Ossification is the process of laying down new bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 material by cells called osteoblast
Osteoblasts are mononucleate cells that are responsible for bone formation; in essence, osteoblasts are specialized fibroblasts that in addition to fibroblastic products, express bone sialoprotein and osteocalcin.Osteoblasts produce a matrix of osteoid, which is composed mainly of Type I collagen...

s. It is synonymous with bone tissue formation. There are two processes resulting in the formation of normal, healthy bone tissue: Intramembranous ossification
Intramembranous ossification
Intramembranous ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike endochondral ossification, which is the other process by which bone tissue is created, cartilage is not present during intramembranous...

 is the direct laying down of bone into the primitive connective tissue (mesenchyme
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...

), while endochondral ossification
Endochondral ossification
Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral...

 involves cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 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 stabilization by metal plate and screws may heal by intramembranous osteogenesis.

Heterotopic ossification
Heterotopic ossification
Heterotopic ossification is the process by which bone tissue forms outside of the skeleton.-Diagnosis:During the early stage, an x-ray will not be helpful because there is no calcium in the matrix...

 is a process resulting in the formation of bone tissue that is often atypical, at an extraskeletal location. Calcification
Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...

 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 vice versa.

The exact mechanisms by which bone development is triggered remains unclear, but it involves growth factor
Growth factor
A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

s and cytokines in some way.

Timetable for human ossification

Time period Bones affected
Third month of embryonic 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 ossifcation 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 25 years Bone of the sternum, clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...

s, and vertebrae become completely ossified
By 25 years Nearly all bones are completely ossified


Several hypotheses have been proposed for how bone evolved as a structural element in vertebrates. One hypothesis is that bone developed from tissues that evolved to store mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s. Specifically, calcium-based minerals were stored in cartilage and bone was an exaptation
Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour...

 development from this calcified cartilage. However, other possibilities include bony tissue evolving as an osmotic barrier, or as a protective structure.

See also

  • Dystrophic calcification
    Dystrophic calcification
    Dystrophic Calcification is the calcification occurring in degenerated or necrotic tissue, as in hyalinized scars, degenerated foci in leiomyomas, and caseous nodules. This occurs as a reaction to tissue damage, including as a consequence of medical device implantation.Dystrophic calcification can...

  • Mechanostat
    The Mechanostat is a model describing bone growth and bone loss. It was promoted by Harold Frost and described extensively in the Utah Paradigm of Skeletal Physiology in the 1960’s...

    , a model describing ossification and bone loss
  • Ossicone
    Ossicones are horn-like protuberances on the heads of giraffes, male okapis, and their extinct relatives, such as Sivatherium, and the climacoceratids, such as Climacoceras. Only giraffids have true ossicones...

    , the horn-like (or antler-like) protuberances on the heads of giraffes and related species
  • Osteogenesis imperfecta
    Osteogenesis imperfecta
    Osteogenesis imperfecta is a genetic bone disorder. People with OI are born with defective connective tissue, or without the ability to make it, usually because of a deficiency of Type-I collagen...

    , a juvenile bone disease
  • Fibrodysplasia ossificans progressiva
    Fibrodysplasia ossificans progressiva
    Fibrodysplasia ossificans progressiva , sometimes referred to as Stone Man Syndrome, is an extremely rare disease of the connective tissue. A mutation of the body's repair mechanism causes fibrous tissue to be ossified when damaged. In many cases, injuries can cause joints to become permanently...

    , an extremely rare genetic disease which causes fibrous tissue (muscle, tendon, ligament etc.) to ossify when damaged
  • Primrose syndrome
    Primrose syndrome
    Primrose syndrome is a rare, slowly progressive genetic disorder that can vary symptomatically between individual cases, but is generally characterised by ossification of the external ears, learning difficulties, and facial abnormalities. It was first described in 1982 in Scotland's Royal National...

    , a rare genetic disease in which cartilage becomes ossified.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.