Bone formation

Bone Formation occurs through three distinct mechanisms:

Examples:
- Long bones in utero.
- Longitudinal growth of long bones.
- Callus formation (the main type).
- Ossification of demineralized bone matrix.
Underlying Principle:
Bone replaces an initial cartilage model, but cartilage is not transformed directly into bone.
Embryonic Bone Formation:
- Starts at 6 weeks gestation.
- Mesenchymal anlage is invaded by osteoprogenitor cells, which differentiate into osteoblasts.
- This leads to the formation of primary ossification centers (physes), usually by the 8th week.
Growth Process:
- The fetal cartilage model increases in size through:
  - Appositional growth: Increase in width.
  - Interstitial growth: Increase in length.
Secondary Ossification Centers (epiphyses) form later, followed by:
- Formation of marrow: Resorption of the central cavity and invasion by myeloid precursors.
Blood Supply during this stage is rich and includes:
- Epiphyseal artery.
- Metaphyseal arteries.
- Perichondral arteries (primary supply to the physis).
- Longitudinal nutrient arteries.

Key Element:
Bone forms directly from undifferentiated cells, not from a pre-existing cartilage model.
Process:
- Undifferentiated cells align in layers (membranes) and differentiate into osteoblasts.
- Osteoblasts lay down bone matrix, which is then mineralized into bone.
Examples:
- Distraction Osteogenesis: A process used in limb lengthening.
- Periphery of fracture callus.
- Flat bones: Such as the pelvis, clavicle, and skull.

Function:
This process is responsible for the increase in width of bones.
Process:
- Osteoblasts align along the bone surface and lay down the bone matrix.
Examples:
- Periosteal bone formation: Occurs on the external surface of bones.
- Fracture healing in children.
- Bone remodeling: As per Wolff’s law, which states that bone adapts to mechanical stress, new bone is laid down on the concave aspect of a deformity.

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