Intramembranous ossification

Intramembranous ossification is a process of bone formation where the bone tissue is directly created from mesenchymal tissue. This process is different from endochondral ossification, where bone is formed by replacing cartilage. Intramembranous ossification primarily occurs during the formation of the flat bones of the skull but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage.

Process

The process of intramembranous ossification begins when mesenchymal cells gather together in the areas of future bones. These cells then differentiate into osteoblasts, which begin to secrete the components of the bone matrix. As the matrix surrounds and isolates the osteoblasts, they become osteocytes, the cells contained within the bone matrix.

Stages

Intramembranous ossification follows four main stages:

1. Mesenchymal cell differentiation: Mesenchymal cells group together, usually in response to certain growth factors, and start to become specialized.
2. Bone matrix deposition: The newly-formed osteoblasts begin to secrete bone matrix, which is initially unmineralized.
3. Matrix mineralization: Calcium and other minerals are deposited into the matrix, hardening the bone.
4. Formation of trabeculae: The small struts of bone are joined together to form spongy bone. Eventually, some of this bone will be replaced with compact bone.

Clinical significance

Abnormalities in intramembranous ossification can lead to a number of diseases, including cleidocranial dysostosis and hypophosphatasia. These conditions can result in a number of symptoms, including craniosynostosis, where the bones of the skull fuse prematurely.

See also

• Endochondral ossification
• Bone development
• Osteoblast
• Osteocyte

References

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  Mesenchymal stem cells at high magnification

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  Bony nidus formation 3