Bone formation. Light micrograph of a longitudinal section through an epiphyseal growth plate in a long bone from a young cat. The long bones of a foetus are initially composed of cartilage, which is progressively replaced by bone in a process called endochondral ossification. The ossification begins in the centre of the bone and gradually spreads outwards. The growth plate marks the transition region where calcification is occuring. The cartillage cells will soon degenerate, leaving cavities called lacunae. Osteogenic cells and capillaries will then invade the lacunae to begin bone formation. Magnification: x250 at 35mm size.
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