Bone formation

The formation of bone is mostly by mineralization of a cartilage model, endochondral ossification and sometimes directly by intramembranous ossification.

1. Endochondral ossification is the principal system of bone formation and growth. Initially a cartilaginous model, derived from mesenchymal tissue condensation, is formed. Vascular invasion, with osteoprogenitor cells differentiating into osteoblasts, leads to the formation of a primary centre of ossification. The epiphysis is a secondary centre at the bone ends, leaving an area between for longitudinal growth - the physis.

The physis consists of three zones for chondrocyte growth and transformation:

• The reserve zone is where resting cells store products required for subsequent maturation. This zone is on the epiphyseal side of the physis. Gaucher's disease affects this zone.

• The proliferative zone is an area of cell proliferation and matrix formation, with longitudinal growth as the chondrocytes become organized or stacked in an area of high oxygen tension and high proteoglycan concentration. Achondroplasia is a defect in this zone.

• The hypertrophic zone has three areas of maturation, degeneration and provisional calcification. In this zone, cells first accumulate calcium as they increase in size and then die, releasing calcium for mineralization. Osteoblasts are active in this zone, with calcification of the cartilage, promoted by low oxygen tension and low proteoglycan. Mucopolysaccharide diseases affect the zone, with chondrocyte degeneration, and in rickets there is reduced provisional calcification. Physeal fractures occur through the zone of provisional calcification.

2. Intramembranous ossification is a process of bone formation without a cartilage precursor during embryonic life, and in some areas during fracture healing. Mesenchymal cells aggregate into layers or membranes, followed by osteoblast invasion and bone formation.

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