Growth that occurs by the addition of matrix to the surface of tissue is appositional growth. The surface of a bone consists of grooves and ridges. Blood vessels in the periosteum tend to lie in the grooves. New bone is added to the ridges (arrows), building them up.

When the bone built on adjacent ridges meets, the groove is transformed into a tunnel. The periosteum of the groove becomes the endosteum of the tunnel. Appositional growth by osteoblasts from the endosteum results in the formation of a new lamella. Additional bone apposition fills in the tunnel, thus completing the osteon.