Matrix

The matrix of osseous tissue is, by dry weight, about one-third organic and two-thirds inorganic matter. The organic matter includes collagen and various protein-carbohydrate complexes such as glycosaminoglycans, proteoglycans, and glycoproteins. The inorganic matter is about 85% hydroxyapatite, a crystallized calcium phosphate salt [Ca10(PO4)6(OH)2], 10% calcium carbonate (CaCO3), and lesser amounts of magnesium, sodium, potassium, fluoride, sulfate, carbonate, and hydroxide ions. Several foreign elements behave chemically like bone minerals and become incorporated into osseous tissue as contaminants, sometimes with deadly results (see insight 7.1).

13 canal = canal, channel + icul = little

14 osteo = bone + clast = destroy, break down

15 J. Howship (1781-1841), English surgeon

Saladin: Anatomy & I 7. Bone Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

222 Part Two Support and Movement

Osteogenic cells

Osteoblasts

Osteocyte

Osteogenic cells

Osteoblasts

Osteocyte

Osteogenic Cells
Figure 7.3 Bone Cells and Their Development. (a) Osteogenic cells give rise to osteoblasts, which deposit matrix around themselves and transform into osteocytes. (b) Bone marrow stem cells fuse to form osteoclasts.

Insight 7.1 Medical History

Bone is somewhat like a fiberglass fishing rod. Fiberglass is a composite made of a ceramic (glass fibers) embedded in a polymer (resin). The polymer alone would be too flexible and limp to serve the purpose of a fishing rod, while the ceramic alone would be too brittle. The composite of the two, however, gives a fishing rod strength and flexibility.

Bone is also a composite of a ceramic (mineral) and a polymer (protein). The mineral component enables bone to support the weight of the body without sagging. If the minerals are dissolved out of a bone with acid, the remaining bone becomes rubbery. When the bones are deficient in calcium salts, they are soft and bend easily. This is the central problem in the childhood disease rickets, in which the soft bones of the lower limbs bend under the body's weight and become permanently deformed.

The protein component gives bone a degree of flexibility. Without protein, a bone is excessively brittle, as in osteogenesis imperfecta, or brittle bone disease (see table 7.4). Without collagen, a jogger's bones would shatter under the impact of running. But normally, when a bone bends slightly toward one side, the tensile strength of the collagen fibers on the opposite side holds the bone together and prevents it from snapping like a stick of chalk.

Unlike fiberglass, bone varies from place to place in its ratio of ceramic to polymer. Osseous tissue is thus adapted to different amounts of tension and compression exerted on different parts of the skeleton.

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