Intramembranous ossification is one of the two essential processes during fetal development of the gnathostome (excluding chondrichthyans such as sharks) skeletal system by which rudimentary bone tissue is created.
Intramembranous ossification is also an essential process during the natural healing of bone fractures and the rudimentary formation of bones of the head.
thumb|264px|right|[[Transmission electron microscopy|Transmission electron micrograph of a mesenchymal stem cell that is displaying typical 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 cells 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 morphological characteristics of a MSC are: A small cell body with a few cell processes that are long and thin; a large, round 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 fibrils.
- Flat bones of the face
- Most of the bones of the skull
- Clavicles
Other bone that formed by intramembranous ossification are: cortices of tubular and flat bones as well as the calvaria, upper facial bones, tympanic temporal bone, vomer, and medial pterygoid process.
See also
- Ossification
Footnotes
References
de:Ossifikation#Desmale Ossifikation