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Intramembranous ossification is one of the two essential processes during fetal development of the gnathostome (excluding chondrichthyans such as sharks) skeletal system by which rudimentary bone tissue is created. Intramembranous ossification is also an essential process during the natural healing of bone fractures〔Brighton, Carl T. and Robert M. Hunt (1991), "Early histological and ultrastructural changes in medullary fracture callus", ''Journal of Bone and Joint Surgery'', 73-A (6): 832-847〕 and the rudimentary formation of bones of the head.〔Netter, Frank H. (1987), ''Musculoskeletal system: anatomy, physiology, and metabolic disorders''. Summit, New Jersey: Ciba-Geigy Corporation ISBN 0-914168-88-6, p.129〕 Unlike endochondral ossification, which is the other process by which bone tissue is created during fetal development, cartilage is not present during intramembranous ossification. ==Creation of bone tissue== Mesenchymal stem cells within mesenchyme or the medullary cavity of a bone fracture initiate the process of intramembranous ossification. A mesenchymal stem cell, or MSC, is an unspecialized cell which can develop into an osteoblast. Before it begins to develop, the morphological characteristics of a MSC are: a small cell body with a few cell processes that are long and thin; a large, round nucleus with a prominent nucleolus that is surrounded by finely dispersed chromatin particles, giving the nucleus a clear appearance; and a small amount of Golgi apparatus, rough endoplasmic reticulum, mitochondria, and polyribosomes. Furthermore, the mesenchymal stem cells are widely dispersed within an extracellular matrix that is devoid of every type of collagen, except for a few reticular fibrils.〔 The process of intramembranous ossification starts when a small group of adjacent MSCs begin to replicate and form a small, dense cluster of cells, a nidus. Once a nidus has been formed the MSCs within it stop replicating. At this point, morphological changes in the MSCs begin to occur: the cell body becomes larger and rounder; the long, thin cell processes are no longer present; and the amount of Golgi apparatus and rough endoplasmic reticulum increases. Eventually, all of the cells within the nidus develop into, and display the morphologic characteristics of, an osteoprogenitor cell.〔 At this stage of development, changes in the morphology of the osteoprogenitor cells occur: their shape becomes more columnar and the amount of Golgi apparatus and rough endoplasmic reticulum increases. Eventually, all of the cells within the nidus develop into, and display the morphologic characteristics of, an osteoblast. Then the osteoblasts create an extracellular matrix containing Type-I collagen fibrils, which is osteoid. The osteoblasts, while lining the periphery of the nodule, continue to form osteoid in the center of the nidus. Some of the osteoblasts become incorporated within the osteoid to become osteocytes.〔 At this point, the osteoid becomes mineralized resulting in a nidus consisting of mineralized osteoid that contains osteocytes and is lined by active osteoblasts. The nidus, that began as a diffuse collection of MSCs, has become rudimentary bone tissue.〔 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「intramembranous ossification」の詳細全文を読む スポンサード リンク
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