Histo.6

• Osteogenesis


  Bone development
• Processes of bone development


  • Intramembranous ossification
  • Endochondral ossification
• Intramembranous ossification

  1. Osteoblasts differentiate directly from mesenchyme and begin secreting osteoid

  2. Woven bone is produced first and is soon replaced by stronger lamellar bone

  3. Areas of woven bone, bone resorption, and lamellar bone exist contiguous to one another during growth
• Intramembranous ossification


  • Occurs within condensed sheets ("membranes") of embryonic mesenchymal tissue
  • Most bones of the skull, jaws, scapula and clavicle are formed this way
• Intramembranous ossification process


  1. Osteoprogenitor cells arise, proliferate, and form incomplete layers of osteoblasts around a network of developing capillaries

  2. Osteoid secreted by osteoblasts calcifies, forming small irregular areas of woven bone

  3. Continued matrix secretion and calcification enlarges these areas and leads to the fusion of neighboring ossification centers

  4. Mesenchymal regions that do not undergo ossification give rise to the endosteum and the periosteum
• Cranial flat bones formed by intramembranous ossification


  • Internal and external plates of compact bone arise, while the central portion (diploë) maintains its cancellous nature
  • Fontanelles or "soft spots" on newborn heads are areas where the membranous tissue is not yet ossified
• Endochondral ossification

  1. Ossification occurs within hyaline cartilage shaped as a small version of the bone to be formed

  2. Osteoblasts in the perichondrium (transitioning to periosteum) produce a bone collar around the cartilage model diaphysis

  3. Chondrocytes in the cartilage model hypertrophy, compress the matrix, and initiate calcification

  4. Chondrocytes die, creating empty spaces

  5. Blood vessels and osteoprogenitor cells from the periosteum penetrate the bone collar, producing woven bone

  6. Primary ossification center forms in the diaphysis, and secondary ossification centers later appear at the epiphyses
• Endochondral ossification


  • Produces cavities filled with bone marrow and cancellous bone trabeculae as the primary and secondary ossification centers expand and remodel
  • Articular cartilage within joints and epiphyseal cartilage (growth plate) remain after ossification
• Epiphyseal cartilage (growth plate)

  1. Allows longitudinal bone growth

  2. Disappears upon completion of bone development (epiphyseal closure) by around age 20
• Zones of the epiphyseal growth plate

  • Zone of reserve (resting) cartilage
  • Proliferative zone
  • Zone of hypertrophy
  • Zone of calcified cartilage
  • Zone of ossification
• Longitudinal growth of a bone occurs by cell proliferation in the epiphyseal plate cartilage
• Chondrocytes in the diaphysis side of the epiphyseal plate undergo hypertrophy, their matrix becomes calcified, and the cells die
• Osteoblasts lay down a layer of new bone on the calcified cartilage matrix
• The epiphyseal plate does not change thickness, but is instead displaced away from the center of the diaphysis as the length of the bone increases
• Appositional growth of long bones


  1. Osteoblasts developing from osteoprogenitor cells in the periosteum form the bone collar on the cartilaginous diaphysis

  2. Osteoclasts in the endosteum enlarge the central marrow cavity as the bone circumference increases