An interactive Flash diagram of the process of bone growth, known as ossification, involving the laying down of calcium and phosphate salts (calcification) to replace either cartilage or membrane. An unborn baby’s bone is largely composed of a cartilaginous model enclosed by perichondrium—the process of ossification includes the production of a bone collar by osteoblasts present in this perichondrium. The cartilage model grows in length and width, and cartilage cells become trapped in small calcified tombs in the diaphysis region, dying and leaving lacunae (small chambers) behind in the bone; blood vessels grow into these chambers, which allows osteoblasts to travel along the tissue from the bone collar to the lacunae, where they form the primary ossification center, and begin producing cancellous (compact) bone. Osteoclasts travel there too, and remove bone in the center of the diaphysis to form the medullary cavity, where bone marrow will develop. Late in fetal development, secondary ossification centres are established in the ends of long bones, and ossification continues throughout a child’s growth—as bones continue to grow in length and width, cartilage is increasingly replaced by bone until only the articular cartilage remains, and the epiphyseal growth plate finally calcifies at adult size. Drag the slider to watch this process in action and discover more details.