chondrocytes die
• Osteoclasts perforate bone collar, periosteal bud consisting of blood vessels, progenitor cells, and HSCs (stem) passes into cartilage model. Progenitors become osteoblasts, which form matrix on remnants of calcified cartilage

2. ) Secondary center of ossification (in epiphysis)
   - Secondary center forms in one epiphysis shortly after birth, another forms in other epiphysis shortly afterwards
   - Bone forms in similar fashion to that in diaphysis (above), but without bone collar.
   - Osteoprogenitor cells populate calcified cartilage and differentiate into osteoblasts.
   - Osteoblasts synthesize osteoid and deposit it on surface of calcified cartilage remnants.
   - Cartilage is replaced by bone except at articular surfaces of epiphyses and at epiphyseal growth plates

Question 4

Describe how bone grows in length

Answer 4

• Long bones grow in length by endochondral bone formation at epiphyseal growth plates, where cartilage maintains thickness until 18-20, then goes away

Question 5

List zones in epiphyseal plate

Answer 5

1. ) zone of reserve cartilage
2. ) zone of proliferation
3. ) zone of hypertrophy
4. ) zone of calcified cartilage
5. ) zone of resorption and ossification

Question 6

Describe structure/function of zones in the epiphyseal growth plate

Answer 6

1. ) Zone of reserve cartilage
   - hyaline cartilage, mitotically active
2. ) Zone of proliferation
   - intensely mitotically active area, forming parallel columns of isogenous cells appearing in stacks (like pennies), cartilage here grows interstitially
3. ) Zone of hypertrophy
   - chondrocytes accumulate glycogen and become vacuolated, no addition of cells or cartilage matrix
4. ) Zone of calcified cartilage
   - lacunae coalesce, calcification of cartilage matrix occurs and chondrocytes die
5. ) Zone of resorption and ossification
   - calcified cartilage remnants form long spicules in direction of diaphysis, blood vessels bring osteoprogenitor cells into this zone, these cells emigrate to matrix area and differentiate into osteoblasts

Question 7

How to differentiate between calcified cartilage and mineralized bone?

Answer 7

• Calcified cartilage is basophilic, does not contain cells (cells have died)
• Mineralized bone is acidophilic, contains living cells

Question 8

Describe how bone grows in width

Answer 8

• Bone grows in width by appositional growth
• Osteoprogenitor cells of inner cellular layer of periosteum differentiates into osteoblasts
• Osteoblasts secrete osteoid on subperiosteal bone surface (external aspect) causing bone diameter to increase
• Osteoclasts resporb bone on internal aspect of subperiosteal bone surface causing bone marrow cavity diameter to increase

Question 9

Describe mineralization of bone

Answer 9

• Minerals delivered by blood vessels
• Osteocalcin and sialoproteins bind calcium
• Elevated calcium activates osteoblasts, release alkaline phosphatase, which elevates local extracellular concentration of phosphate ions, causing further elevation of calcium
• Osteoblasts release matrix vesicles into matrix, which contains high concentrations of calcium and phosphate ions. These vesicles regulate initial site of mineral deposition in osteoid
• Vesicles contain pumpls that transport more and more minerals into the vesicles
• Crystallization of calcium and phosphate occurs in vesicles forming hydroxyapatite crystals, as they enlarge they damage the vesicle’s membrane and are deposited into the matrix

Question 10

Describe bone remodeling

Answer 10

• Youth: bone growth rate»_space;> bone resorption rate
• Adult: bone growth rate = bone resorption rate
• Surface remodeling occurs by deposition on outer surface, resorption on inner surface. Occurs via PTH and calcitonin

Question 11

Describe bone repair

Answer 11

• Bone is repaired by intramembranous bone formation, cartilage formation and endochondral bone formation
  1. ) Trauma: damage occurs to bone matrix, osteocytes die, damage to periosteum and endosteum occurs, hemorrhage and clot formation
  2. ) Clean up: by neutrophils and macrophages
  3. ) New CT: blood vessels and fibroblasts grow into blood clot, fibroblasts form granulation tissue, cartilage and dense collagenous CT form – granulation tissue and cartilage/dense CT = soft callus that acts as bridge
  4. ) Formation of bone: progenitor – osteoblasts – osteoid, bone forms shell on fibrocartilagenous callus, cartilage calcifies, dies and is replaced by bone, bony callus replaces fibrocartilagenous callus, spongy bone forms, replaced by compact, bone remodeled by osteoclasts to original shape

Question 12

Describe achondroplasia

Answer 12

• Genetic disorder of bone growth that causes the most common type of dwarfism
• Characterized by decrease in production of cartilage cells in epiphyseal growth plates of long bones. Cartilage is replaced by bones at very slow rate. Zones of proliferation and hypertrophy are slender and disorderly.