Throughout the body, cells that form bone are called osteoblasts.

This irritates the old bone, and stimulates it to form new bone.

And if they added vitamin D3, which is needed to form bone, the cells turned into bone.

"The most amazing thing to me is that fat tissue has the capacity to form bone," he said.

But some stem cells move to where they are needed, from the bone marrow to form muscle, bone or adipose (fat) tissue.

As the older chondrocytes degenerate, osteoblasts ossify the remains to form new bone.

Later, it is revealed that the metal has grown out from where Nathan's amputated leg once was, forming metallic bone, muscles, and flesh.

As the embryo develops, the cartilage hardens to form bone.

Osteoblasts also work in groups to form new bone.

Facial ectomesenchyme of the pharyngeal arches forming skeletal muscle, bone, and cartilage in the face.