Bone Development and Osteogenesis

Bone Development

• Bone development or osteogenesis occurs through two processes: intramembranous ossification and endochondral ossification.

Intramembranous Ossification

• Intramembranous ossification forms flat bones, such as those in the skull and jaws, scapula, and clavicle.
• It occurs within condensed sheets of embryonic mesenchymal tissue.
• Osteoprogenitor cells arise, proliferate, and form incomplete layers of osteoblasts around a network of developing capillaries.
• Osteoid secreted by osteoblasts calcifies, forming small irregular areas of woven bone with osteocytes in lacunae and canaliculi.
• Continued matrix secretion and calcification enlarge these areas, leading to the fusion of neighboring ossification centers.
• The anatomical bone forms gradually as woven bone matrix is replaced by compact bone that encloses a region of cancellous bone with marrow and larger blood vessels.
• Mesenchymal regions that do not undergo ossification give rise to the endosteum and periosteum of the new bone.

Endochondral Ossification

• Endochondral ossification forms most bones of the body, especially long bones.
• It takes place within a hyaline cartilage model of the bone to be formed.
• Ossification first occurs within a bone collar produced by osteoblasts that differentiate within the perichondrium (transitioning to periosteum) around the cartilage model diaphysis.
• The bone collar impedes diffusion of oxygen and nutrients into the underlying cartilage, causing local chondrocytes to swell up (hypertrophy), compress the surrounding matrix, and initiate its calcification.
• The hypertrophic chondrocytes eventually die, creating empty spaces within the calcified matrix.
• One or more blood vessels from the perichondrium (now the periosteum) penetrate the bone collar, bringing osteoprogenitor cells to the porous central region.
• Along with the vasculature, newly formed osteoblasts move into all available spaces and produce woven bone.
• The process in the diaphysis forms the primary ossification center, beginning in many embryonic bones as early as the first trimester.

Epiphyseal Growth Plate

• The epiphyseal cartilage is responsible for the growth in length of the bone and disappears upon completion of bone development at adulthood.
• Elimination of these epiphyseal plates (“epiphyseal closure”) occurs at various times with different bones and by about age 20 is complete in all bones.
• The epiphyseal growth plate shows distinct regions of cellular activity, including:
  • The zone of reserve (or resting) cartilage
  • The proliferative zone
  • The zone of hypertrophy
  • The zone of calcified cartilage
  • The zone of ossification

Bone Growth

• Longitudinal growth of a bone occurs by cell proliferation in the epiphyseal plate cartilage.
• At the same time, chondrocytes in the diaphysis side of the 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.
• Growth in the circumference of long bones does not involve endochondral ossification but occurs through the activity of osteoblasts developing from osteoprogenitor cells in the periosteum by a process of appositional growth.

Learn about the two processes of bone development: intramembranous ossification and endochondral ossification, and how osteoblasts produce osteoid in each process.