Intramembranous ossification

Bernard, G. W. An electron microscopic study of the initial intramembranous ossification. Amer. J. Anat. 125, 211 (1969)... 

Ossification is the process of the synthesis of bone from cartilage. There are two types of ossification—in-tramembranous and endochondral ossification. Bone may be synthesized by intramembranous ossification, endochondral ossification, or a combination of the two. 

Intramembranous ossification is the transformation of the mesenchyme, cells of an embryo into bone. During early development of vertebrate animals, the embryo consists of three primary cell layers ectoderm on the outside, mesoderm in the middle, and endoderm on the inside. Mesenchyme cells constitute part of the embryo s mesoderm and develop into connective tissue such as bone and blood. The bones of the skull derive directly from mesenchyme cells by intramembranous ossification. 

Intramembranous ossification is responsible for most of the mineralization of the skull, including the maxilla and mandible. It begins with the differentiation and activation of osteoblasts from fibroblast-related precursors within a region of connective tissue that demarcates where the bone will develop. The osteoblasts secrete a nonmineralized protein-rich (osteoid) matrix and, as they move away, the matrix mineralizes (Fig. 9.3a). The periosteum remains uncalcified and contains latent and undifferentiated osteoblasts for bone remodeling. Odontoblasts (Ob) and cementoblasts secrete an osteoid-like matrix similar to that of intramembraneous ossification. 

Paradoxically, whereas aging is associated with a decrease in bone formation [121], it is also linked to vascular calcification [122]. Vascular calcification can form by intramembranous ossification or by endochondral bone formation, and eventually lead to the appearance of mature bone, with bone marrow, in the vasculature... 

The bones of skull and rib have different developmental paths, resulting in different concentrations of calcium and sodium. The flat bones of the skull developed by intramembranous ossification and in an embryo 16 days old these fetal bones have begun to ossify and concentrate calcium (23). In contrast, rib bones develop by endochondral ossification. The rib bones of the embryos under examination are still primarily cartilage (23), which consists mainly of a matrix of collagen and proteoglycans (24) with a high concentration of Na ions. 

Tortelli, F., Tasso, R., Loiacono, F., and Cancedda, R. 2010. The development of tissue-engineered bone of different origin through endochondral and intramembranous ossification foUowmg the Implantation of mesenchymal stem cells and osteoblasts in a murine model. Biomaterials 31 242-9. 

Most bones arise from a combination of intramembranous and endochondral ossification. In this process, mesenchyme cells develop into chondroblasts and increase in number by cell division. Then, the chrondrob-lasts enlarge and excrete a matrix which hardens due to... 

Osteogenic differentiation induced by BMPs is dependent on the regulating growth factor as well as on the carrier specifications. Murata et al. [69] observed that in a synthetic HA carrier, BMP induced ossification was predominantly intramembranous, whereas enchondral bone formation was observed in a nonceramic carrier. Direct bone formation on BMP loaded Ca-P ceramics was also observed by others [70-73]. 

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