Bone (osseous tissue) is the most rigid connective tissue. Its hardness is largely due to mineral salts, such as calcium phosphate and calcium carbonate, in its matrix. This intercellular material also contains a great amount of collagen, whose fibers flexibly reinforce the mineral components of bone.

Bone internally supports body structures. It protects vital structures in the cranial and thoracic cavities and is an attachment for muscles. Bone also contains red marrow, which forms blood cells, and it stores and releases inorganic salts.

Bone matrix is deposited by bone cells, osteocytes (osote-o-sitz), in thin layers called lamellae, which form concentric patterns around capillaries located within tiny longitudinal tubes called central (Haversian) canals. Osteocytes are located in lacunae that are rather evenly spaced between the lamellae. Consequently, osteocytes also form concentric circles (fig. 5.26).

In a bone, the osteocytes and layers of intercellular material, which are concentrically clustered around a central canal, form a cylinder-shaped unit called an osteon (oscte-on) (Haversian system). Many of these units cemented together form the substance of bone (see chapter 7, page 198).

Each central canal contains a blood vessel, so every bone cell is fairly close to a nutrient supply. In addition, the bone cells have many cytoplasmic processes that extend outward and pass through minute tubes in the matrix called canaliculi. Gap junctions attach these cellular processes to the membranes of nearby cells (see chapter 3, page 70). As a result, materials can move rapidly between blood vessels and bone cells. Thus, in spite of its inert appearance, bone is a very active tissue. Injured bone heals much more rapidly than does injured cartilage.

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