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Bone, or osseous tissue, is characterized by its marble-like hardness, making it highly suitable to serve as the body’s supporting and protective skeletal framework. Despite its unique hardness, bone exhibits all the characteristics of a typical connective tissue. It has more extracellular material than cells, it is vascular, and it is well innervated. Collagen fibers are the most abundant fiber type in the extracellular matrix. Unlike other connective tissues, however, most of the interfibrillar substance of the matrix is replaced with hard calcium phosphate crystals, which are deposited on and between the collagen fibers. As with the preceding connective tissues, the distribution and arrangement of bone’s structural components account for its functional properties. This highly dynamic, living tissue has a combination of exceptional tensile strength (provided by collagen) and compressive strength (provided by the inorganic crystals), with significant resilience (also provided by the collagen). If bones were composed entirely of calcium phosphate crystals, they would be brittle, like pieces of chalk. But because they are composite tissues they have incredible strength. Bones have the structural strength approaching that of reinforced concrete, yet they are not brittle and are much lighter in weight as a result of the structural blending of a collagen-rich organic matrix hardened by precipitation of inorganic crystals. Bone has a remarkable growth and regenerative capacity, which is essential to its function as a supportive skeletal tissue. Effective mending permits bone to resume its weight-bearing responsibilities and to participate in accomplishing body movements. In addition to its function as a support tissue, bone also provides protective functions (such as the skull surrounding and protecting the fragile brain) and serves as a storehouse for calcium. When the plasma concentration of calcium starts to fall too low, some of the calcium is “borrowed” from the bone to restore the plasma calcium level to normal. Like other connective tissues, the components of bone show various forms of arrangement and structure. This variability leads to two different subcategories of bone tissue, compact bone and spongy bone.


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