Most bones begin as cartilage that is gradually replaced by bone.
As the embryonic bones grow in length and diameter, chondroblasts continually divide and form new cartilage at the active growth sites. After chondroblasts have laid down their cartilage, they mature into chondrocytes and take on the responsibility of nourishing and maintaining the cartilage surrounding them until it becomes ossified. The cartilage begins to ossify, or turn to bone, when its chondrocytes mature and die. 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.