Endochondral ossification

The periosteal band, the future diaphysis, surrounds the spongy bone and myeloid tissue located in the center of the bone. This band was formed by the perichondrium-turned-periosteum that originally surrounded the cartilage template. As the bone grows, this band will increase in thickness, and the woven bone comprising it will be replaced by lamellar bone.

Red marrow fills the spaces between the centrally located spicules. Red marrow contains blood vessels and myeloid (hemopoietic) tissue that produces blood cells.

The blue-staining central cores of the spicules are composed of calcified cartilage (blue arrows) formed by the chondrocytes in the epiphyseal plate. These cores form a framework on which woven bone (black arrows) can be deposited by osteoblasts.

Because the spicules are composed of calcified cartilage and bone, this image illustrates either the zone of ossification or the zone of resorption. Because osteoclasts cannot be seen in the surrounding endosteum, the zone of ossification is represented here.

The right image is quite similar to that on the left with the exception that several osteoclasts (arrows) are present in the endosteum surrounding the spicules of calcified cartilage plus bone. The presence of these cells indicates that this must be the zone of resorption.