Intramembranous ossification
- A DIRECT form of ossification mesenchymal cells directly differentiate to osteoblasts (no cartilaginous model is first formed), such as occurs with flat bones (the skull bones).
Endochondral ossification
- An INDIRECT form of ossification, wherein a hyaline cartilaginous model (template) is replaced with bone, such as occurs with long bones (eg, the femur).
Intramembranous vs Endochondral
Major Bones & Their Development
Intramembranous Ossification
- Cranial Vault
- Maxilla/Mandible
- Clavicles
Consider that the skull bones must ossify prior to delivery of the fetus, so the brain isn't squashed during childbirth to help us remember that intramembranous ossification is a more direct form of ossification.
Endochondral ossification
- Skull base
- Vertebrae
- Pelvis
- Long Bones
They grow extensively throughout pediatric development and require an amount of pliability via their cartilaginous template prior to committing to ossification too soon. Consider that for a time-period, children fall often and thus must bounce and not break!