Step 1) Endochondral ossification starts when chondrocytes within the cartilage model enlarge and the surrounding matrix begins to calcify. The chondrocytes die because the diffusion of nutrients slow through the calcified matrix
Step 2) Bone formation first occurs at the shaft surface. Blood vessels invade the perichondrium, and cells of its inner layer differentiate into osteoblasts that begin producing bone matrix
Step 3) Blood vessels invade the inner region of the cartilage, and newly differentiated osteoblasts form spongy bone within the center of the shaft at a primary ossification center. Bone development proceeds toward either end, filling the shaft with spongy bone
Step 4) As the bone enlarges, osteoclasts break down some of the spongy bone and create a marrow cavity. The cartilage model does not completely fill with bone because the epiphyseal cartilages, or epiphyseal plates, on the ends continue to enlarge, increasing the length of the developing bone. Although osteoblasts from the shaft continuously invade the epiphyseal cartilages, the bone grows longer because new cartilage is continuously added in front of the advancing osteoblasts. This situation is like a pair of joggers, one in front of the other: as long as they run at the same speed, the one in back will never catch the one in front, no matter how far they travel
Step 5) The centers of the epiphyses begin to calcify. As blood vessels and osteoblasts enter theses areas, secondary ossification centers form, and the epiphyses eventually become filled with spongy bone. A thin cap of the original cartilage model remains exposed to the joint cavity as the articular cartilage. At this stage, the bone of the shaft and the bone of each epiphysis are still separated by epiphyseal cartilage. As long as the rate of the cartilage growth keeps pace with the rate of osteoblast invasion, the epiphyseal cartilage persists, and the bone continues to grow longer