OssificationOssification is the formation of new bones. There are two major forms of ossification: intramembranousand endochondral.Intramembranous OssificationIntramembranous ossification produces the flat bones of the skull and the clavicle. Embryonically, it begins as a flat sheet of soft tissue called mesenchyme. This mesenchyme condenses and transforms into a network of soft trabeculae. Osteoblasts gather on the trabeculae to form osteoid tissue. Calcium phosphate is deposited in the osteoid tissue, transforming the osteoblasts into osteocytes. The last steps include osteoclast remodeling the center of the flat bone to contain marrow spaces and osteoblasts remodeling the surface to form compact bone. The remaining mesenchyme at the surface gives rise to periosteum. The significant features of this process is that it occurs simultaneously throughout the flat bones and it results in an internal and external layer of compact bone.
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Intramembranous ossification, ossification center