Anatomy and Physiology: Two Ways of Bone Growth and Development

Bones can develop embryonically or postembryonically.  Bones use hyaline cartilage structures as "models" during bone formation or ossification.  The picture below shows the process of bone growth, both embryonic and postembryonic.

(This picture is taken from this site: http://training.seer.cancer.gov/anatomy/skeletal/growth.html).

During the embryonic development of the bone, the hyaline cartilage model is covered by oseoblasts (bone-forming cells).  The cartilage model would remain there until it is digested away and a medullary cavity opens up within the new bone.

During postembryonic development (bone development that happens after birth), most hyaline cartilage models are replaced by bone except the articular cartilages, which cover the bone ends, and the epiphyseal plates. The articular cartilages are never converted into bone because they reduce friction at the joints.  During long bone growth after birth, the long bones lengthen and widen.  When a long bone lengthens, new cartilage is always being formed on external side of the articular cartilage and the epiphyseal plate while the old cartilage is broken down and replaced by bony matrix on the internal side of the articular cartilage and medullary cavity.  When a long bone widens, it undergoes appositional growth, in which the osteoblasts in the periosteum add bone tissue to the outer surface of the diaphysis (shaft of the bone) while the osteoclasts (bone-destroying) cells break down bone tissue in the inner surface of the diaphysis.

Cartilage and bone tissue is constantly being replaced during  a bone's appositional growth. (This picture is taken from this site: http://www.rci.rutgers.edu/~uzwiak/AnatPhys/APFallLect8.html).

Sources:
Elaine N. Marieb's Essentials of Human Anatomy & Physiology
Dawn A. Tamarkin's website http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/bone/bonedev.htm