Intramembranous ossification

Intramembranous ossification is characterized by the formation of bone directly from connective tissue - endemically.

Process
The first step of this process is the development of a condensed layer of  mesenchyme. In this structure, the cells become rounded, differentiate their organelle system and become osteoprogenitor cells. Simultaneously, the vascularization of this tissue is carried out. Along the capillaries, osteoprogenitor cells give rise to osteoblast,which will thereupon differentiate and proliferate. Osteoblast synthesize bone mass (matrix), deposit inorganic materials in it and create islets of woven bone developing the first bone beams, which will then act as ossification centers.

The structure grows apically, and osteoblast are aligned on the surface of the bone lacunae so that they can form another matrix. Converging trabeculae and islets of bone together with blood vessels forms the spongy structure of the primary bone. The ossification center enlarges and expands towards the periphery (circumference) of the bone until it  replaces the original connective tissue.

The most superficial layers of the connecting tissue, which do not undergo ossification turn into the periosteum. New bone formation takes place in the periosteum all throughout life, but most intensively during the growth period. This appositional growth is complemented by resorption of bone to preserve the shapes and proportions of the body.

The original woven bone is then transformed into lamellar bone.

Examples
This type of ossification is characteristic of cranial flat bones, the facial part of the skull and clavicle.

Related articles

 * Bone
 * Endochondral ossification
 * Bone growth and healing