Bone Formation by Endochondral Ossification

In the embryonic skeleton, the hyaline cartilage surrounded by perichondrium forms the general shape of bones, called the cartilaginous model.

Most bones, including long bones, replace this cartilage with bone tissue during development through endochondral ossification.

It begins with the perichondrium developing into a periosteum and forming a collar of bone matrix around the diaphysis of the model.

Simultaneously, the chondrocytes at the center become enlarged and direct the calcification of the surrounding matrix.

Because calcification prevents nutrient diffusion, the chondrocytes die, leaving behind cavities.

Next, osteogenic and hematopoietic cells infiltrate these cavities along with the nutrient vessels and nerves.

The osteogenic cells differentiate into osteoblasts, which deposit new bone matrix in these cavities and form the primary ossification center.

As the bone grows outward from the ossification center, the hematopoietic cells give rise to osteoclasts. These osteoclasts degrade the central matrix and form the medullary cavity.

By the twelfth week of gestation, most of the diaphysis is ossified. But, the ossification of epiphyses begins only after birth.