Bone Formation: Endochondrial and Intramembranous Ossification

Bone Formation

• Most bones in the body form within hyaline cartilage that develops from mesenchyme and replaces cartilage with osseous tissue.

Intramembranous Ossification

• Has four steps:
  • Ossification centers form where osteoblasts differentiate and begin secreting bone matrix.
  • Calcification occurs as osteocytes extend out into the osteon's canaliculi and deposit minerals that calcify.
  • Trabeculae formation creates spongy bone with abundant blood vessels and red bone marrow.
  • Periosteum development around the periphery of the spongy bone creates a compact bone to surround the underlying spongy bone.

Endochondral Ossification

• Replaces cartilage with bone matrix for most bones.
• Process includes:
  • Cartilage model development as chondroblasts secrete cartilage matrix to form a cartilage model.
  • Cartilage model growth as the matrix calcifies and chondroblasts die, creating lacunae.
  • Primary ossification center develops as a blood vessel stimulates cells to become osteoblasts, replacing cartilage matrix with bone matrix.
  • Ossification grows exponentially towards both ends of the cartilage model.

Endochondral Ossification (continued)

• Process continues with:
  • Medullary cavity development as osteoclasts digest spongy bone to form the medullary cavity inside the diaphysis.
  • Secondary ossification centers form as blood vessels enter the epiphyses, with ossification traveling from the inner to the outer surface.
  • Articular cartilage formation from hyaline cartilage and the epiphyseal plate is the "growth plate" for long bones.

Epiphyseal Plate

• Consists of four zones:
  • Zone of resting cartilage.
  • Zone of proliferation cartilage.
  • Zone of hypertrophic cartilage.
  • Zone of calcified cartilage.

Bone Growth

• Growth in length occurs at the epiphyseal plate.
• Two major events:
  • Growth of cartilage (epiphyseal side of plate).
  • Replacement of cartilage with osseous tissue (diaphyseal side of plate).

Growth in Width/Thickness

• Occurs through deposition of osseous tissue on the outer surface of bone (periosteum) by osteoblasts.
• As new osseous tissue is deposited, osteoclasts destroy osseous tissue lining the medullary cavity.

Comparison of Growth

• Growth in length vs growth in width of bone:
  • Growth in length occurs at the epiphyseal plate.
  • Growth in width occurs through deposition and resorption of bone tissue.