DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Foetal bone formation. Light micrograph of a longitudinal section through a foetal finger, showing bone formation (ossification). The finger bones (phalanges) are initially formed of cartilage. The ossification is endochondral (as opposed to intramembranous) and starts from the middle of the cartilage and proceeds outwards. Osteoblasts secrete collagen fibres to form a matrix with calcium salts, forming the trabeculae of spongy bone (red-orange mesh). The articular cartilage (yellow) at the ends of the bone, undergoes a secondary ossification, and also forms cartilage discs for the joint. Magnification: x2 when printed 10 centimetres wide.
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