Early in the development of a human fetus, the skeleton is made entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification. This is a process in which mineral deposits replace cartilage. Ossification of long bones, which are found in the arms and legs, begins at the center of the bones and continues toward the ends. By birth, several areas of cartilage remain in the skeleton, including the ends of the long bones. This cartilage grows as the long bones grow, so the bones can keep increasing in length during childhood.
It will be worth relating this aspect of one growth to the young students. Given that the children participating and learning from this unit of study will have larger growth plates (the growing cartilaginous bone at each end of the long bone), it is important to focus on this aspect.
In the late teens or early twenties, a person reaches skeletal maturity. By then, the cartilage has been replaced by bone, so no further growth in bone length is possible. However, bones can still increase in thickness. This may occur in response to increased load placed on the bone itself due to attached muscle activity, such as weight training.4
