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Ossification (also calledosteogenesis orbone mineralization) inbone remodeling is the process of laying down newbone material bycells namedosteoblasts. It is synonymous withbone tissue formation.[1] There are two processes resulting in the formation of normal,healthybone tissue:[2]Intramembranous ossification is the direct laying down of bone into the primitive connective tissue (mesenchyme), whileendochondral ossification involvescartilage as a precursor.
Infracture healing, endochondralosteogenesis is the most commonly occurring process, for example in fractures of longbones treated byplaster of Paris, whereas fractures treated byopen reduction and internal fixation withmetal plates,screws, pins, rods and nails may heal by intramembranousosteogenesis.
Heterotopic ossification is a process resulting in the formation ofbone tissue that is often atypical, at an extraskeletal location.Calcification is often confused with ossification. Calcification is synonymous with the formation ofcalcium-based salts and crystals withincells andtissue. It is a process that occurs during ossification, but not necessarilyvice versa.
The exact mechanisms by whichbone development is triggered remains unclear, butgrowth factors andcytokines appear to play a role.
| Time period[3] | Bones affected[3] |
|---|---|
| Third month of fetal development | Ossification in long bones beginning |
| Fourth month | Most primaryossification centers have appeared in the diaphyses of bone. |
| Birth to five years | Secondary ossification centers appear in the epiphyses |
| five 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 andscapulae becoming completely ossified |
| 18 to 23 years | Bone of the lower limbs andos coxae become completely ossified |
| 23 to 26 years | Bone of thesternum,clavicles, andvertebrae become completely ossified |
| By 25 years | Nearly all bones are completely ossified |
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Intramembranous ossification forms theflat bones of theskull,mandible andhip bone.
Osteoblasts cluster together to create an ossification center. They then start secreting osteoid, an unmineralized collagen-proteoglycan matrix that has the ability to bind calcium. As calcium binds to the osteoid, the matrix hardens, and the osteoblasts become entrapped, transforming into osteocytes.
As osteoblasts continue to secreteosteoid, it surrounds blood vessels, leading to the formation of trabecular (cancellous or spongy) bone. These blood vessels will eventually develop into red bone marrow. Mesenchymal cells on the bone surface form a membrane known as the periosteum. Osteoblasts secrete osteoid in parallel with the existing matrix, creating layers of compact (cortical) bone.[4]
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Endochondral ossification is the formation of long bones and other bones.This requires ahyaline cartilage precursor. There are two centers of ossification forendochondral ossification.
The primary center
In long bones, bone tissue first appears in thediaphysis (middle of shaft).Chondrocytes multiply and form trebeculae. Cartilage is progressively eroded and replaced by hardened bone, extending towards theepiphysis. Aperichondrium layer surrounding the cartilage forms theperiosteum, which generates osteogenic cells that then go on to make a collar that encircles the outside of the bone and remodels the medullary cavity on the inside.
The nutrient artery enters via thenutrient foramen from a small opening in the diaphysis. It invades the primary center of ossification, bringing osteogenic cells (osteoblasts on the outside,osteoclasts on the inside.) The canal of thenutrient foramen is directed away from more active end of bone when one end grows more than the other. When bone grows at same rate at both ends, the nutrient artery is perpendicular to the bone.
Most other bones (e.g.vertebrae) also have primary ossification centers, and bone is laid down in a similar manner.
Secondary centers
The secondary centers generally appear at theepiphysis. Secondary ossification mostly occurs after birth (except for distal femur and proximaltibia which occurs during 9th month of fetal development). The epiphyseal arteries and osteogenic cells invade theepiphysis, depositingosteoclasts andosteoblasts which erode the cartilage and build bone, respectively. This occurs at both ends of long bones but only one end of digits and ribs.
Several hypotheses have been proposed for how bone evolved as a structural element invertebrates. One hypothesis is that bone developed from tissues that evolved to storeminerals. Specifically, calcium-based minerals were stored in cartilage and bone was anexaptation development from this calcified cartilage.[5] However, other possibilities include bony tissue evolving as anosmotic barrier, or as a protective structure.
