Ossification

Heterotopic ossification is a process resulting in the formation of bone tissue that is often atypical, at an extraskeletal location.

The exact mechanisms by which bone development is triggered remains unclear, but growth factors and cytokines appear to play a role.

As osteoblasts continue to secrete osteoid, it surrounds blood vessels, leading to the formation of trabecular (cancellous or spongy) bone.

Osteoblasts secrete osteoid in parallel with the existing matrix, creating layers of compact (cortical) bone.

A perichondrium layer surrounding the cartilage forms the periosteum, which generates osteogenic cells that then go on to make a collar that encircles the outside of the bone and remodels the medullary cavity on the inside.

It invades the primary center of ossification, bringing osteogenic cells (osteoblasts on the outside, osteoclasts on the inside.)

The epiphyseal arteries and osteogenic cells invade the epiphysis, depositing osteoclasts and osteoblasts which erode the cartilage and build bone, respectively.

Bone is broken down by osteoclasts, and rebuilt by osteoblasts, both of which communicate through cytokine ( TGF-β , IGF ) signalling.
Diagram showing stages of endochondral ossification
Microscopic image of the growth plate
A spotted gar larva at 22 days stained for cartilage (blue) and bone (red).