Editing Intramembranous ossification

{{short description|Mesenchymal bone development that forms the non-long bones}}
'''Intramembranous ossification''' is one of the two essential processes during [[fetal]] development of the [[gnathostome]] (excluding [[Chondrichthyes|chondrichthyans]] such as [[shark]]s) [[skeleton|skeletal system]] by which rudimentary [[bone tissue]] is created.
Intramembranous ossification is also an essential process during the natural healing of [[bone healing|bone fractures]]<ref name= "Brighton">{{cite journal |author1=Brighton, Carl T. |author2=Robert M. Hunt |year=1991 |title=Early histological and ultrastructural changes in medullary fracture callus |journal=[[Journal of Bone and Joint Surgery]] |volume=73-A |issue=6 |pages=832-847}}</ref> and the rudimentary formation of [[bone]]s of the [[Human skull|head]].<ref name="Netter">{{cite book |author-link=Frank H. Netter |author=Netter, Frank H. |year=1987 |title=Musculoskeletal system: anatomy, physiology, and metabolic disorders |place=Summit, New Jersey |publisher=Ciba-Geigy Corporation |ISBN=0-914168-88-6 |page=129}}</ref>

[[Image:MSC high magnification.jpg|thumb|264px|right|[[Transmission electron microscopy|Transmission]] [[electron micrograph]] of a [[mesenchymal stem cell]] that is displaying typical [[Ultrastructure|ultrastructural]] characteristics.]]
Unlike [[endochondral ossification]], which is the other process by which bone tissue is created during fetal development, [[cartilage]] is not present during intramembranous ossification.

==Formation of woven bone==
[[Mesenchymal stem cell]]s within [[mesenchyme]] or the [[medullary cavity]] of a bone fracture initiate the process of intramembranous ossification. A mesenchymal stem cell, or MSC, is an unspecialized cell that can develop into an [[osteoblast]]. Before it begins to develop, the [[Morphology (biology)|morphological]] characteristics of a MSC are: A small [[cell body]] with a few cell processes that are long and thin; a large, round [[Cell nucleus|nucleus]] with a prominent [[nucleolus]] that is surrounded by finely dispersed [[chromatin]] particles, giving the nucleus a clear appearance; and a small amount of [[Golgi apparatus]], [[rough endoplasmic reticulum]], [[mitochondria]], and [[polyribosomes]]. Furthermore, the mesenchymal stem cells are widely dispersed within an [[extracellular matrix]] that is devoid of every type of [[collagen]], except for a few [[reticular fiber|reticular]] fibrils.<ref name= "Brighton"/>

[[Image:Bony nidus 1.jpg|thumb|right|264px|[[Light micrograph]] of a nidus consisting of [[osteoprogenitor]] cells that are displaying a prominent [[Golgi apparatus]].]]
The process of intramembranous ossification starts when a small group of adjacent MSCs begin to [[Replicate (biology)|replicate]] and form a small, dense cluster of cells that is called a ''nidus''.{{efn|''Nidus'' is Latin for "nest". In tissue, a ''nidus'' resembles a nest morphologically, with respect to its appearance, and functionally, because it is a site where cell development occurs.}} Once a nidus has been formed the MSCs within it stop replicating.
At this point, morphological changes in the MSCs begin to occur: The cell body is now larger and rounder; the long, thin cell processes are no longer present; and the amount of Golgi apparatus and rough endoplasmic reticulum increases. Eventually, all of the cells within the nidus develop into, and display the morphologic characteristics of, an [[osteoprogenitor|osteoprogenitor cell]].<ref name= "Brighton" />

[[Image:Bony nidus 2.jpg|thumb|left|264px|[[Light micrograph]] of a nidus consisting of  [[osteoblast]]s, many are  displaying a prominent [[Golgi apparatus]], that have created [[osteoid]] at its center.]]
{{nowrap|At this stage of development,}} changes in the morphology of the osteoprogenitor cells occur: Their shape becomes more columnar and the amount of Golgi apparatus and rough endoplasmic reticulum increases. Eventually, all of the cells within the nidus develop into, and display the morphologic characteristics of, an [[osteoblast]].
Then the osteoblasts create an extracellular matrix containing [[Type-I collagen]] fibrils, which is [[osteoid]]. The osteoblasts, while lining the periphery of the nidus, continue to form osteoid in the center of the nidus. Some of the osteoblasts become incorporated within the osteoid to become [[osteocyte]]s.<ref name= "Brighton" />

[[Image:Bony nidus 3.jpg|thumb|right|264px|[[Light micrograph]] of an undecalcified nidus consisting of rudimentary [[bone tissue]] that is lined by numerous [[osteoblast]]s.]]
At this point, the osteoid becomes mineralized resulting in a nidus consisting of mineralized osteoid that contains osteocytes and is lined by active osteoblasts. The nidus, that began as a diffuse collection of MSCs, has developed into woven bone, the most rudimentary [[bone tissue]].<ref name= "Brighton" />

==Formation of lamellar bone==
The first step in the process is the formation of bone [[Process (anatomy)|spicules]] which eventually fuse with each other and become [[trabeculae]]. The [[periosteum]] is formed and bone growth continues at the surface of trabeculae. Much like spicules, the increasing growth of trabeculae result in interconnection and this network is called ''woven bone''. Eventually, woven bone is replaced by ''[[lamellar bone]]''.

===Formation of bone spicules===
[[Embryo]]logic [[mesenchymal cell]]s (MSC) condense into layers of vascularized primitive [[connective tissue]]. Certain  mesenchymal cells group together, usually near or around blood vessels, and differentiate into osteogenic cells which deposit bone [[Matrix (biology)|matrix]] [[Secretory pathway|constitutively]]. These aggregates of bony [[Matrix (biology)|matrix]] are called bone spicules. Separate mesenchymal cells differentiate into [[osteoblast]]s, which line up along the surface of the spicule and secrete more [[osteoid]], which increases the size of the spicule.

===Formation of trabecular bone===
As the spicules continue to grow, they fuse with adjacent spicules and this results in the formation of ''[[trabecula]]e''. When [[osteoblast]]s become trapped in the matrix they secrete, they differentiate into [[osteocyte]]s. Osteoblasts continue to line up on the surface which increases the size. As growth continues, trabeculae become interconnected and ''trabecular bone'' is formed. The term ''primary spongiosa'' is also used to refer to the initial trabecular network.

===Primary centre of ossification===
The ''[[periosteum]]'' is formed around the trabeculae by differentiating mesenchymal cells. The primary center of ossification is the area where bone growth occurs between the [[periosteum]] and the bone. Osteogenic cells that originate from the periosteum increase appositional growth and a [[bone collar]] is formed. The bone collar is eventually mineralized and ''lamellar bone'' is formed.

===Formation of osteons===
[[Osteon]]s are components or principal structures of compact bone. During the formation of bone spicules, [[cytoplasmic]] processes from osteoblasts interconnect. This becomes the '''canaliculi''' of osteons. Since bone spicules tend to form around [[blood vessel]]s, the perivascular space is greatly reduced as the bone continues to grow. When replacement to compact bone occurs, this blood vessel becomes the central canal of the osteon.

== Examples in the human body ==
The following bones develop in humans via ''Intramembranous ossification'':<ref>{{Cite web |url=https://opentextbc.ca/anatomyandphysiology/chapter/6-4-bone-formation-and-development/ |title=6.4 Bone Formation and Development – Anatomy and Physiology |website=opentextbc.ca |language=en-US |access-date=2018-05-05 |df=dmy-all}}</ref> 
* [[Facial skeleton|Flat bones]] of the face
* Most of the bones of the [[skull]]
* [[Clavicle|Clavicles]]

Other bone that formed by intramembranous ossification are: cortices of tubular and flat bones as well as the [[calvaria (skull)|calvaria]], upper facial bones, tympanic temporal bone, vomer, and medial pterygoid process.<ref>{{Cite journal |last=Ihde |first=Lauren L. |last2=Forrester |first2=Deborah M. |last3=Gottsegen |first3=Christopher J. |last4=Masih |first4=Sulabha |last5=Patel |first5=Dakshesh B. |last6=Vachon |first6=Linda A. |last7=White |first7=Eric A. |last8=Matcuk |first8=George R. |date=November 2011|title=Sclerosing Bone Dysplasias: Review and Differentiation from Other Causes of Osteosclerosis |url=http://pubs.rsna.org/doi/10.1148/rg.317115093 |journal=RadioGraphics |language=en |volume=31 |issue=7 |pages=1865–1882 |doi=10.1148/rg.317115093 |issn=0271-5333|url-access=subscription }}</ref>

== See also ==
*[[Ossification]]

==Footnotes==
{{notelist|1}}

==References==
{{Reflist|25em}}

* {{cite book |author1=Martin, R.B. |author2=Burr, D.B. |author3=Sharkey, N.A. |year=1998 |title=Skeletal Tissue Mechanics |at=Chapter&nbsp;2 |publisher=Springer-Verlag}}

{{Bone and cartilage}}

[[Category:Vertebrate developmental biology]]
[[Category:Skeletal system]]

[[de:Ossifikation#Desmale Ossifikation]]