The long bones grow in parts. Early in fetal development, the bones are formed from cartilage. Bone tissue forms as special cells (called osteoblasts) lay down mineralized channels into the cartilage. Initially, the shafts, or diaphyses of the long bones begin to ossify. Later, the articular ends of the bone form their own centers of ossification, called epiphyses. Between the diaphysis and epiphyses remains a thin plate of cartilage, called the metaphysis.
As the bone grows, the metaphysis constantly adds new cartilage, and the diaphysis continues to ossify into this cartilage. So the bone can grow even as parts of it have already become mineralized tissue.
During the course of development, the bone tissue is recycled, gradually altering its shape. The hard cortical tissue can be invaded by cells that destroy the bone, called osteoclasts, only to have new bone laid down by secondary osteoblasts. The surface of the bone can be altered by having bone gradually removed, a process called resorption. Thus, bones remain living organs that can change their shape gradually, heal themselves, and adapt to new habits and needs.