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Osteoblast (from the Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cells with single nuclei that synthesize bone. However, in the process of bone formation, osteoblasts function in groups of connected cells. Individual cells cannot make bone, and the group of organized osteoblasts together with the bone made by a unit of cells is usually called the osteon; the basis of this is discussed in "Organization and ultrastructure of osteoblasts" below. Osteoblasts are specialized, terminally differentiated products of mesenchymal stem cells.〔Pittenger MF, Mackay AM, Beck SC, Jaiswal RK, Douglas R, Mosca JD, Moorman MA, Simonetti DW, Craig S, Marshak DR. (1999). "Multilineage potential of adult human mesenchymal stem cells". ''Science''. 284: 143-7. PMID 10102814.〕 They synthesize very dense, crosslinked collagen, and several additional specialized proteins in much smaller quantities, including osteocalcin and osteopontin, which compose the organic matrix of bone. In organized groups of connected cells, osteoblasts produce a calcium and phosphate-based mineral that is deposited, in a highly regulated manner, into the organic matrix forming a very strong and dense mineralized tissue - the mineralized matrix. This is further discussed in "Mineralization of bone" below. The mineralized skeleton is the main support for the bodies of air breathing vertebrates. It also is an important store of minerals for physiological homeostasis including both acid-base balance and calcium or phosphate maintenance.〔Arnett T. (2003). Regulation of bone cell function by acid-base balance. Proc Nutr Soc. 62: 511-20. PMID 14506899.〕〔Blair HC, Zaidi M, Huang CL, Sun L. (2008). The developmental basis of skeletal cell differentiation and the molecular basis of major skeletal defects. Biol Rev Camb Philos Soc. 83: 401-15. doi: 10.1111/j.1469-185X.2008.00048.x. PMID 18710437.〕 ==Definitions of bone-forming and -degrading cells== Osteoblasts are the major cellular component of bone. The functional part of bone, the ''bone matrix'', is entirely extracellular (outside of the cells). The bone matrix consists of protein and mineral. The protein is called the ''organic matrix''; it is synthesized first, and then the mineral is added. The vast majority of the organic matrix is collagen, which provides tensile strength. Then the matrix is mineralized by deposition of a calcium-phosphate-hydroxide salt called hydroxyapatite (alternative name, hydroxylapatite). This mineral is very hard, and provides compressive strength. Thus, the collagen and mineral together are a composite material with excellent tensile and compressive strength, which can bend under a strain and recover its shape without damage. This is called elastic deformation. Forces that exceed the capacity of bone to behave elastically may cause failure, typically bone fractures. Normally, almost all of the bone matrix, in the air breathing vertebrates, is mineralized by the osteoblasts. Before the organic matrix is mineralized, it is called osteoid. Osteoblasts that are buried in matrix are called osteocytes. During active bone formation, the surface layer of osteoblasts consists of cuboidal cells, called ''active osteoblasts''. When the bone-forming unit is not actively synthesizing bone, the surface osteoblasts are flattened and are called ''inactive osteoblasts''. Osteocytes remain alive and are connected by cell processes to a surface layer of osteoblasts. Osteocytes have important functions in maintenance of the skeleton. Bone is a dynamic tissue that is constantly being reshaped by osteoblasts, which produce and secrete matrix proteins and transport mineral into the matrix, and osteoclasts, which break down the tissue. The osteoclasts are the only cell type other than osteoblasts and osteocytes that are structural components of bone, although within the hollow inside of bones are many other cell types of the bone marrow. Components of the bone marrow that are essential for osteoblast bone formation include mesenchymal stem cells, the precursors of osteoblasts, and blood vessels which supply oxygen and nutrients for bone formation. Bone is a highly vascular tissue, and active formation of blood vessel cells, also from mesenchymal stem cells, is essential to support the metabolic activity of bone. The balance of bone formation and bone resorption tends to be negative with age, particularly in post-menopausal women,〔Nicks KM, Fowler TW, Gaddy D. (2010). "Reproductive hormones and bone." Curr Osteoporos Rep. 8: 60-7. doi: 10.1007/s11914-010-0014-3. PMID 20425612.〕 often leading to a loss of bone serious enough to cause fractures, which is called osteoporosis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Osteoblast」の詳細全文を読む スポンサード リンク
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