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The bone, cartilage, tendons, and ligaments of the skeletal system are all connective tissues. Their characteristics are largely deter-mined by the composition of their extracellular matrix. The matrix always contains collagen, ground substance, and other organic molecules, as well as water and minerals. But the types and quan-tities of these substances differ in each type of connective tissue.
Collagen (kol′lă-jen;koila,glue+-gen,producing) is atough, ropelike protein. Proteoglycans (prō ′ tē -ō -glı̄ ′ kanz; proteo, protein + glycan, polysaccharide) are large molecules consisting of polysaccharides attached to core proteins, similar to the way needles of a pine tree are attached to the tree’s branches. The proteoglycans form large aggregates, much as pine branches combine to form a whole tree. Proteoglycans can attract and retain large amounts of water between their polysaccharide “needles.”
The extracellular matrix of tendons and ligaments contains large amounts of collagen fibers, making these structures very tough, like ropes or cables. The extracellular matrix of cartilage (kar′ ti-lij) contains collagen and proteoglycans. Collagen makes cartilage tough, whereas the water-filled proteoglycans make it smooth and resilient. As a result, cartilage is relatively rigid, but it springs back to its original shape after being bent or slightly compressed. It is an excellent shock absorber.
The extracellular matrix of bone contains collagen and min-erals, including calcium and phosphate. The ropelike collagen fibers, like the reinforcing steel bars in concrete, lend flexible strength to the bone. The mineral component, like the concrete itself, gives the bone compression (weight-bearing) strength. Most of the mineral in bone is in the form of calcium phosphate crystals called hydroxyapatite (hı̄ -drok′ s̄e -ap-ă -tı̄ t).
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