Bone Formation by Intramembranous Ossification

The human embryo begins bone formation around the eighth week of development.

The embryonic skeleton is initially composed of fibrous mesenchymal membranes and hyaline cartilage on which the bone matrix is deposited.

Bone formation occurs through two processes. While most bones develop from the cartilage by endochondral ossification, flat bones, such as the cranial bones, develop from the mesenchyme by intramembranous ossification.

During intramembranous ossification, clusters of mesenchymal cells differentiate into osteoblasts, which then secrete the unmineralized organic matrix.

This organic osteoid then calcifies into a hard bone matrix. The bone grows outward from such ossification centers and around the surrounding blood vessels.

These vessels provide oxygen and nutrition for further bone formation.

As the matrix grows further, it also surrounds some osteoblasts, which differentiate into mature osteocytes.

Eventually, the growth from various ossification centers interconnects, forming the spongy bone.

Towards the cortex, the osteoblasts secrete bone matrix layers that arrange into compact bone.

At the surface, the mesenchyme develops into the periosteum, resulting in a flat bone.