Endochondral ossification- Step 1: As the cartilage enlarges, chondrocytes near the center of the shaft increase greatly in size. The matrix is reduced to a series of small struts that soon begin to calcify. The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage. Step 2: Blood vessels grow around the edges of the cartilage, and the cells of the perichondrium convert to osteoblasts. The shaft of the cartilage then becomes ensheathed in a superficial layer of bone. Step 3: Blood vessels penetrate the cartilage and invade the central region. Fibroblasts migrating with the blood vessels differentiate into osteoblasts and begin producing spongy bone at a primary center of ossification. Bone formation then spreads along the shaft toward both ends. Step 4: Remodeling occurs as growth continues, creating a medullary cavity. The bone of the shaft becomes thicker, and the cartilage near each end epiphysis is replaced by shafts of bone. Further growth involves increasing in length and diameter. Step 5: Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers. Step 6: Soon the epiphyses are filled with spongy bone. An articular cartilage remains exposed to the joint cavity; over time it will be reduced to a thin superficial layer. Intramembraneous ossification- Step 1: Mesenchymal cells aggregate, differentiate into osteoblasts, and begin the ossification process. The bone expands as a series of spicules that spread into surrounding tissues. Step 2: As the spicules interconnect, they trap blood vessels within the bone. Step 3: Over time, the bone assumes the structure of spongy bone may later be removed, creating madullary cavities. Through remodeling, spongy bone formed in this way can ne converted to compact bone.