Study Question Sixteen & Seventeen

• Q16. Outline the processes of intramembranous and endochondral ossification.
• Intramembranous ossification is the bone developing from mesenchyme or fibrous connective tissue
• Intramembranous ossification forms bones such as the clavicle, mandible, and flat bones of the face and skull
• Endochondral ossification is the bone replacing existing cartilage
• Endochondral ossification forms bones such as the limbs and vertebral column
• Step I of the Intramembranous Ossification: Differentation Of Embryonic Mesenchyme
• Step I of the Intramembranous Ossification: Differentation Of Embryonic Mesenchyme

  • mesenchyme differentiates to osteoblast to make bone
  • osteoblasts secret osteoid
  • osteoid will calcify
  • osteoblasts get trapped in lacunae
  • osteoblasts differentiate into osteocytes
  • blood vessels grow around primary ossification centers
• Step II of the Intramembranous Ossification: Spicule Development
• Step II of the Intramembranous Ossification: Spicule Development

  • bone centers connect to each other
  • create spicules which look like tiny trabeculae
  • blood vessels get trapped between spicules
  • feed tissue and bring in osteoclasts
• Step III of the Intramembranous Ossification: Woven (Lamellar) Bone
• Step III of the Intramembranous Ossification: Woven (Lamellar) Bone

  • create young and weak spongy and compact bone
  • bone starts to become recognizable
• Step IV of the Intramembranous Ossification: Expansion And Increase In Ossification Centers
• Step IV of the Intramembranous Ossification: Expansion And Increase In Ossification Centers

  • compact-spongy-compact sandwich known as diploe
  • exterior compact is periosteum
  • interior compact is endosteum
  • creates only skull
• Step I of the Endochondral Ossification: Make Cartilage
• Step I of the Endochondral Ossification: Make Cartilage

  • mesoderm differentiates to mesenchyme
  • mesenchyme wants to differentiate into chondroblasts (to build cartilage) or fibroblasts (to be part of perichondrium)
  • chondroblasts when trapped in lacuna, make chondrocytes and fibroblasts when metabolism slows make fibrocytes
• Step II of the Endochondral Ossification: Chondrocyte Mitosis And Calcification
• Step II of the Endochondral Ossification: Chondrocyte Mitosis And Calcification

  • avascular tissue which means starving cells
  • calcification drops pH killing cells
  • acid denatures proteins
• Step III of the Endochondral Ossification: Cellular Differentiation
• Step III of the Endochondral Ossification: Cellular Differentiation

  • mesenchyme differentiates to osteoblasts
  • create bone around cartilage
• Step IV of the Endochondral Ossification: Angiogenesis
• Step IV of the Endochondral Ossification: Angiogenesis

  • create blood vessels
  • break through bone collar and deliver bone cells
• Step V of the Endochondral Ossification: Bone Remodeling And Medullary Cavity
• Step V of the Endochondral Ossification: Bone Remodeling And Medullary Cavity

  • create primary ossification centers
• Step VI of the Endochondral Ossification: Secondary Ossification Centers Develop
• Step VI of the Endochondral Ossification: Secondary Ossification Centers Develop

  • area of cartilage in bones for childhood growth
  • Reserve Zone. Chondrocytes and chondroblasts waiting
  • Zone of Proliferation. Mitosis occurs
  • Zone of Hypertrophy. Enlarge cartilage matrix from cells made
  • Zone of Calcification. Calcium salts build up so cartilage hardens
  • Zone of Ossification. Osteoblasts replace cartilage with bone
• Step VII of the Endochondral Ossification: Appositional Growth
• Step VII of the Endochondral Ossification: Appositional Growth

  • enlarges bone diameter
  • blood vessels grow around bone and get trapped
• Step VIII of the Endochondral Ossification: Epiphyseal Closure
• Step VIII of the Endochondral Ossification: Epiphyseal Closure

  • all cartilage is replaced by none
  • growth cannot occur anymore
• Q17. What are key similarities and differences in the different ossification processes above?
• Similarities between intramembranous ossification and endochondral ossification:

  • Result in the formation of bone tissue
  • Involve the differentiation of mesenchymal cells
  • Involve the activation of osteoblasts, responsible for laying down new bone matrix
  • Bone matrix is mineralized with calcium phosphate to form mature bone
• Differences between intramembranous ossification and endochondral ossification:

  • Intramembranous begins with mesenchymal tissue that condenses to form a fibrous membrane. Endochondral begins with a hyaline cartilage model, which is gradually replaced by bone
  • Intramembranous tends to form flat bones. Endochondral tends to form long bones
  • Intramembranous has mesenchymal cells which directly differentiate into osteoblasts and start forming bone within the fibrous membrane. Endochondral has hyaline cartilage first then bone, progressing outward
• Intramembranous ossification...

  • direct transformation of mesenchyme into bone
  • occurs in flat bones
  • faster and simpler
• Endochondral ossification...

  • replacement of cartilage with bone
  • occurs in long bones
  • involves multiple stages and a cartilage template