The development of a bone is traditionally classified as endochondral or intramembranous
The distinction between endochondral and intramembranous formation rests on whether a cartilage model serves as the precursor of the bone ^endochondral ossification,) or the bone is formed by a simpler method, without the intervention of a cartilage precursor ('intramembranous ossification,). The bones of the extremities and those parts of the axial skeleton that bear weight (e.g., vertebrae) develop by endochondral ossification. The flat bones of the skull and face, the mandible, and the clavicle develop by intramembranous ossification.
The existence of two distinct types of ossification does not imply that existing bone is either membrane bone or endochondral bone. These names refer only to the mechanism by which a bone is initially formed. Because of the remodeling that occurs later, the initial bone tissue laid down by endochondral formation or by intramembranous formation is soon replaced. The replacement bone is established on the preexisting bone by appositional growth and is identical in both cases. Although the long bones are classified as being formed by endochondral formation, their continued growth involves the histogenesis of both endochondral and intramembranous bone, with the latter occurring through the activity of the periosteal (membrane) tissue.
In intramembranous ossification, bone is formed by differentiation of mesenchymal cells into osteoblasts
The first evidence of intramembranous ossification occurs around the eighth week of gestation in humans. Some of the pale-staining, elongate mesenchymal cells within the mesenchyme migrate and aggregate in specific areas, the sites where bone is destined to form. This condensation of cells within the mesenchymal tissue is the membrane referred to in the term intramembranous ossification (Fig. 8.13). ^As the process continues, the newly organized tissue at the presumptive bone site becomes more vascularized, and the aggregated mesenchymal cells become larger and rounded. The cytoplasm of these cells changes from eosinophilic to basophilic, and a clear Golgi area becomes evident. These cytologic changes result in the differentiated osteoblast, which then secretes the collagens and other components of the bone matrix (osteoid). The osteoblasts within the bone matrix become increasingly separated from one another as the matrix is produced, but they remain attached by thin cytoplasmic processes. Because of the abundant collagen content, the bone matrix appears denser than the surrounding mesenchyme, in which the intercellular spaces reveal only delicate connective tissue fibers.
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