Bone is mineralized connective tissue that during development may start as cartilage and become mineralized (endochondral ossification) or arise in mineralization centres within mesodermal tissue as in the case of the skull bones. Dysostosis, hypophosphatasia, and rickets are failures of mineralization (see LIM mineralization protein; vitamin D). Excessive mineralization is a feature of osteopetrosis. The organic matrix is of type I collagen (defective in Caffey's disease), glycosaminoglycans, matrix Gla protein, osteocalcin, osteonectin, osteopontin, and tenascin W. Osteoblasts are involved in synthesis and remodelling of bone; osteoclasts break down bone and are regulated by osteoclast differentiation factor, osteoprotegerin, and parathyroid hormone. Many skeletal abnormalities are a result of developmental defects in bone morphogenesis, which is under the control of various hormones and growth factors (especially bone morphogenetic proteins, BMPs). Antagonists of BMPs affect bone development and include chordin, gremlin, noggin, and sclerostin. Bone disorders include acromegaly, acromesomelic dysplasia, ankylosing spondylitis, atelosteogenesis, brachydactyly, campomelic dysplasia, Camurati–Engelmann disease, various chondrodysplasias, ectromelia, hypochondrogenesis, Mona, osteodystrophia fibrosa, otosclerosis, Paget's disease of bone, Rothmund–Thomson syndrome, Saethre–Chotzen syndrome, sclerosteosis, and van Buchem's disease. Age-related disorders include osteoporosis, the latter often being treated with bisphosphonates. Neoplastic disorders affecting bone include chordoma, enchondroma, Ewing's sarcoma, exostosis, and sarcoma (osteosarcoma). See also progressive osseous heteroplasia.
Subjects: Medicine and Health.