Ossification

Bone is broken down by osteoclasts, and rebuilt by osteoblasts, both of which communicate through cytokine (TGF-β, IGF) signalling.

Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation.^[1] There are two processes resulting in the formation of normal, healthy bone tissue:^[2] Intramembranous ossification is the direct laying down of bone into the primitive connective tissue (mesenchyme), while endochondral ossification involves cartilage as a precursor.

In fracture healing, endochondral osteogenesis is the most commonly occurring process, for example in fractures of long bones treated by plaster of Paris, whereas fractures treated by open reduction and internal fixation with metal plates, screws, pins, rods and nails may heal by intramembranous osteogenesis.

Heterotopic ossification is a process resulting in the formation of bone tissue that is often atypical, at an extraskeletal location. Calcification is often confused with ossification. Calcification is synonymous with the formation of calcium-based salts and crystals within cells and tissue. It is a process that occurs during ossification, but not necessarily vice versa.

The exact mechanisms by which bone development is triggered remains unclear, but growth factors and cytokines appear to play a role.

Time period^[3]	Bones affected^[3]
Third month of fetal development	Ossification in long bones beginning
Fourth month	Most primary ossification centers have appeared in the diaphyses of bone.
Birth to 5 years	Secondary ossification centers appear in the epiphyses
5 years to 12 years in females, 5 to 14 years in males	Ossification is spreading rapidly from the ossification centers and various bones are becoming ossified.
17 to 20 years	Bone of upper limbs and scapulae becoming completely ossified
18 to 23 years	Bone of the lower limbs and os coxae become completely ossified
23 to 26 years	Bone of the sternum, clavicles, and vertebrae become completely ossified
By 25 years	Nearly all bones are completely ossified

^ "bone formation | Definition & Physiology". Encyclopedia Britannica. Retrieved 2021-01-22.
^ Caetano-Lopes J, Canhão H, Fonseca JE (2007). "Osteoblasts and bone formation". Acta reumatológica portuguesa. 32 (2): 103–10. PMID 17572649.
^ ^a ^b Emily Morey-Holton. "Predicting Height from the Length of Limb Bones". Examining Effects of Space Flight on the Skeletal System. Moffett Field, California: NASA Ames Research Center. Archived from the original on 2012-03-01.

[1] "bone formation | Definition & Physiology". Encyclopedia Britannica. Retrieved 2021-01-22.

[2] Caetano-Lopes J, Canhão H, Fonseca JE (2007). "Osteoblasts and bone formation". Acta reumatológica portuguesa. 32 (2): 103–10. PMID 17572649.

[Morey-Holton-3] Emily Morey-Holton. "Predicting Height from the Length of Limb Bones". Examining Effects of Space Flight on the Skeletal System. Moffett Field, California: NASA Ames Research Center. Archived from the original on 2012-03-01.

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