Chondrocytes on the advancing edge of ossification enlarge die, and leave large holes in the cartilage. Blood vessels find their way to the area, bringing osteoblasts and osteoclasts with them to form and remodel bone on the thin columns of remaining cartilage. The original bone is woven and later remodeled into lamellar bone. The outer layers are remodeled into compact bone, while the bone within the central cavity becomes spongy bone. Primitive mesenchymal cells trapped within the spongy bone specialize into bone marrow.
Bone marrow is responsible for producing most of our blood cells.
In the newborn, all bones contain bone marrow. By adulthood, most of the marrow in long bones is replaced by fat. Long bones are made up of a long central shaft with an articulating knob on either end. A long bone begins to ossify at the center of its shaft. This is called its primary ossification center.