Intramembranous ossification
Intramembranous ossification is one of the two essential processes during
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
The process of intramembranous ossification starts when a small group of adjacent MSCs begin to replicate and form a small, dense cluster of cells that is called a nidus.[a] 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
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
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
Formation of lamellar bone
The first step in the process is the formation of bone
Formation of bone spicules
Formation of trabecular bone
As the spicules continue to grow, they fuse with adjacent spicules and this results in the formation of trabeculae. When osteoblasts become trapped in the matrix they secrete, they differentiate into osteocytes. 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 osteon
Osteons 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 vessels, 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:[3]
- 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.[4]
See also
Footnotes
- ^ 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.
References
- ^ Journal of Bone and Joint Surgery. 73-A (6): 832–847.
- ISBN 0-914168-88-6.
- ^ "6.4 Bone Formation and Development – Anatomy and Physiology". opentextbc.ca. Retrieved 5 May 2018.
- ISSN 0271-5333.
- Martin, R.B.; Burr, D.B.; Sharkey, N.A. (1998). Skeletal Tissue Mechanics. Springer-Verlag. Chapter 2.