The hard connective tissue of which the skeleton of most vertebrates is formed. It comprises a matrix of collagen fibres (30%) impregnated with bone salts (70%), mostly calcium phosphate (hydroxyapatite, Ca10(PO4)6(OH)2), in which are embedded bone cells: osteoblasts (which secrete the matrix) and osteocytes. Bone generally replaces embryonic cartilage and is of two sorts—compact bone and spongy bone. The outer compact bone is formed as concentric layers (lamellae) that surround small holes (Haversian canals): see illustration. The inner spongy bone is chemically similar but forms a network of bony bars. The spaces between the bars may contain bone marrow or (in birds) air for lightness. Bone is resorbed by cells called osteoclasts, which work in dynamic interaction with the bone-forming osteoblasts to remodel bone during growth or repair, e.g. following injury or in response to changes in loads. See also bone morphogenetic protein; cartilage bone; membrane bone; periosteum.
Structure of compact bone
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Bone/Bone.htm Descriptions and images of bone tissue from the School of Anatomy and Human Biology, University of Western Australia
The skeletal tissue of vertebrates. Bone is composed of about 70% inorganic calcium salts, mostly hydroxyapatite but carbonate, citrate, and fluoride amines are also present. The organic component is mostly made up of the structural protein collagen.
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